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Thermochromic Inks

Thermochromic inks

Thermochromism is the ability of a substance to change colour due to a change in temperature. A mood ring is an excellent example of this, but it has many other uses. Thermochromism is one of several types of chromism. __NOTOC__

Principles

Liquid crystals

Some liquid crystals are capable of displaying different colors at different temperatures. This change is dependent on selective reflection of certain wavelengths by the crystallic structure of the material, as it changes between the low-temperature crystallic phase, through anisotropic chiral or twisted nematic phase, to the high-temperature isotropic liquid phase. Only the nematic mesophase has thermochromic properties; this restricts the effective temperature range of the material. The twisted nematic phase has the molecules oriented in layers with regularly changing orientation, which gives them periodic spacing. The light passing the crystal undergoes Bragg diffraction on these layers, and the wavelength with the greatest constructive interference is reflected back, which is perceived as a spectral color. As the crystal undergoes changes in temperature, thermal expansion occurs, resulting in change of spacing between the layers, and therefore in the reflected wavelength. The color of the thermochromic liquid crystal can therefore continuously range from black through the spectral colors to black again, depending on the temperature. Some such materials are cholesteryl nonanoate or cyanobiphenyls. Liquid crystals used in dyes and inks often come microencapsulated, in the form of suspension.

Leuco dyes

Thermochromic dyes are based on mixtures of leuco dyes with suitable other chemicals, displaying a color change (usually between the colorless leuco form and the colored form) in dependence on temperature. The dyes are rarely applied on materials directly; they are usually in the form of microcapsules with the mixture sealed inside. An illustrative example is the Hypercolor fashion, where microcapsules with crystal violet lactone, weak acid, and a dissociable salt dissolved in dodecanol are applied to the fabric; when the solvent is solid, the dye exists in its lactone leuco form, while when the solvent melts, the salt dissociates, the pH inside the microcapsule lowers, the dye becomes protonated, its lactone ring opens, and its absorption spectrum shifts drastically, therefore it becomes deeply violet. In this case the apparent thermochromism is in fact halochromism. The dyes most commonly used are spirolactones, fluorans, spiropyrans, and fulgides. The weak acids include bisphenol A, parabens, 1,2,3-triazole derivates, and 4-hydroxycoumarin and act as proton donors, changing the dye molecule between its leuco form and its protonated colored form; stronger acids would make the change irreversible.

Materials

Inks

Thermochromic inks or dyes are temperature sensitive compounds, developed in the 1970s, that temporarily change color with exposure to heat. They come in two forms, liquid crystals and leuco dyes. Liquid crystals are used in mood rings. Leuco dyes are easier to work with and allow for a greater range of applications. These applications include: flat thermometers, battery testers, clothing, and the indicator on bottles of maple syrup that change color when the syrup is warm. The most well-known line of clothing utilizing thermochromics was Hypercolor. The thermometers are often used on the exterior of aquariums, or to obtain a body temperature via the forehead.

Paints

Thermochromic paint is a relatively recent development in the area of color-changing pigments. It involves the use of liquid crystal or leuco dye technology. After absorbing a certain amount of light or heat, the crystallic or molecular structure of the pigment reversibly changes in such a way that it absorbs and emits light at a different wavelength than at lower temperatures. Thermochromic paints are seen quite often as a coating on coffee mugs, whereby once hot coffee is poured into the mugs, the thermochromic paint absorbs the heat and becomes colored or transparent, thus changing the appearance of the mug.

Papers

Thermochromic papers are used for thermal printers. One example is the paper impregnated with the solid mixture of a fluoran dye with octadecylphosphonic acid. This mixture is stable in solid phase; however, when the octadecylphosphonic acid is melted, the dye undergoes chemical reaction in the liquid phase, and assumes the protonated colored form. This state is then conserved when the matrix solidifies again, if the cooling process is fast enough. As the leuco form is more stable in lower temperatures and solid phase, the records on thermochromic papers slowly fade out over years; this may lead to interesting effects in combination with accounting records, receipts from a thermal printer, and a tax audit.

Others

Another good example of this is the color indicators on batteries. The indicator turns green if the battery still possesses a charge. This works by passing the charge of the battery through a small resistor on the battery, and causes the pigment to absorb heat. Once the paint has absorbed enough heat from the current of the battery, it changes from black to green (usually), thus indicating that the battery still has a fair amount of charge left in it. A simple to make thermochromic compound is zinc oxide this is white at room temperature, but when it is heated to changes to yellow due to various types of crystal lattice defects, on cooling the zinc oxide reverts to white. Also lead(II) oxide has a similar colour change on heating. These solids are technically semiconductors, and the colour change is linked to their electronic properties. Copper mercury iodide undergoes a phase transition at 55 °C, reversibly changing from a solid material at low temperature to a dark brown solid at high temperature. Other such material is mercury iodide. Yet another example is nickel sulfate, green at room temperature but becoming yellow at 155 °C. Thermochromic solid semiconductor materials investigated for commercial use are _x_1-x_y_1-y (x=0.5...1, y=0.5...1), Zn_xCd_y_1-x-y_aS_bSe_c_1-a-b-c (x=0...0.5, y=0.5...1, a=0...0.5, b=0.5...1, c=0...0.5), Hg_xCd_yZn_1-x-yS_bSe_1-b (x=0...1, y=0...1, b=0.5...1). [http://www.patentstorm.us/patents/5499597.html]

External links

- [http://www.mutr.co.uk/pdf_files/SMARTCOL.pdf/ Thermochromic pigments]
- [http://www.artrend.com.hk/products/matsui/matsuiMenu.htm Matsui Shikiso Chemical]
- [http://www.colorchange.com/ Color Change Corporation]
- [http://www.t-m-c.com/ Liquid Crystal Resources LLC]
- [http://www.alsacorp.com/products/xposurepaint/xposurepaint_prodinfo.htm ALSA Corporation Exotic Paint]
- [http://jchemed.chem.wisc.edu/HS/Journal/Issues/1999/Sep/clicSubscriber/V76N09/p1201.pdf Thermochromism in commercial products, PDF] Category:inks Category:paints Thermochromism Category:Chromism

Colour

Color

Temperature

Temperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter. Physically, temperature is a measure of the random agitation of matter and ambiant photons, under the effect of thermal fluctuations. It is a fundamental parameter in thermodynamics and it is conjugate to entropy. More quantitatively, the order of magnitude of the fluctuations of the energy associated with an atom, molecule or another elementary constituant of a physical system is

k_B T

, where

k_B

is Boltzmann's constant, and T is temperature, expressed in Kelvins.

Overview

The formal properties of temperature are studied in thermodynamics and statistical mechanics. The temperature of a system at thermodynamic equilibrium is defined by a relation between the amount of heat

\delta Q

incident on the system during an infinitesimal quasistatic transformation, and the variation

\delta S

of its entropy during this transformation. :

\delta S = \frac

Contrarly to entropy and heat, whose microscopic definitions are valid even far away from thermodynamic equilibrium temperature can only be defined at thermodynamic equilibrium, or local thermodynamic equilibrium (see below). As a system receives heat its temperature rises, similarly a loss of heat from the system tends to decrease its temperature (at the - uncommon - exception of negative temperature, see below). When two systems are at the same temperature, no heat transfer occurs between them. When a temperature difference does exist, heat will tend to move from the higher-temperature system to the lower-temperature system, until they are at thermal equilibrium. This heat transfer may occur via conduction, convection or radiation (see heat for additional discussion of the various mechanisms of heat transfer). Temperature is also related to the amount of internal energy and enthalpy of a system. The higher the temperature of a system, the higher its internal energy and enthalpy are. Temperature is an intensive property of a system, meaning that it does not depend on the system size or the amount of material in the system. Other intensive properties include pressure and density. By contrast, mass and volume are extensive properties, and depend on the amount of material in the system.

Role of temperature in nature

Temperature plays an important role in almost all fields of science, including physics, chemistry, and biology. Many physical properties of materials including the phase (solid, liquid, gaseous or plasma), density, solubility, vapor pressure, and electrical conductivity depend on the temperature. Temperature also plays an important role in determining the rate and extent to which chemical reactions occur. This is one reason why the human body has several elaborate mechanisms for maintaining the temperature at 37 °C, since temperatures only a few degrees higher can result in harmful reactions with serious consequences. Temperature also controls the type and quantity of thermal radiation emitted from a surface. One application of this effect is the incandescent light bulb, in which a tungsten filament is electrically heated to a temperature at which significant quantities of visible light are emitted. Temperature-dependence of the speed of sound in air c, density of air ρ and acoustic impedance Z vs. temperature °C

Temperature measurement

Main article: Temperature measurement Temperature measurement using modern scientific thermometers and temperature scales goes back at least as far as the early 18th century, when Gabriel Fahrenheit adapted a thermometer (switching to mercury) and a scale both developed by Ole Christensen Rømer. Fahrenheit's scale is still in use, alongside the Celsius scale and the Kelvin scale.

Units of temperature

The basic unit of temperature (symbol: T) in the International System of Units (SI) is the kelvin (K). One kelvin is formally defined as 1/273.16 of the temperature of the triple point of water (the point at which water, ice and water vapor exist in equilibrium). The temperature 0 K is called absolute zero and corresponds to the point at which the molecules and atoms have the least possible thermal energy. An important unit of temperature in theoretical physics is the Planck temperature (1.4 × 10³² K). In the field of plasma physics, because of the high temperatures encountered and the electromagnetic nature of the phenomena involved, it is customary to express temperature in electronvolts (eV) or kiloelectronvolts (keV), where 1 eV = 11,605 K. In the study of QCD matter one routinely meets temperatures of the order of a few hundred MeV, equivalent to about 10¹² K. For everyday applications, it is often convenient to use the Celsius scale, in which 0 °C corresponds to the temperature at which water freezes and 100 °C corresponds to the boiling point of water at sea level. In this scale a temperature difference of 1 degree is the same as a 1 K temperature difference, so the scale is essentially the same as the kelvin scale, but offset by the temperature at which water freezes (273.15 K). Thus the following equation can be used to convert from degrees Celsius to kelvins. :

\mathrm

In the United States, the Fahrenheit scale is widely used. On this scale the freezing point of water corresponds to 32 °F and the boiling point to 212 °F. The following formula can be used to convert from Fahrenheit to Celsius: :

\mathrm

See temperature conversion formulas for conversions between most temperature scales. ¹ Only the kelvin, Celsius, Fahrenheit, and Rankine scales are in use today.
² Some numbers in this table have been rounded off.
³ Normal human body temperature is 36.8 °C ±0.7 °C, or 98.2 °F ±1.3 °F.

Negative temperatures

:See main article: Negative temperature. For some systems and specific definitions of temperature, it is possible to obtain a negative temperature. A system with a negative temperature is not colder than absolute zero, but rather it is, in a sense, hotter than infinite temperature (sic).

Articles about temperature ranges:

- 10⁻¹² K = 1 picokelvin (pK)
- 10⁻⁹ K = 1 nanokelvin (nK)
- 10⁻⁶ K = 1 microkelvin (µK)
- 10⁻³ K = 1 millikelvin (mK)
- 10⁰ K = 1 kelvin
- 10¹ K = 10 kelvins
- 10² K = 100 kelvins
- 10³ K = 1,000 kelvin = 1 kilokelvin (kK)
- 10⁴ K = 10,000 kelvins = 10 kK
- 10⁵ K = 100,000 kelvins = 100 kK
- 10⁶ K = 1 megakelvin (MK)
- 10⁹ K = 1 gigakelvin (GK)
- 10¹² K = 1 terakelvin (TK) See Orders of magnitude (temperature).

Theoretical foundation of temperature

Zeroth-law definition of temperature

While most people have a basic understanding of the concept of temperature, its formal definition is rather complicated. Before jumping to a formal definition, let us consider the concept of thermal equilibrium. If two closed systems with fixed volumes are brought together, so that they are in thermal contact, changes may take place in the properties of both systems. These changes are due to the transfer of heat between the systems. When a state is reached in which no further changes occur, the systems are in thermal equilibrium. Now a basis for the definition of temperature can be obtained from the so-called zeroth law of thermodynamics which states that if two systems, A and B, are in thermal equilibrium and a third system C is in thermal equilibrium with system A then systems B and C will also be in thermal equilibrium (being in thermal equilibrium is a transitive relation; moreover, it is an equivalence relation). This is an empirical fact, based on observation rather than theory. Since A, B, and C are all in thermal equilibrium, it is reasonable to say each of these systems shares a common value of some property. We call this property temperature. Generally, it is not convenient to place any two arbitrary systems in thermal contact to see if they are in thermal equilibrium and thus have the same temperature. Also, it would only provide an ordinal scale. Therefore, it is useful to establish a temperature scale based on the properties of some reference system. Then, a measuring device can be calibrated based on the properties of the reference system and used to measure the temperature of other systems. One such reference system is a fixed quantity of gas. The ideal gas law indicates that the product of the pressure and volume (P · V) of a gas is directly proportional to the temperature: :

P \cdot V = n \cdot R \cdot T

(1) where 'T is temperature, n is the number of moles of gas and R is the gas constant. Thus, one can define a scale for temperature based on the corresponding pressure and volume of the gas: the temperature in kelvins is the pressure in pascals of one mole of gas in a container of one cubic metre, divided by 8.31... In practice, such a gas thermometer is not very convenient, but other measuring instruments can be calibrated to this scale. Equation 1 indicates that for a fixed volume of gas, the pressure increases with increasing temperature. Pressure is just a measure of the force applied by the gas on the walls of the container and is related to the energy of the system. Thus, we can see that an increase in temperature corresponds to an increase in the thermal energy of the system. When two systems of differing temperature are placed in thermal contact, the temperature of the hotter system decreases, indicating that heat is leaving that system, while the cooler system is gaining heat and increasing in temperature. Thus heat always moves from a region of high temperature to a region of lower temperature and it is the temperature difference that drives the heat transfer between the two systems.

Temperature in gases

As mentioned previously for a monatomic ideal gas the temperature is related to the translational motion or average speed of the atoms. The kinetic theory of gases uses statistical mechanics to relate this motion to the average kinetic energy of atoms and molecules in the system. For this case 7736 K = 7463 degrees Celsius corresponds to an average kinetic energy of one electronvolt; to take room temperature (300 K) as an example, the average energy of air molecules is 300/7736 eV, or 0.0388 electronvolt. This average energy is independent of particle mass, which seems counterintuitive to many people. Although the temperature is related to the average kinetic energy of the particles in a gas, each particle has its own energy which may or may not correspond to the average. However, after an examination of some basic physics equations it makes perfect sense. The second law of thermodynamics states that any two given systems when interacting with each other will later reach the same average energy. Temperature is a measure of the average kinetic energy of a system. The formula for the kinetic energy of an atom is:
:

K_t = \begin \frac \end mv^2

(Note that a calculation of the kinetic energy of a more complicated object, such as a molecule, is slightly more involved. Additional degrees of freedom are available, so molecular rotation or vibration must be included.)

Thus, particles of greater mass (say a neon atom relative to a hydrogen molecule) will move slower than lighter counterparts, but will have the same average energy. This average energy is independent of the mass because of the nature of a gas, all particles are in random motion with collisions with other gas molecules, solid objects that may be in the area and the container itself (if there is one). A visual illustration of this [http://intro.chem.okstate.edu/1314F00/Laboratory/GLP.htm from Oklahoma State University] makes the point more clear. Not all the particles in the container have different velocities, regardless of whether there are particles of more than one mass in the container, but the average kinetic energy is the same because of the ideal gas law. In a gas the distribution of energy (and thus speeds) of the particles corresponds to the Boltzmann distribution. An electronvolt is a very small unit of energy, approximately 1.602×10^-19 joule.

Temperature of the vacuum

When a satellite in empty space is heated by sunshine and cooled by radiating energy away it is not in thermodynamic equilibrium and has no well-defined temperature. A system in a vacuum will radiate its thermal energy, i.e. convert heat into electromagnetic waves. If vacuum is filled with electromagnetic waves (say, radiation from walls of vacuum chamber, or relic microwave radiation in space) then the system will exchange by energy with these waves and thermally equilibrates at some finite (non zero) temperature. Cosmic microwave background radiation being remnant of radiation of hot early universe when radiation was in thermal equilibrium with matter has Planck spectrum (black body spectrum) with the temperature (at present) of about 2.7 K.

Second-law definition of temperature

In the previous section temperature was defined in terms of the Zeroth Law of thermodynamics. It is also possible to define temperature in terms of the second law of thermodynamics, which deals with entropy. Entropy is a measure of the disorder in a system. The second law states that any process will result in either no change or a net increase in the entropy of the universe. This can be understood in terms of probability. Consider a series of coin tosses. A perfectly ordered system would be one in which every coin toss would come up either heads or tails. For any number of coin tosses, there is only one combination of outcomes corresponding to this situation. On the other hand, there are multiple combinations that can result in disordered or mixed systems, where some fraction are heads and the rest tails. As the number of coin tosses increases, the number of combinations corresponding to imperfectly ordered systems increases. For a very large number of coin tosses, the number of combinations corresponding to ~50% heads and ~50% tails dominates and obtaining an outcome significantly different from 50/50 becomes extremely unlikely. Thus the system naturally progresses to a state of maximum disorder or entropy. Now, we have stated previously that temperature controls the flow of heat between two systems and we have just shown that the universe, and we would expect any natural system, tends to progress so as to maximize entropy. Thus, we would expect there to be some relationship between temperature and entropy. In order to find this relationship let's first consider the relationship between heat, work and temperature. A heat engine is a device for converting heat into mechanical work and analysis of the Carnot heat engine provides the necessary relationships we seek. The work from a heat engine corresponds to the difference between the heat put into the system at the high temperature, q_H and the heat ejected at the low temperature, q_C. The efficiency is the work divided by the heat put into the system or: :

\textrm = \frac  = \frac = 1 - \frac

(2) where w_cy is the work done per cycle. We see that the efficiency depends only on q_C/q_H. Because q_C and q_H correspond to heat transfer at the temperatures T_C and T_H, respectively, q_C/q_H should be some function of these temperatures: :

\frac = f(T_H,T_C)

(3) Carnot's theorem states that all reversible engines operating between the same heat reservoirs are equally efficient. Thus, a heat engine operating between T₁ and T₃ must have the same efficiency as one consisting of two cycles, one between T₁ and T₂, and the second between T₂ and T₃. This can only be the case if: :

q_ = \frac

which implies: :

q_13 = f(T_1,T_3) = f(T_1,T_2)f(T_2,T_3)

Since the first function is independent of T₂, this temperature must cancel on the right side, meaning f(T₁,T₃) is of the form g(T₁)/g(T₃) (i.e. f(T₁,T₃) = f(T₁,T₂)f(T₂,T₃) = g(T₁)/g(T₂)· g(T₂)/g(T₃) = g(T₁)/g(T₃)), where g is a function of a single temperature. We can now choose a temperature scale with the property that: :

\frac = \frac

(4) Substituting Equation 4 back into Equation 2 gives a relationship for the efficiency in terms of temperature: :

\textrm = 1 - \frac = 1 - \frac

(5) Notice that for T_C = 0 K the efficiency is 100% and that efficiency becomes greater than 100% below 0 K. Since an efficiency greater than 100% violates the first law of thermodynamics, this implies that 0 K is the minimum possible temperature. In fact the lowest temperature ever obtained in a macroscopic system was 20 nK, which was achieved in 1995 at NIST. Subtracting the right hand side of Equation 5 from the middle portion and rearranging gives: :

\frac  - \frac = 0

where the negative sign indicates heat ejected from the system. This relationship suggests the existence of a state function, S, defined by: :

dS = \frac

(6) where the subscript indicates a reversible process. The change of this state function around any cycle is zero, as is necessary for any state function. This function corresponds to the entropy of the system, which we described previously. We can rearranging Equation 6 to get a new definition for temperature in terms of entropy and heat: :

T = \frac

(7) For a system, where entropy S may be a function S(E) of its energy E, the temperature T is given by: :

\frac = \frac

(8) The reciprocal of the temperature is the rate of increase of entropy with energy.

References

External links

- [http://www.unitconversion.org/unit_converter/temperature.html Online Temperature Converter] - convert between various units of temperature, such as kelvin, Celsius, Fahrenheit, Rankine, Reaumur, and even Triple point of water
- [http://www.unitconversion.org/unit_converter/temperature-v.html Interactive Temperature Conversion Table] - convert selected unit to all other units of temperature
- [http://www.indiana.edu/~animal/fun/conversions/temperature.html Temperature Conversions: Celsius, Fahrenheit, Kelvin, Réaumur and Rankine]
- [http://www.unidata.ucar.edu/staff/blynds/tmp.html An elementary introduction to temperature aimed at a middle school audience]
- [http://www.straightdope.com/mailbag/mtempscales.html Why do we have so many temperature scales?]
- [http://thermodynamics-information.net A Brief History of Temperature Measurement] Category:Meteorology Category:Physical quantity Category:Thermodynamics Category:Heat ko:온도 ja:温度 th:อุณหภูมิ

Mood ring

A mood ring is a novelty ring which changes color in response to body temperature, using a thermochromic liquid crystal. A form of biofeedback, they supposedly indicated the temperament of the wearer, indicated by the ring's color. Mood rings were a fad whose popularity peaked in the United States in the 1970s, and they are now seen as an icon of 1970s culture. Not everyone agrees on what the colors mean, but generally black stands for stressed or anxious and blue represents happy or relaxed. This is derived from the temperature of extermities depending on stress-related vasoconstriction. However the temperature of the environment and individual variations will greatly skew the measurements and make the mood ring readouts rather unreliable. There are many different types of mood ring, and generally depends on the manufacturer of what the colors are, although, the most common ones are listed below. They come in necklaces, earrings, toerings, and finger rings. Most commonly though, the finger rings and the earrings are the most popular. The Mood Ring was invented by Joshua Reynolds, who also invented the Thighmaster. Reynolds is heir to the fortune of Richard Joshua Reynolds, tobacco company founder.

In popular culture

- A mood ring plays a central part in the movie My Girl.
- Relient K released a popular song called "Mood Rings" that suggested, in a tongue-in-cheek manner, giving mood rings to overly emotional girls.

The meanings of various colors

- Black: Tense, nervous, harrassed, overworked.
- Gray: Anxious, nervous, strained.
- Amber: Nervous, emotions mixed, unsettled, cool
- Green: Average reading. Active, not under great stress
- Blue-green: Emotionally charged, somewhat relaxed
- Blue: Relaxed, at ease, calm
- Dark blue: Very happy See also: Thermochromics

External links

- [http://www.super70s.com/Super70s/Culture/Fads/Mood_Ring.asp Super70s.com] Contains more information about the mood ring and other 1970s fads.
- [http://home.howstuffworks.com/question443.htm How stuff works] Category:Rings Category:1970s fads Category:Thermochromism

Liquid crystal

Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal. For instance, a liquid crystal (LC) may flow like a liquid, but have the molecules in the liquid arranged and oriented in a crystal-like way. There are many different types of LC phase, which can be distinguished based on their different optical properties (such as birefringence). Viewed in a microscope under polarized light illumination, a liquid crystal material will appear to have a distinct texture. Each 'patch' in the texture corresponds to a domain where the LC molecules are oriented in a different direction. Within a domain, however, the molecules are well ordered. Liquid crystal materials may not always be in an LC phase (just as water is not always in the liquid phase: it may also be found in the solid or gas phase). Liquid crystals can be divided into thermotropic and lyotropic LCs. Thermotropic LCs exhibit a phase transition into the LC phase as temperature is changed, whereas lyotropic LCs exhibit phase transitions as a function of concentration.

Mesogens

Molecules that exhibit liquid crystal phases are called mesogens. For a molecule to display an LC phase, it must generally be rigid and anisotropic (i.e. longer in one direction than another). Most mesogens fall into the 'rigid-rod' class (calamitic mesogens), which orient based on their long axis. Disk-like (discotic) mesogens are also known, and these orient in the direction of their short axis. In addition to molecules, polymers and colloidal suspensions can also form LC phases. For instance, micrometre-sized objects (such as anisotropic colloids, latex particles, clay platelets, and even some viruses, such as the tobacco mosaic virus) can organize themselves in liquid crystal phases.

Liquid crystal phases

The various LC phases (called mesophases) can be characterized by the type of ordering that is present. One can distinguish positional order (whether or not molecules are arranged in any sort of ordered lattice) and orientational order (whether or not molecules are pointing in the same direction), and moreover order can be either short-range (only between molecules close to each other) or long-range (extending to larger, sometimes macroscopic, dimensions). Most thermotropic LCs will have an isotropic phase at high temperature. That is, heating will eventually drive them into a conventional liquid phase characterized by random and isotropic molecular ordering (little to no long-range order), and fluid-like flow behavior. Under other conditions (for instance, lower temperature), an LC might inhabit one or more phases with significant anisotropic orientational structure and long-range orientational order while still having an ability to flow. The orientational order may be quasicrystalline. The ordering of liquid crystalline phases is extensive on the molecular scale. This order extends up to the entire domain size, which may be on the order or micrometres, but usually does not extend to the macroscopic scale as often occurs in classical crystalline solids. However, some techniques (such as the use of boundaries or an applied electric field) can be used to enforce a single ordered domain in a macroscopic liquid crystal sample. The ordering in a liquid crystal might extend along only one dimension, with the material being essentially disordered in the other two directions.

Thermotropic liquid crystals

Thermotropic phases are those that occur in a certain temperature range. If the temperature is raised too high, thermal motion will destroy the delicate cooperative ordering of the LC phase, pushing the material into a conventional isotropic liquid phase. At too low a temperature, most LC materials will form a conventional (though anisotropic) crystal. Many thermotropic LCs exhibit a variety of phases as temperature is changed. For instance, a particular mesogen may exhibit various smectic and nematic (and finally isotropic) as temperature is increased.

Nematic phase

One of the most common LC phases is the nematic, where the molecules have no positional order, but they do have long-range orientational order. Thus, the molecules flow and are randomly distributed as in a liquid, but they all point in the same direction (within each domain). Most nematics are uniaxial: they have one axis that is longer and preferred, with the other two being equivalent (can be approximated as cylinders). Some liquid crystals are biaxial nematics, meaning that in addition to orienting their long axis, they also orient along a secondary axis.

Smectic phase

The smectic phase is one where in addition to orientation order, the mesogens are grouped into layers, enforcing long-range positional order in one direction. In the smetic A phase, the molecules point perpendicular to the layer planes, whereas in the smectic C phase, the molecules are tilted with respect to the layer planes. In hexatic phases, the mesogens in a particular layer take on a roughly hexagonal close-packed ordering, with typically no registry between adjacent smectic layers. It is also possible to find examples of liquid crystals where the registry between layers is fairly strong, hence there is three dimensional positional (and possibly even orientational) order. These phases are called crystal mesophases, and are in fact nearly as ordered as solid crystals (although they still exhibit fluid-like flow).

Chiral phases

The chiral nematic phase exhibits chirality (handedness). This phase is often called the cholesteric phase because it was first observed for cholesterol derivatives. Only chiral molecules (i.e.: those that lack inversion symmetry) can give rise to such a phase. This phase exhibits a twisting of the molecules along the director, with the molecular axis perpendicular to the director. The finite twist angle between adjacent molecules is due to their asymmetric packing, which results in longer-range chiral order. In the smectic C
- phase, the molecules orient roughly along the director, with a finite tilt angle, and a twist relative to other mesogens. This results in, again, a spiral twisting of molecular axis along the director. The chiral pitch refers to the distance (along the director) over which the mesogens undergo a full 360º twist (but note that the structure repeats itself every half-pitch, since the positive and negative directions along the director are equivalent). The pitch may be varied by adjusting temperature or adding other molecules to the LC fluid. For many types of liquid crystals, the pitch is on the same order as the wavelength of visible light. This causes these systems to exhibit unique optical properties, such as selective reflection. These properties are exploited in a number of optical applications.

Discotic phases

Disk-shaped mesogens can orient themselves in a layer-like fashion known as the discotic nematic phase. If the disks pack into stacks, the phase is called a discotic columnar. The columns themselves may be organized into rectangular or hexagonal arrays. Chiral discotic phases, similar to the chiral nematic phase, are also known.

Lyotropic liquid crystals

A lyotropic liquid crystal consists of two or more components that exhibit liquid-crystalline properties in certain concentration ranges. In the lyotropic phases, solvent molecules fill the space around the compounds to provide fluidity to the system. In contrast to thermotropic liquid crystals, these lyotropics have another degree of freedom of concentration that enables them to induce a variety of different phases. A compound which has two immiscible hydrophilic and hydrophobic parts within the same molecule is called an amphiphilic molecule. Many amphiphilic molecules show lyotropic liquid-crystalline phase sequences depending on the volume balances between the hydrophilic part and hydrophobic part. These structures are formed through the micro-phase segregation of two incompatible components on a nanometer scale. Soap is a everyday example of a lyotropic liquid crystal. The content of water or other solvent molecules changes the self-assembled structures. At very low amphiphile concentration, the molecules will be dispersed randomly without any ordering. At slightly higher (but still low) concentration, amphiphilic molecules will spontaneously assemble into micelles or vesicles. This is done so as to 'hide' the hydrophic tail of the amphiphile inside the micelle core, exposing a hydrophilic (water-soluble) surface to aqueous solution. These spherical objects do not order themselves in solution, however. At higher concentration, the assemblies will become ordered. A typical phase is a hexagonal columnar phase, where the amphiphiles form long cylinders (again with a hydrophilic surface) that arrange themselves into a roughly hexagonal lattice. This is called the middle soap phase. At still higher concentration, a lamellar phase (neat soap phase) may form, wherein extended sheets of amphiphiles are separated by thin layers of water. For some systems, a cubic (also called viscous isotropic) phase may exist between the hexagonal and lamellar phases, wherein spheres are formed that create a dense cubic lattice. These spheres may also be connected to one another, forming a bicontinuous cubic phase. The objects created by amphiphiles are usually spherical (as in the case of micelles), but may also be disc-like (bicelles), rod-like, or biaxial (all three micelle axes are distinct). These anisotropic self-assembled nano-structures can then order themselves in much the same way as liquid crystals do, forming large-scale versions of all the thermotropic phases (such as a nematic phase of rod-shaped micelles). For some systems, at high concentration, inverse phases are observed. That is, one may generate an inverse hexagonal columnar phase (columns of water encapsulated by amphiphiles) or an inverse micellar phase (a bulk liquid crystal sample with spherical water cavities). A generic progression of phases, going from low to high amphiphile concentration, is:
- Discontinuous cubic phase (micellar phase)
- Hexagonal columnar phase (middle phase)
- Bicontinuous cubic phase
- Lamellar phase
- Bicontinuous cubic phase
- Reverse hexagonal columnar phase
- Inverse cubic phase (Inverse micellar phase) Even within the same phases, their self-assembled structures are tunable by the concentration: for example, in lamellar phases, the layer distances increase with the solvent volume. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is more difficult to analyze their structures and properties than those of thermotropic liquid crystals. Similar phases and characteristics can be observed in immiscible diblock copolymers.

Biological liquid crystals

Lyotropic liquid-crystalline nanostructures are abundant in living systems. Accordingly, lyotropic liquid crystals attract particular attention in the field of biomimetic chemistry. In particular, biological membranes are a form of liquid crystal. Their constituent rod-like molecules (e.g., phospholipids) are organized perpendicularly to the membrane surface, yet the membrane is fluid and elastic. The constituent molecules can flow in-plane quite easily, but tend not to leave the membrane, and can flip from one side of the membrane to the other with some difficulty. These liquid crystal membrane phases can also host important proteins such as receptors freely "floating" inside, or partly outside, the membrane. Many other biological structures exhibit LC behavior. For instance, the concentrated protein solution that is extruded by a spider to generate silk is, in fact, a liquid crystal phase. The precise ordering of molecules in silk is critical to its renowned strength. DNA and many polypeptides can also form LC phases. Since biological mesogens are usually chiral, chirality often plays a role in these phases.

Theoretical treatment of liquid crystals

Microscopic theoretical treatment of fluid phases can become quite involved, owing to the high material density, which means that strong interactions, hard-core repulsions, and many-body correlations cannot be ignored. In the case of liquid crystals, anisotropy in all of these interactions further complicate analysis. There are a number of fairly simple theories, however, that can at least predict the general behavior of the phase transitions in liquid crystal systems.

Order parameter

The description of liquid crystals involves an analysis of order. To make this quantitative, an orientational order parameter is usually defined based on the average of the second Legendre polynomial: :

S = \langle P_2(\cos \theta) \rangle = \left \langle \frac \right \rangle

where

\theta

is the angle between the mesogen molecule axis and the local director (which is the 'preferred direction' in a liquid crystal sample). This definition is convenient, since for a completely random and isotropic sample, S=0, whereas for a perfectly aligned sample S=1. For a typical liquid crystal sample, S is on the order of 0.3 to 0.8, and generally decreases as the temperature is raised. In particular, a sharp drop of the order parameter to 0 is observed when one undergoes a phase transition from an LC phase into the isotropic phase. The order parameter can be measured experimentally in a number of ways. For instance, diamagnetism, birefringence, Raman scattering, and NMR can also be used to determine S. One could also characterize the order of a liquid crystal using other even Legendre polynomials (all the odd polynomials average to zero since the director can point in either of two antiparallel directions). These higher-order averages are more difficult to measure, but can yield additional information about molecular ordering.

Onsager hard-rod model

A very simple model which predicts lyotropic phase transitions is the hard-rod model proposed by Lars Onsager. This theory considers the volume excluded from the center-of-mass of one idealized cylinder as it approaches another. Specifically, if the cylinders are oriented parallel to one another, there is very little volume that is excluded from the center-of-mass of the approaching cylinder (it can come quite close to the other cylinder). If, however, the cylinders are at some angle to one another, then there is a large volume surrounding the cylinder where the approaching cylinder's center-of-mass cannot enter (due to the hard-rod repulsion between the two idealized objects). Thus, this angular arrangement sees a decrease in the net positional entropy of the approaching cylinder (there are fewer states available to it). The fundamental insight here is that that while parallel arrangements of anisotropic objects leads to a decrease in orientational entropy, there is an increase in positional entropy. Thus in some case greater positional order will be entropically favorable. This theory thus predicts that a solution of rod-shaped objects will undergo a phase transition, at sufficient concentration, into a nematic phase. Although this model is conceptually helpful, its mathematical formulation makes several assumptions that limit its applicability to real systems.

Maier-Saupe mean field theory

This statistical theory includes contributions from an attractive intermolecular potential. The anisotropic attraction stabilizes parallel alignment of neighboring molecules, and the theory then considers a mean-field average of the interaction. Solved self-consistently, this theory predicts thermotropic phase transitions, consistent with experiment.

Elastic continuum theory

In this formalism, a liquid crystal material is treated as a continuum; molecular details are entirely ignored. Rather, this theory considers perturbations to a presumed oriented sample. One can identify three types of distortions that could occur in an oriented sample: (1) twists of the material, where neighboring molecules are forced to be angled with respect to one another, rather than aligned; (2) splay of the material, where bending occurs perpendicular to the director; and (3) bend of the material, where the distortion is parrallel to the director and mesogen axis. All three of these types of distortions incur an energy penalty. They are defects that often occur near domain walls or boundaries of the enclosing container. The response of the material can then be decomposed into terms based on the elastic constants corresponding to the three types of distortions.

Effect of chirality

As already described, chiral mesogens usually give rise to chiral mesophases. For molecular mesogens, this means that the molecule must possess an asymmetric carbon atom. An additional requirement is that the system not be racemic: a mixture of right- and left-handed versions of the mesogen will cancel the chiral effect. Due to the cooperative nature of liquid crystal ordering, however, a small amount of chiral dopant in an otherwise achiral mesophase is often enough to select out one domain handedness, making the system overall chiral. Chiral phases usually have a helical twisting of the mesogens. If the pitch of this twist is on the order of the wavelength of visible light, then interesting optical interference effects will be observed. The chiral twisting that occurs in chiral LC phases also makes the system respond differently to right- and left-handed circularly polarized light. These materials can thus be used as polarization filters. It is possible for chiral mesogens to produce essentially achiral mesophases. For instance, in certain ranges of concentration and molecular weight, DNA will form an achiral line hexatic phase. A curious recent observation is of the formation of chiral mesophases from achiral mesogens. Specifically, bent-core molecules (sometimes called banana liquid crystals) have been shown to form liquid crystal phases that are chiral. In any particular sample, various domains will have opposite handedness, but within any given domain, strong chiral ordering will be present. The appearance mechanism of this macroscopic chirality is not yet entirely clear. It appears that the molecules stack in layers and orient themselves in a tilted fashion inside the layers. These liquid crystals phases are ferroelectric and antiferroelectric, both of which are of interest for applications.

Applications of liquid crystals

Liquid crystals find wide use in liquid crystal displays, which rely on the optical properties of certain liquid crystalline molecules in the presence or absence of an electric field. In a typical device, a liquid crystal layer sits between two polarizers that are crossed (oriented at 90° to one another). The liquid crystal is chosen so that its relaxed phase is a twisted one. This twisted phase reorients light that has passed through the first polarizer, allowing it to be transmitted through the second polarizer and reflected back to the observer. The device thus appears clear. When an electric field is applied to the LC layer, all the mesogens align (and are no longer twisting). In this aligned state, the mesogens do not reorient light, so the light polarized at the first polarizer is absorbed at the second polarizer, and the entire device appears dark. In this way, the electric field can be used to make a pixel switch between clear or dark on command. Color LCD systems use the same technique, with color filters used to generate red, green, and blue pixels. Similar principles can be used to make other liquid crystal based optical devices. Thermotropic chiral LCs whose pitch varies strongly with temperature can be used as crude thermometers, since the color of the material will change as the pitch is changed. Liquid crystal color transitions are used on many aquarium and pool thermometers. Other liquid crystal materials change color when stretched or stressed. Thus, liquid crystal sheets are often used in industry to look for hot spots, map heat flow, measure stress distribution patterns, and so on. Liquid crystal in fluid form is used to detect electrically generated hot spots for failure analysis in the semiconductor industry. It is also worth noting that many common fluids are in fact liquid crystals. Soap, for instance, is a LC, and forms a variety of LC phases depending on its concentration in water.

References

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External links

- [http://plc.cwru.edu/tutorial/enhanced/files/textbook.htm An introduction to liquid crystals]
- [http://www.physlink.com/Education/AskExperts/ae303.cfm What are liquid crystals made of?] ja:液晶

Anisotropic

Anisotropy (the opposite of isotropy) is the property of being directionally dependent. In the field of computer graphics, an anisotropic surface will change in appearance as it is rotated about its geometric normal, as is the case with velvet. Anisotropic scaling occurs when something is scaled by different amounts in different directions. An example is down-scaling a 64×64-pixel texture to cover a 12×34-pixel rectangle; this is anisotropic filtering. An anisotropic filter, on the other hand, is a filter with increasingly smaller interstitial spaces in the direction of filtration so that the proximal regions filter out larger particles and distal regions increasingly remove smaller particles, resulting in greater flow-through and more efficient filtration. Cosmologists use the term to describe the fluctuations in the background radiation left over after the big bang. The term refers to the difference in the temperature of the cosmic microwave background radiation with direction. An anisotropic liquid is one which has the fluidity of a normal liquid, but, unlike water or chloroform, which contain no structural ordering of the molecules, they have an average structural order relative to each other along their molecular axis. Liquid crystals are examples of anisotropic liquids. Some materials conduct heat in a way that is isotropic, that is independent of spatial orientation around the heat source. It is more common for heat conduction to be anisotropic, which implies that detailed geometric modeling of typically diverse materials being thermally managed is required. The materials used to transfer and reject heat from the heat source in electronics are often anisotropic. Many crystals are anisotropic to light, and exhibit properties such as birefringence. Crystal optics describes light propagation in these media. Category:Orientation

Nematic

In nematic liquid crystals the centers of gravity of molecules have no long-range order. The molecules tend to be parallel to a common axis, labeled by a unit vector (director)

\hat

. Opposite orientations of the director are indistinquishible, even if the molecules are asymmetric. Hence, the nematic order parameter is a second-rank tensor :

Q_=\frac\langle 3 n_\alpha n_\beta-\delta_\rangle

where

n_\alpha

are the components of a unit vector along the major symmetry axis of the molecules The transition from the isotropic liquid state to the nematic phase is a (often weak) first order phase transition. The word nematic comes from the Greek νημα = thread. It refers to thread-like topological defects observed in nematics. They are called 'disclinations'. Hedgehog is the other type of topological defects in nematics. Nematics have fluidity similar to that of ordinary (isotropic) liquids but they can be easily aligned by an external magnetic or electric field. An aligned nematic has the optical properties of a uniaxial crystal and this makes them extremely useful in Liquid Crystal Displays (LCD). Category:Liquid crystals ja:ネマティック液晶

Isotropic

Isotropy (the opposite of anisotropy) is the property of being independent of direction. Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented.
- Mathematics: Isotropy is also a concept in mathematics. Some manifolds are isotropic, meaning that the geometry on the manifold is the same regardless of direction. A similar concept is homogeneity. A manifold can be homogeneous without being isotropic.
- Radio broadcasting: In radio, an isotropic antenna is an idealized "radiating element" used as a reference; an antenna that broadcasts power equally (calculated by the poynting vector) in all directions. In practice, an isotropic antenna cannot exist, as equal radiation in all directions would be a violation of the Helmholtz Wave Equation. The gain of an arbitrary antenna is usually reported in Decibels relative to an isotropic antenna, and is expressed as dBi or dB(i).
- Physiology: In skeletal muscle cells (a.k.a. muscle fibers), the term "isotropic" refers to the light bands (I bands) that contribute to the striated pattern of the cells.
- Materials: In the study of mechanical properties of materials, "isotropic" means having identical values of a property in all crystallographic directions. Category:Orientation ko:등방성

Interference

Interference is the superposition of two or more waves resulting in a new wave pattern. As most commonly used, the term usually refers to the interference of waves which are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency. Two non-monochromatic waves are only fully coherent with each other if they both have exactly the same range of wavelengths and the same phase differences at each of the constituent wavelengths. The principle of superposition of waves states that the resultant displacement at a point is equal to the sum of the displacements of different waves at that point. If a crest of a wave meets a crest of another wave at the same point then the crests interfere constructively and the resultant wave amplitude is greater. If a crest of a wave meets a trough then they interfere destructively, and the overall amplitude is decreased. Interference is involved in Thomas Young's double-slit experiment where two beams of light which are coherent with each other interfere to produce an interference pattern (the beams of light both have the same wavelength range and at the center of the interference pattern they have the same phases at each wavelength, as they both come from the same source). More generally, this form of interference can occur whenever a wave can propagate from a source to a destination by two or more paths of different length. Two or more sources can only be used to produce interference when there is a fixed phase relation between them, but in this case the interference generated is the same as with a single source; see Huygens' principle. When a single source interferes with itself, the principle of conservation of energy dictates that the energy "missing" from the darkened regions of an interference pattern where destructive interference has taken place will be found in the brightened portions where constructive interference has taken place. Light from any source can be used to obtain interference patterns, for example, Newton's rings can be produced with sunlight. However, in general white light is less suited for producing clear interference patterns, as it is a mix of a full spectrum of colours, that each have different spacing of the interference fringes. Sodium light is close to monochromatic and is thus more suitable for producing interference patterns. Most suitable is laser light because that is almost perfectly monochromatic.

Constructive and destructive interference

laser When two waves superimpose, the resulting waveform depends on the frequency, (or wavelength) amplitude and relative phase of the two waves. If the two waves have the same amplitude A and wavelength the resultant waveform will have amplitude between 0 and 2A depending on whether the two waves are in phase or out of phase. Consider two waves that are in phase,with amplitudes A₁ and A₂. Their troughs and peaks line up and the resultant wave will have amplitude A = A₁ + A₂. This is known as constructive interference. If the two waves are 180° out of phase, then one wave's crests will coincide with another wave's troughs and so will tend to cancel out. The resultant amplitude is A = |A₁ − A₂|. If A₁ = A₂ the resultant amplitude will be zero. This is known as destructive interference.

External links

- [http://www.falstad.com/ripple/ex-2source.html Java demonstration of interference] Category:Optics Category:Wave mechanics ja:干渉 (物理学)

Thermal expansion

In physics, thermal expansion is the tendency of matter to increase in volume or pressure when heated. For liquids and solids the amount of expansion will normally vary depending on the material's coefficient of thermal expansion. While for Gases the change in volume or pressure is related to the container that the gas is in, this can be easily estimated by the ideal gas law. To accurately calculate thermal expansion of a substance a more advanced Equation of state must be used. This equation would be able to calculate thermal expansion among with many other state functions. Most materials expand when heated and contract when cooled. The amount a material expands or contracts is estimated by the formula:

(L_ - L_) / L_ = \alpha (T_ - T_)

where

\alpha

is the coefficient of thermal expansion in inverse kelvin. A number of materials have been discovered to exhibit negative thermal expansion, they contract on heating. In materials engineering, the three primary types of materials have well defined rates of expansion. Polymers expand as much as 10 times more than metals, which expand more than ceramics. Thermal expansion generally increases with bond energy. See PVT relation. Category:Thermodynamics

Spectral color

A spectral color is a color that is part of the optical spectrum. The spectral colors are: # Red # Orange # Yellow # Green # Cyan # Blue # Violet (For historical reasons, it is common to exclude cyan.) Among some of the colors that are not spectral colors are:
- Grayscale colors, such as White, Silver, Gray, and Black
- Any color obtained by mixing a grayscale color and a spectral color
- Magenta, which, in the color wheel, goes between violet and red. Category:Color

Leuco dye

A leuco dye is a dye whose molecules can acquire two forms, one of which is colorless. An example of a leuco dye is the crystal violet lactone, which in its lactone form is colorless or slightly yellowish, but in low pH, when it is protonated, it becomes intensely violet. Other examples are phenolphthalein and thymolphthalein, colorless in acidic to neutral pH, but becoming pink and blue in alcaline environment. Other example are many redox indicators, which undergo reversible color change between colored and colorless form at a specific electrode potential. Leuco dyes are a key component of some thermochromic dyes and thermal printer papers, and of the Flexplay DVD discs with limited play time, where eg. leuco form of methylene blue is used. methylene blue Category:Dyes

Hypercolor

Hypercolor was a brand of clothing, mainly T-shirts and shorts, that changed color with heat. They were manufactured by Generra (now a division of Public Clothing Company) and marketed in the United States as Generra Hypercolor or Generra Hypergrafix and outside the US as Global Hypercolor. They contained a thermochromic (temperature sensitive) pigment made by Matsui Shikiso Chemical of Japan, that changed between two colors–one when cold, one when warm. The shirts were produced with several color change choices from the late 1980s until the early 1990s, and were predominantly tie-dye in pattern. Unfortunately the effect could easily be permanently damaged, particularly when the clothing was placed in a hotter than recommended wash. tie-dye

Principle

The color change of Hypercolor shirts is based on combination of two colors: the color of the dyed fabric, which remained constant, and the color of the thermochromic dye. The dye is enclosed in microcapsules, tiny (few micrometers in diameter) drops of liquid sealed in a transparent shell, bound to the fibers of the fabric. The liquid is a leuco form of a dye (in this case crystal violet lactone), a weak acid (1,2,3-benzotriazole), and a quaternary ammonium salt of a fatty acid (myristylammonium oleate) dissolved in a solvent (lauryl alcohol). At low temperatures, the weak acid forms a colored complex with the leuco dye, interrupting the lactone ring. At high temperatures, above 24-27 °C, the solvent melts and the salt dissociates, reversibly reacts with the weak acid and increases the pH. The pH change leads to closing of the lactone ring of the dye, which then regains its colorless (leuco) form. Therefore at the low temperature the color of the shirt is the combination of the color of the microcapsules with the color of the dyed fabric, while at higher temperatures the capsules become colorless and the color of the fabric prevails.

External links

- [http://www.publicclothing.com/generra.html Generra] Category:Clothing Category:1980s fads Category:1990s fads Category:Thermochromism

Crystal violet lactone

Crystal violet lactone (CVL) is a leuco dye, a lactone derivate of crystal violet 10B. In pure state it is a slightly yellowish crystalline powder, soluble in nonpolar or slightly polar organic solvents. Its chemical formula is ₂₆₂₉₃₂ and its CAS number is . The central carbon in the leuco form is in a tetraedric configuration, forming four covalent bonds. In acidic environment the lactone ring is broken, the central carbon loses one valence and becomes planar, interconnecting the π systems of the aromatic rings to form one large conjugated system acting as a chromophore with strong absorption in visible spectrum. It was the first dye used in carbonless copy papers, and it is still widely used in this application. It is also the leuco dye component in some thermochromic dyes, eg. in the Hypercolor line of clothing. One of its novel uses is a security marker for fuels. It may cause allergic contact dermatitis in people handling the carbonless copy paper. Category:Triarylmethane dyes

Dodecanol

Dodecanol, also known by its IUPAC name 1-dodecanol or dodecan-1-ol, and by its trivial name dodecyl alcohol and lauryl alcohol, is a fatty alcohol. Its CAS number is 112-53-8. Its chemical formula is ₃(CH₂)₁₁OH. Dodecanil is a colourless, water insoluble solid of melting point 24-27 °C and boiling point 260-262 °C. It has a floral odor. It can be obtained from coconut oil fatty acids. Dodecanol is used to make surfactants, lubricating oils, and pharmaceuticals. In cosmetics, dodecanol is an emulsifier and emollient.

External links

MSDS Data:
- http://physchem.ox.ac.uk/MSDS/DO/1-dodecanol.html
- http://www.jtbaker.com/msds/englishhtml/d8784.htm Category:Alcohols

Halochromism

A halochromic material is a material which changes colour when pH changes occur. The term ‘chromic’ is defined as materials that can change colour reversibly with the presence of a factor. In this case, the factor is pH. The pH indicators have this property. Halochromic substances are suited for use in environments where pH changes occur frequently, or places where changes in pH are extreme. Halochromic substances detect alterations in the acidity of substances, like detection of corrosion in metals. Category:Chromism

Bisphenol A

to get this template --> Bisphenol A is a chemical compound that is prepared by reaction of two equivalents of phenol with one equivalent of acetone. Bisphenol A belongs to the phenol class of aromatic organic compounds; it has two phenol functional groups in its molecule.

History and use

Bisphenol A was first synthesized by Dianin in 1891^[1],^[2]. Bisphenol A was investigated in the 1930s during the search for synthetic estrogens. At that time, another synthesized compound, diethylstilbestrol, turned out to be more powerful an estrogen, so bisphenol A was not used as a synthetic estrogen. Its current main uses are as a monomer in the manufacture of polycarbonate plastic and in the manufacture of epoxy resins. Bisphenol A is also used as an antioxidant in plasticizers and in PVC, and as a polymerization inhibitor in PVC. Polycarbonates are widely used in many consumer products, from sunglasses and CDs to water and food containers and shatter-resistant baby bottles. Some polymers used in dental fillings also contain bisphenol A, while epoxy resins containing bisphenol A are popular coatings for the inside of cans used for canning food.

Possible health risks

BPA has been known to leach from plastics which are cleaned with harsh detergents or used to contain acidic or high temperature liquids. The chemical has been found in nearly every human tested in the United States. The first evidence of bisphenol A's estrogenicity came from experiments in the 1930s in which it was fed to ovariectomised rats^[4],^[5]. Some hormone disrupting effects in studies on animals and human cancer cells have been shown to occur at levels as low as 2-5 ppb (parts per billion). It has been claimed that these effects lead to health problems such as, in men, lowered sperm count and infertile sperm. The plastics industry has long claimed that bisphenol A is safe at typical levels of human exposure, minimizing or discounting all tests to the contrary. Eleven industry-funded studies found no risk from bisphenol A, while 90% of 104 independent studies showed possible risks, says a December 2004 report from scientists Frederick vom Saal and Claude Hughes^{[http://www.latimes.com/news/nationworld/nation/la-na-plastics13apr13,1,3167913.story?ctrack=2&cset=true 6]}. A previous report, released by the Harvard Center for Risk Analysis and funded by the American Plastics Council, called the evidence for risks "weak" and "inconsistent".

References

- 1: Dianin, Zhurnal russkogo fiziko-khimicheskogo obshchestva, 23 (1891), pp. 492 ff.
- 2: Th. Zincke, Mittheilungen aus dem chemischen Laboratorium der Universität Marburg, Justus Liebigs Annalen der Chemie, 343 (1905), pp. 75-131.
- 3: Handbook of Chemistry and Physics, CRC Press, 76th edition (1995-1996).
- 4: E. C. Dodds and Wilfrid Lawson, Nature, 137 (1936), 996.
- 5: E. C. Dodds and W. Lawson, Proceedings of the Royal Society of London, Series B, Biological Sciences, 125, #839 (27-IV-1938), pp. 222-232.

External links

- 6: [http://www.latimes.com/news/nationworld/nation/la-na-plastics13apr13,1,3167913.story?ctrack=2&cset=true "Study Cites Risk of Compound in Plastic Bottles" - LA Times]
- [http://www.bisphenol-a.org Plastics Industry Bisphenol A information site]
- [http://www.ourstolenfuture.org/NewScience/oncompounds/bisphenola/bpauses.htm Bisphenol Info page of a site about dangers of endocrine disruptors ]
- [http://www.epa.gov/iris/subst/0356.htm EPA page on Bisphenol A]
- [http://www.eeletter.com/bpareport.pdf An Endocrine/Estrogen Letter special Report on BPA] Category:Phenols Category:Plasticizers

Paraben

Parabens are a group of chemicals widely used as preservatives in the cosmetic and pharmaceutical industries. They can be found in shampoos, shaving gels, cleansing gels, deodorants, personal lubricants, and topical pharmaceuticals. They are also used as food additives. Parabens are esters of para-hydroxybenzoic acid. Common parabens include methylparaben, ethylparaben (E214), propylparaben (E126), butylparaben, and benzylparaben. The general chemical structure of a paraben is shown at right, where R symbolizes an organic group such as methyl, ethyl, propyl, butyl, or benzyl. Parabens have been linked to breast cancer, but so far there is no scientific evidence to support this claim. Parabens have been found in 20 samples of breast tumors [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14745841&dopt=Abstract], but it is unknown if this would be the same for healthy breast tissue. Further research is necessary to establish the significance of parabens in breast tumors and to establish a causal link between parabens in cosmetics and breast cancer. Tests on animals involving oral administration and injection of parabens have shown weak oestrogenic activity, acting as xenoestrogens [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11867263&dopt=Abstract]. Oestrogen is known to drive the growth of tumors. However, there is no evidence that underarm cosmetics containing parabens pose a health risk, because of the low doses involved and the fact that parabens are unlikely to penetrate into the tissue and to accumulate there (enzymes in skin and subcutaneous fat cells are capable of breaking down parabens). Parabens are extremely effective as preservatives in all types of formulas. For example, you could use parabens to preserve a shampoo, a lotion, and a personal lubricant, but you would have to use a different preservative system in each of those products in order to replace the parabens. Combine this with their low cost, and the unproven efficacy of natural preservatives like Grapefruit seed extract, and it's easy to see why parabens are so commonplace.

External links

- [http://www.nicnas.gov.au/news/20040123-parabenbreastcancer.asp Parabens in deodorants and antiperspirants linked to breast cancer] (NICNAS)
- [http://www.cancer.org/docroot/MED/content/MED_6_1x_Antiperspirants.asp?sitearea=MED Antiperspirants and breast cancer] (American Cancer Society)
- [http://www.antiperspirantsinfo.com Antiperspirantsinfo.com] Category:Preservatives Category:Cleaning product components Category:Phenols Category:Esters

Compound

- A compound is an area of land that is surrounded by fences, walls, or barbed wire and is used for a particular purpose, especially an area containing buildings and where the entry and exit of people is controlled. (For example, the sprawling estate located in southern Maine which houses the Bush Family residence is known as the Bush Compound.)
- In chemistry, a compound (chemical compound) is a chemical combination of two or more elements. See list of compounds.
- In linguistic morphology, a compound is a word that consists of more than one radical element, for example summertime. See also English compound. This is not to be confused with a complex phrase.
- In botany, compound is a quality of leaves. Leaves that are compound are in an array of small, symmetrically-arranged leaflets on each stem. In contrast, a plant with simple leaves has one leaf per stem.
- In economics, Compound Annual Growth Rate (CAGR) is the average annual growth rate of a value over a given number of years.
- In finance, compound Interest is interest that is paid on both the principal and interest earned.
- In music, a compound is an attribute of an interval or time signature. An interval that is compound is an interval which exceeds or is wider than one octave, whereas a simple interval lies within one octave. A time signature that is compound is one based on groups of three notes (most often quavers or eighth notes) whereas a simple time signature is one based on groups of two notes (most often crotchets or quarter notes).
- In steam locomotive engineering, a compound locomotive has steam that is passed that has already passed through one cylinder is then passed through another; i.e. the cylinders are in series as opposed to the normal arrangement of a simple locomotive in which the cylinders are in parallel.
- In the art world, Compound is an international exhibition space which was part of the Portland millennial art renaissance see Compound gallery for more information.
- In geometry, a polyhedral compound is a polyhedron which is itself composed of several other polyhedra sharing a common centre, the three-dimensional analogs of polygonal compounds. simple:Compound

Color

Color or colour is the perception of the frequency (or wavelength) of light, and can be compared to how pitch (or a musical note) is the perception of the frequency or wavelength of sound. It is a perception which in humans derives from the ability of the fine structures of the eye to distinguish (usually three) differently filtered analyses of a view. The perception of color is influenced by biology (some people are born seeing colors differently or not at all; see color blindness), long-term history of the observer, and also by short-term effects such as the colors nearby. (This is the basis of many optical illusions.) The science of color is sometimes called chromatics. It includes the perception of color by the human eye, the origin of color in materials, color theory in art, and the physics of color in the electromagnetic spectrum.

Physics of color

The colors of the visible light spectrum.

color	wavelength interval	frequency interval
red	~ 625-740 nm	~ 480-405 THz
orange	~ 590-625 nm	~ 510-480 THz
yellow	~ 565-590 nm	~ 530-510 THz
green	~ 500-565 nm	~ 600-530 THz
cyan	~ 485-500 nm	~ 620-600 THz
blue	~ 440-485 nm	~ 680-620 THz
violet	~ 380-440 nm	~ 790-680 THz
Continuous optical spectrum Image:Spectrum441pxWithnm.png Designed for monitors with gamma 1.5.
Computer "spectrum" Image:Computerspectrum.png The bars below show the relative intensities of the three colors mixed to make the color immediately above.

Color, frequency, and energy of light.

Color	$\lambda \,\!$ /nm	$\nu \,\!$ /10¹⁴ Hz	$\nu_b \,\!$ /10⁴ cm^-1	$E \,\!$ /eV	$E \,\!$ /kJ mol^-1
Infrared	>1000	<3.00	<1.00	<1.24	<120
Red	700	4.28	1.43	1.77	171
Orange	620	4.84	1.61	2.00	193
Yellow	580	5.17	1.72	2.14	206
Green	530	5.66	1.89	2.34	226
Blue	470	6.38	2.13	2.64	254
Violet	420	7.14	2.38	2.95	285
Near ultraviolet	300	10.0	3.33	4.15	400
Far ultraviolet	<200	>15.0	>5.00	>6.20	>598

Electromagnetic radiation is a mixture of radiation of different wavelengths and intensities. When this radiation has a wavelength inside the human visibility range (approximately from 380 nm to 740 nm), it is known as light within the (human) visible spectrum. The light's spectrum records each wavelength's intensity. The full spectrum of the incoming radiation from an object determines the visual appearance of that object, including its perceived color. As we will see, there are many more spectra than color sensations; in fact one may formally define a color to be the whole class of spectra which give rise to the same color sensation, although any such definition would vary widely among different species and also somewhat among individuals intraspecifically. A surface that diffusely reflects all wavelengths equally is perceived as white, while a dull black surface absorbs all wavelengths and does not reflect (for mirror reflection this is different: a proper mirror also reflects all wavelengths equally, but is not perceived as white, while shiny black objects do reflect). The familiar colors of the rainbow in the spectrum—named from the Latin word for appearance or apparition by Isaac Newton in 1671—contains all those colors that consist of visible light of a single wavelength only, the pure spectral or monochromatic colors. The frequencies are approximations and given in terahertz (THz). The wavelengths, valid in vacuum, are given in nanometers (nm). A list of other objects of similar size is available.

Important note

The color table should not be interpreted as a definite list – the pure spectral colors form a continuous spectrum, and how it is divided into distinct colors is a matter of taste and culture. Similarly, the intensity of a spectral color may alter its perception considerably; for example, a low-intensity orange-yellow is brown, and a low-intensity yellow-green is olive-green.

Spectral versus non-spectral colors

Most light sources are not pure spectral sources; rather they are created from mixtures of various wavelengths and intensities of light. To the human eye, however, there is a wide class of mixed-spectrum light that is perceived the same as a pure spectral color. In the table above, for instance, when your computer screen is displaying the "orange" patch, it is not emitting pure light at a fixed wavelength of around 600 nm (which is something most computer screens are unable to do). Rather, it is emitting a mixture of about two parts red to one part green light. Were you to print this page on a color printer, the orange patch on the paper, when lit with white light, would reflect yet another, more continuous spectrum. We cannot see those differences (although many animals can), and the reason has to do with the pigments that make up our color vision cells (see below). A useful quantification of this property is the dominant wavelength, which matches a wavelength of spectral light to a non-spectral source that evokes the same color perception. Dominant wavelength is the formal background for the popular concept of hue. In addition to the many light sources that can appear to be pure spectral colors but are actually mixtures, there are many color perceptions that by definition cannot be pure spectral colors due to desaturation or because they are purples (which are a mixture of red and violet light, from either end of the spectrum). Some examples of necessarily non-spectral colors are the achromatic colors (black, gray and white) and other colors such as pink, tan and magenta. See metamerism (color) for a basic introduction as to why color matching challenges exist.

Physical basis of color

A light wave can be analyzed as a superposition of sine waves, each of which has a specific frequency and wavelength. The eye gives limited information about the relative intensities of these sine waves (but not their phases — the eye is even more blind to phase than the ear, which can detect phase relationships of sounds only in certain very specific contexts). To understand which particular color perception will arise from a particular physical spectrum requires knowledge of the physiology of the retina. The human eye is also insensitive to polarization in most cases (though see Haidinger's brush), whereas some fish and mollusks can perceive it.

Color vision

Though the exact status of color is a matter of current philosophical dispute, color is arguably a psychophysical phenomenon that exists only in our minds. (See Qualia, for some of that dispute.) A "red" apple does not give off "red light", and it is misleading to think of things that we see, or of light itself, as objectively colored at all. Rather, the apple simply absorbs light of various wavelengths shining on it to different degrees, in such a way that the unabsorbed light which it reflects is perceived as red. An apple is perceived to be red only because normal human color vision perceives light with different mixes of wavelengths differently—and we have language to describe that difference. language In 1931, an international group of experts called the Commission Internationale d'Eclairage (CIE) developed a mathematical color model. The premise used by the CIE is that color is the combination of three things: a light source, an object, and an observer. The CIE tightly controlled each of these variables in an experiment that produced the measurements for the system. Although Aristotle and other ancient scientists speculated on the nature of light and color vision, it was not until Newton that light was correctly identified as the source of the color sensation. Goethe studied the theory of colors, and in 1801 Thomas Young proposed his trichromatic theory which was later refined by Hermann von Helmholtz. That theory was confirmed in the 1960s and will be described below. Hermann von Helmholtz The retina of the human eye contains three different types of color receptor cells, or cones. One type, relatively distinct from the other two, is most responsive to light that we perceive as violet, with wavelengths around 420 nm (cones of this type are sometimes called short-wavelength cones, S cones, or, most commonly but quite misleadingly, blue cones). The other two types are closely related genetically, chemically and in response. Each type is most responsive to light that we perceive as green or greenish. One of these types (sometimes called long-wavelength cones, L cones, or, misleadingly, red cones) is most sensitive to light we perceive as yellowish-green, with wavelengths around 564 nm; the other type (sometimes called middle-wavelength cones, M cones, or misleadingly green cones) is most sensitive to light perceived as green, with wavelengths around 534 nm. The term "red cones" for the long-wavelength cones is deprecated as this type is actually maximally responsive to light we perceive as greenish, albeit longer wavelength light than that which maximally excites the mid-wavelength/"green" cones. The sensitivity curves of the cones are roughly bell-shaped, and overlap considerably. The incoming signal spectrum is thus reduced by the eye to three values, sometimes called tristimulus values, representing the intensity of the response of each of the cone types. Because of the overlap between the sensitivity ranges, some combinations of responses in the three types of cone are impossible no matter what light stimulation is used. For example, it is not possible to stimulate only the mid-wavelength/"green" cones: the other cones must be stimulated to some degree at the same time, even if light of some single wavelength is used (including that to which the target cones are maximally sensitive). The set of all possible tristimulus values determines the human color space. It has been estimated that humans can distinguish roughly 10 million different colors, although the identification of a specific color is highly subjective, since even the two eyes of a single individual perceive colors slightly differently. This is discussed in more detail below. The rod system (which vision in very low light relies on exclusively) does not by itself sense differences in wavelength; therefore it is not normally implicated in color vision. But experiments have conclusively shown that in certain marginal conditions a combination of rod stimulation and cone stimulation can result in color discriminations not based on the mechanisms described above. While the mechanisms of color vision at the level of the cones in the retina are well described in terms of tristimulus values (see above), color processing and perception above that base level are organized differently. A dominant theory of the higher neural mechanisms of color vision proposes three opponent processes, or opponent channels, constructed out of the raw input from the cones: a red-green channel, a blue-yellow channel, and a black-white ("luminance") channel. This theory tries to account for the structure of our subjective color experience (see discussion below). Blue and yellow are considered complementary colors, or opposites: you could not experience a bluish yellow (or a greenish red), any more than you could experience a dark brightness or a hot coldness. The four "polar" colors proposed as extremes in the two opponent processe