Paints that change color depending on temperature. Chemical essence of color
Thermochromic paint is a modern material that is used to create unusual coatings that can change color under the influence of different temperatures. Thanks to this effect, thermally sensitive compounds have found wide application in various industries, from the production of souvenirs to car painting.
Properties of the active substance
The active component in the composition is a thermochromic pigment. It is this that ensures the reaction of the coating to heating or cooling, accompanied by a change in color. The amplitude of temperature fluctuations is 15–70 °C.
The value at which the reaction begins is individual for each specific composition.
KATO_Katosha - Chameleon Hair (PRAVANA VIVIDS Mood Color)
PRAVANA VIVIDS Mood Color is the world's first pigment that changes the color of your hair depending on the temperature. THIS...
Thermochromic pigments are contained in the material in the form of liquid crystals enclosed in microcapsules, which allows them to be mixed with various solutions, for example, oil-based, rubber or acrylic-based paints. The active ingredient usually ranges from 5 to 30% of the total mass of the coloring agent facilities; this figure depends on what result is required.
Types of thermal paints
Thermochromic compounds are divided into two groups:
- returnable,
- non-refundable.
The first include those coatings that provide a reversible visual effect, that is, they are able to change shade and return to their original state when the temperature returns to normal. This “trick” is repeated a large number of times.
In the second case, the paint changes color once and finally, the coating will no longer react to heat or cold.
Areas of use
Returnable thermochromic paints are used more widely than their “disposable” counterparts. These materials have gained great popularity among car owners who want to make their car original in its external design.
Covering for cars
Heat-sensitive paint is a godsend for those who like to experiment and take a creative approach to car care. Everyone can create a new interesting image for their iron horse with their own hands, because working with paint that changes color is not difficult. It can even be applied with a regular paint brush or roller, although the best option for painting a car body is, of course, a paint sprayer.
Thermochromic material can not only be a highlight of the decor, but also have an important practical function: if, when heated, the car’s coating begins to turn white or another light shade, then in hot weather the body will be able to reflect the sun’s rays and surface the car will overheat less.
To create a complex visual effect, you can use the following technique: paint the car in several layers of thermal paint, using compositions with different temperature thresholds. How clear wall from old paint at home? Drawings made using a stencil or applied by hand (if you have the makings of an artist) will help add “magic.”
Paints that change color depending on temperature? A car, for the design of which heat-sensitive paints were expertly used, simply cannot go unnoticed in the stream of other cars!
While you rejoice at the opportunity to decorate your vehicle, you should still know that thermochromic paint also has some disadvantages:
- low light resistance: to protect the car body coating from the destructive effects of ultraviolet radiation, you will have to apply a layer of special varnish and equip the parking area with a canopy (the best option is a garage);
- in case of mechanical damage, a complete repainting of the car will be required;
- difficulties when registering a car that does not have a permanent color;
- heat sensitive dye- expensive material.
Dishes that change color
A tea or coffee mug, on the surface of which a funny inscription or design appears when a hot drink gets into it, is a good memorable gift. An appetizer dish with an emerging pattern is an interesting detail in table setting. Paint that changes color depending on temperature for painting?
And a variety of baby dishes that give a visual signal when porridge or milk is too hot is a useful thing in the everyday life of young mothers.
Important: thermochromic paints do not contain toxic substances, and dishes painted using these materials are safe for health.
Cloth
The textile industry also uses compounds that change color depending on temperature. So, a plain T-shirt worn on the body can surprise you with the fashionable print that appears, and a stylish pattern or label will appear on your jeans.
Souvenirs and decorative elements
In this industry, there is an unusually wide scope for the use of thermochromic materials: Christmas tree decorations and garlands, other holiday paraphernalia, original lamps and candlesticks, key rings, gift stationery, etc. The great thing is that you can do and paint a lot of things yourself, for example, draw a picture or create a panel with a “secret”.
Printed products
Business cards that “come to life” from the touch of warm hands, advertising brochures or magazines promoting perfume (rub the page!), children’s picture books, postcards - all this is often produced using heat-sensitive compounds, fortunately their color palette is quite rich.
In general, anyone can find original uses for these unusual materials in everyday life on their own, using imagination and a little effort.
Additional Information:
Another advantage that thermochromic paint has is price. It is quite low, taking into account the properties of this material (1,500 rubles for a 25-gram jar, which lasts for a long time). Such solutions attract customers and are an excellent advertising ploy.
- Below + 20 degrees – for applying the substance to dishes that will be used for soft drinks.
- + 29… + 31 degrees – suitable for surfaces that will change color when exposed to body temperature (when touched). The use of this effect is widely used for advertising purposes, on T-shirts, magazines and booklets.
- Above + 43 degrees – materials intended for products that will interact with hot temperatures (dishes for hot drinks). In this case, the color changing effect performs not only a decorative, but also a warning function.
To apply to mugs, use thermochromic paint with a barrier below +20 degrees Celsius
Typically, thermochromic pigments are toxic and have only limited use, but the developers of The Unseen managed to get rid of this problem by finding and synthesizing similar, but harmless substances. How to change paint color at home? A change in temperature causes these molecules to take on one or another spatial conformation, changing the spectrum of absorbed radiation.
Depending on the specific paint in the kit, this may occur at different temperatures. For example, “cold” blue and white change into each other around 15 °C, and “hot” red and black - at 31 °C.
Boker developed several dyes that change color in different temperature ranges. Transition points correspond to the transition between room and street temperatures, or correspond to the temperature of the human body. Among the developed compositions there is a black paint that changes its color to red under the influence of hot air, there are paints that change from black to white, from silver to pale blue, from blue to white and from black to yellow.
To create a complex visual effect, you can use the following technique: paint the car in several layers of thermal paint, using compositions with different temperature thresholds. Drawings made using a stencil or applied by hand (if you have the makings of an artist) will help add “magic.” A car, for the design of which heat-sensitive paints were expertly used, simply cannot go unnoticed in the stream of other cars!
But even the first samples in promotional videos allow us to imagine the effect of using such hair dye. When the curls, under the influence of the temperature from the hair dryer, change shades from dark, almost black with a slight reddish shimmer to bright red and even light red.
Looks quite interesting. In addition, the creators of the dye promise its maximum safety: that it will not be harmful than conventional hair dyes that are sold today.
Thermochromic (heat-sensitive) paints are very popular in the food industry. An image coated with such paint and placed on the product informs the consumer whether the product, for example, in a refrigerator or oven, has reached the desired temperature. Thermochromic paint is also used by manufacturers of beer, liquor and vodka products (bottles, labels, stickers, etc.), where it signals that the drink is chilled, in the manufacture of ceramic dishes (cups, glasses, plates), and is also used in various types of plastic PP, PVC, ABS, silicone rubber and other transparent or translucent plastic materials for injection molding, extrusion, offset, screen printing, silk-screen printing, flexography.
A new way to minimize damage from failures in various structures would be to develop improved methods for detecting damage before it becomes critical. And this is where materials that change color when damaged can come to the rescue.
Adding special nanoparticles to a transparent polymer resin results in the creation of a "smart" material that changes color when damaged or when it is close to destruction. Such materials are called “materials with a changeable character” (English). mood ring materials ", literally - materials for a mood ring that changes color depending on a person's temperature," explained Cole Brubaker, a doctoral student in the Laboratory of Reliability Systems (LASIR) at Vanderbilt University.
The material changes color in response to mechanical stress.
Intelligent monitoring technologies are currently one of the most studied issues in civil, mechanical and aerospace engineering. These issues are largely addressed by developing networks of physical sensors that are attached to structures of interest. But this approach has disadvantages such as the high cost of equipment and complex processing of the obtained data.
LASIR researchers have taken a different route and incorporate luminescent nanoparticles into the material itself, which respond to mechanical stress by changing their optical properties. This approach makes it possible to create a new type of monitoring system that is effective and cost-effective.
“Currently, there are two ways to keep all infrastructure, from bridges to aircraft, safe,” the researchers say. “One of them is when people constantly conduct direct inspection of structures. The problem with this is that this method is labor intensive and people cannot see very small cracks. Another way is to implement complex networks of sensors into the controlled object that continuously evaluate condition of the structure and look for small cracks and detect them before they become too large and begin to affect the safety of the structure. The problem is that such networks are very expensive and, in the case of aircraft, add a lot of weight. So we need some kind of We can then change the materials we use to reveal these tiny cracks."
The team's initial studies showed that adding tiny concentrations of special nanoparticles (1 to 5 percent by weight) to an optically transparent polymer matrix results in a characteristic change in the material's optical properties when subjected to a wide range of compressive and tensile loads.
A group of researchers at Vanderbilt University aren't the only ones using nanoparticles to create smart materials, but they have an advantage. They use a special type of nanoparticle called a white light quantum dot. These quantum dots are unique because they emit white light where other quantum dots only emit light at specific wavelengths.
These special quantum dots were accidentally discovered in 2005 at Vanderbilt University while studying cadmium selenide quantum dots.
White light quantum dots have unique optical properties compared to other nanoparticles because the emission of white light is a surface phenomenon. When such nanoparticles are placed in a material, they react to what happens around them.
In preliminary tests, glass fibers and aluminum strips were coated with a polymer coating containing white light quantum dots and subjected to external loads of varying intensities. They found that the intensity of the spectrum of radiation emitted by quantum dots decreases as the load increases.
The graph shows that the white light spectrum of quantum dots in epoxy resin on aluminum strips decreases as the tensile load on the strip increases.
(LASIR Lab/Vanderbilt)
"There is still much that is unclear about the mechanism of the phenomenon, but we have shown that adding these quantum dots to ultrathin polymer coatings on metal surfaces can provide early warning when the underlying metal suffers any damage," the researchers said.
Researchers believe quantum dots emit light across a broad spectrum because more than 80 percent of the atoms lie on the surface. They also know that the bond between surface atoms and the molecules around them is critical.
Thus, the researchers confirmed that the material can act as a new type of strain gauge that constantly records mechanical stress on it.
The researchers also encountered a number of difficulties. For example, in a number of tests, epoxy cylinders deformed into a barrel shape when compressed, and the emission spectrum actually increased rather than decreased. The researchers speculate that this happened because the deformation squeezed the nanoparticles closer together and their concentration in the deformation region increased.
Besides this, there is one more problem that they will have to solve in order to make a workable damage detection system. Quantum dots suffer from light exposure. That is, when they are exposed to light, they gradually reduce their glow over time. As a result, such material must be protected from external light.
"There are many challenges that need to be solved before we can create a smart material that is ready for real-world applications, but the trend is positive," the researchers say.
Man and all animals (insects, inhabitants of the seas and oceans, even the simplest microorganisms) have vision of varying degrees of resolution, and in many cases color vision.
As a result of the interaction of light rays of a certain length (380–700 nm), corresponding to the visible part of the solar spectrum, with transparent and opaque objects containing inorganic and organic substances of a certain chemical structure (dyes and pigments) or objects with a strictly organized structure of nanoparticles (structural coloring) selective absorption of rays of a certain wavelength occurs and, accordingly, the remaining (less absorbed) rays are reflected (opaque object) or transmitted (transparent object). These rays enter the eye of an animal with color vision, onto biosensors and cause a chemical impulse corresponding to the energy of the quanta of light rays hitting the retina, and are transmitted by the nervous system to a certain part of the brain responsible for visual perception, and there a sensation of a color image of the surrounding world is formed .
In order for each of us to see the world as beautiful in all its diversity of colors, a combination of certain physical, chemical, biochemical, and physiological conditions that are met on our planet is necessary. Or maybe on some others?
- The presence in the solar spectrum of rays (visible part of the spectrum) reaching the Earth's surface with a wavelength of 380–700 nm. Not all rays of the solar spectrum reach the surface of the earth. So the ozone layer absorbs hard (high energy that kills living organisms) ultraviolet radiation (< 290 нм), благодаря чему на планете Земля существует жизнь.
- Nature, and then man, created many substances and materials, thanks to their chemical structure and physical structure, capable of selectively absorbing rays of the visible part of the spectrum. We call such substances and materials colored and colored.
- The evolution (many millions of years) of living matter has endowed living beings with biosensors (“biospectrophotometers”) - vision, capable of selectively responding to quanta of visible rays, a nervous system and brain structure (higher animals), transforming photoimpulses into biochemical ones, which create a color picture in our brain.
Traditionally, for a long time (many thousands of years), imitating nature (in the daytime, almost everything is colored, colored, all the colors of the rainbow), learned to produce colored and dyed materials, and succeeded in many ways. In the middle of the century before last (1854), William Perkin, a 3rd year student at King's College (England, London), synthesized the first synthetic dye - mauvais. This marked the beginning of the formation of the aniline dye industry (the first industrial revolution). Before this, for many thousands of years, people used natural colored substances (dyes, pigments).
But in nature, dyes and pigments not only perform a very important and multi-purpose function of coloring natural objects, but also a number of other tasks: protection from harmful microorganisms (in plants), converting light energy into biochemical energy (chlorophyll, rhodopsin), etc.
Chromium dyes and coloring (dyes, pigments, nanostructures)
Once again, it should be emphasized that there are two mechanisms for the appearance of color:
- Due to the presence in the substrate of colored (dyes, pigments) substances of a certain chemical structure;
- Due to the physical structure of ordered nanolayers, nanohoneycombs, nanoparticles (molecules, supramolecules, crystals, liquid crystals), on which the phenomena of interference, diffraction, multiple reflection, refraction, etc. occur.
For the coloration of the first and second mechanisms of its formation, chromium may be observed. What is chromium, which an ordinary person encounters quite often, and a color chemist not only constantly encounters this phenomenon, but is also forced to fight it, or in any case is obliged to take it into account, and even better, use it (this remains to be discussed).
Chromia- This reversible change in color (color, shade, intensity) under the influence of some external physical, chemical and physico-chemical impulses.
Chromia should not be confused with irreversible changes when destruction of the colored system occurs. These irreversible changes in color are scored as color stability to various factors.
The following types of chromium are distinguished depending on which factor or impulse causes a reversible color change: photo-, thermo-, chemo-, solvato-, mechano-, electro-, magnetochromia.
Photochromia(reversible change in color or light transmission) - under the influence of electromagnetic radiation, including natural (sunlight) or artificial radiation sources. Color chemists encounter this negative phenomenon when they use dyes with a high tendency to photochromia. Products made from material painted with such dyes when exposed to bright sunlight noticeably changes its color shade, but it is reversible, and in the dark (in a closet, at night) the color returns to its original color. However, this phenomenon is hysteretic and after a certain number of cycles the color loses its intensity (photodestruction). As a rule, dyes prone to photochromia do not have high enough light fastness.
The tendency of dyes to photochromia is assessed according to the ISO standard.
Thermochromia– a reversible change in color (color, shade) when a painted object is heated. We observe this phenomenon in everyday life when we iron dyed textiles; Thermochromia is especially pronounced if the products are moistened before ironing. After a certain time after cooling, the color returns to its original color. Each dye has a different tendency to thermochromia; on fabrics made of synthetic fibers it is more pronounced.
Chemochromia– reversible color change under the action of chemical reagents (change in pH, action of oxidizing agents and reducing agents).
Which chemist did not use color reactions of indicator dyes to determine the pH of a medium? All indicator dyes are chemochromes.
The technology of coloring with vat pigments (usually called dyes) is based on reversible redox processes: first, the conversion of an insoluble colored pigment into a weaker colored leuco form using reducing agents in an alkaline medium, and then again into a colored pigment by oxidation.
Solvatochromia– reversible color change when changing the solvent (polar to non-polar and vice versa).
Mechanochromia– reversible change in color (color) under deformation loads on the painted material.
Electrochromia and magnetochromia– a reversible change in color when passing various types of current and the action of a magnetic field on a painted object.
General mechanisms of chromia
All these types of chromium have a common mechanism, but specific features associated with the nature (physics, chemistry, physical chemistry) of the impulse itself are also obvious.
As was said earlier, coloring, color under all other necessary conditions (they have already been discussed) are determined by the chemical structure of the substance or the physical nanostructure that makes the substance, object, material colored and colored. In the case of coloring, the formation of which involves colored substances (dyes, pigments), the molecules of these substances must have a specific structure responsible for the selective absorption of rays of the visible part of the spectrum. In the case of organic dyes and pigments, the part of their molecule that determines this property is called a chromophore. According to the theory of color, a chromophore in organic substances is a structure with a fairly extended system of conjugated double bonds (conjugation).
The longer the chain of conjugations, the deeper the color of substances built from such molecules.
The conjugated bond system is characterized by a certain density of π- and d-electrons and, as a result, when interacting with rays of sunlight (its visible part), the substance is able to selectively absorb some of them.
Consequently, the phenomenon of chromism is necessarily associated with the reversible formation or change of the chromophore structure. If coloring is due to the presence of a strictly organized nanostructure (structural coloring), then chromism is associated with the reversible organization or disorganization of this structure under the influence of external impulses. Under the influence of external factors, a reversible chemical modification of the molecule does not necessarily have to occur, but very often this is associated with spatial isomerism (for example, cis-trans isomerism of azo dyes), a transition from an amorphous to a crystalline state (pots at the stage of soaping with boiling surfactant solutions), etc.
The specifics of the mechanism of chromia, depending on the nature and type of impulses causing it, will be outlined when considering each type of chromia.
Photochromia
The most studied type of chromia. Photophysical and photochemical transformations of dyes became objects of study by outstanding physicists and chemists of the last few hundred years, as soon as the foundations of physical and chemical ideas about the world began to form (I. Newton, A. Einstein, N. Vavilov, N. Terenin, etc.).
Photochromia, as part of a broader scientific and practical direction - photonics, underlies the properties of many natural and man-made phenomena and materials.
So rhodopsin– a natural visual pigment (chromoprotein), a highly chromic photoactive substance contained in the retinal rods of mammals and humans. It is essentially a visual photosensor. If its photoactivity were irreversible, then it would not be able to perform this function. The evolution of living nature created and selected this substance for effective vision at the very beginning of evolution (~ 2.8 billion years ago). This dye, rhodopsin, is present in archaic (original), primitive bacteria Halobacterium halolium, which convert light energy into biochemical energy.
The mechanism of rhodopsin photochromy involves very complex biochemical transformations.
In the case of photochromia during the transition from a colorless compound to a colored one, the transition diagram can be represented as follows:
Figure 1. On the absorption spectra, the reversible transition will be reflected in the shape of curves A and B.
Colorless substance A intensively absorbs light in the near UV (~ 300 nm), passes into a photoexcited state, the energy of which is spent on photochemical transformations of substance A into substance B with a chromophore that absorbs in the visible part of the spectrum. The reverse transformation can occur in the dark or when heated. Return to the original state occurs either spontaneously (due to the supply of heat) or under the influence of light (hυ2). When moving from compound A to B, its electron density changes and molecule B acquires the ability to absorb photons of lower energy, that is, absorb rays of the visible part of the spectrum. From the photoexcited state, molecule B is able to return again to the colorless state A. As a rule, forward reaction 1 proceeds much faster than reverse reaction 2.
It is necessary to distinguish between the physical and chemical mechanisms of photochromia. Physical photochromia is based on the transition of a molecule of a substance for some time to a photoexcited state, which has an absorption spectrum different from the initial state. Chemical photochromia is based on deep intramolecular rearrangements under the influence of light, passing through the stages of photoexcitation.
The chemical photochromia of colored substances is based on the following transformations caused by the absorption of light quanta by the molecule and its transition to a photoexcited state:
- redox reactions;
- tautomeric prototropic transformations;
- cis-trans isomerism;
- photo rearrangements;
- photolysis of covalent bonds;
- photodimerization.
Currently, many photochromic substances of inorganic and organic nature are known and studied. Inorganic photochromes: metal oxides, compounds of titanium, copper, mercury, some minerals, compounds of transition metals.
These interesting photochromes are unfortunately not very suitable for fixation on textile materials due to the lack of affinity for fibers. But they are successfully used as such or on substrates of various natures.
Organic photochromes are more suitable for fixation on textiles (they have affinity) and are less environmentally harmful.
These are mainly spiropyrans and their derivatives, spirooxazines, diarylethanes, triarylmethane dyes, stylenes, and quinones. Let us give an example of photoinitiated photochromic transformations of spiropyran, as the most studied photochrome. The photochromism of spiropyrans and their derivatives is based on reversible reactions: the rupture of covalent bonds in the molecule under the influence of UV and their restoration under the influence of rays of quanta of the visible part of the spectrum or due to heating. Figure 2 shows a diagram of the photochromic transformations of spiropyrans and their derivatives.
As can be seen, the original form of spiropyran does not have a conjugated double bond system and, accordingly, these compounds are colorless. Photoexcitation initiates the cleavage of the weak spiro-(C-O) bond, as a result of which the new two forms (cis- and trans-) cyanine derivatives acquire a conjugated system of double bonds and, accordingly, color.
Thermochromia– reversible color change when heated; When cooled, the color returns to its original color. As in the case of photochromia, this is associated with reversible changes in the structure of the molecule and, accordingly, with changes in the absorption spectrum and color.
Thermochromes can be, as in the case of photochromes, inorganic and organic.
Among the inorganic thermochromes are indium and zinc oxides, complexes of chromium and aluminum oxides, etc. The mechanism of thermochromia is a change in the state of aggregation or geometry of the ligand in a metal complex under the influence of temperature.
Inorganic complexes are not suitable for textiles, since they require high temperatures to change color, at which the textile material is thermally degraded.
Organic thermochromes can reversibly change color by two mechanisms: direct or sensitized. The direct mechanism usually requires relatively high temperatures (not suitable for textiles), leading to the breaking of chemical bonds or molecular conformations. Both lead to the appearance or change of color. When heated, structural, phase changes can also occur, for example, a transition to a liquid crystalline state and, as a consequence, the appearance of structural color due to purely physical, optical phenomena (interference, refraction, diffraction, etc.).
The breaking of chemical bonds, leading to the reversible appearance of color, as in the case of photochromia, is associated with the formation of a chain of conjugated double bonds. This is how spiropyran derivatives behave (60° – red, 70° – blue).
Stereoisomerization when heated requires relatively high temperatures (>100°C). When ironing textiles based on synthetic fibers dyed with azo dyes, the consumer often observes a reversible change in color shade, as a result of cis-trans isomerism of azo compounds.
Another reason for direct thermochromia may be isomerism associated with the transition from a planar (coplanar) form of a molecule to a volumetric one.
Particular attention should be paid to the thermochromia of crystalline structures, a reversible transition to the liquid crystalline form. Liquid crystals: an intermediate state of matter between solid crystalline and liquid; the transition between which occurs with a change in temperature. A certain degree of ordering of molecules in the liquid crystalline state causes them to display a structural color that depends on temperature. Coloring in liquid crystal form depends on the refractive index, which in turn depends on the specifics of this structure (the orientation and thickness of the layers, the distance between them). Similar behavior (structural coloring) is demonstrated by certain structures of living and inanimate nature: opals, the color of the plumage of birds, sea creatures, butterflies, etc. True, this is not always a liquid crystalline form, but more often photonic crystals. Liquid crystal structures change color in the range of –30 – +120°C and are sensitive to very small temperature changes (Δ 0.2°C), which makes them potentially interesting in various fields of technology.
These were all examples of the direct thermochromic mechanism, requiring high temperatures and therefore of little use for textiles.
The mechanism of indirect (sensitized) thermochromia is that substances that do not have thermochromic properties are capable of triggering the chromium mechanism of other substances when heated. Of interest are systems with a negative thermochromic effect, when the color appears at room temperature or lower, and when heated, the color disappears reversibly.
This thermochromic system consists of 3 components:
- A dye or pigment sensitive to changes in pH (indicator dye), for example, spiropyrans;
- Hydrogen donors (weak acids, phenols);
- Polar, non-volatile solvent for dye and hydrogen donor (hydrocarbons, fatty acids, amides, alcohols).
In such a 3-component system at low temperatures, the dye and the hydrogen donor are in close contact in the solid state and the color appears. When heated, the system melts, and the interaction between the main partners disappears along with the color.
Electrochromia occurs due to the addition or donation of electrons by molecules (redox reactions). The initiation of these reactions and the development of color can be achieved using a low current (just a few volts, ordinary batteries will do). At the same time, depending on the strength of the current, the color changes color and shade (a find for fashionable clothing - “chameleon”).
Electrochromes (of course, they must be conductive conductors): metal oxides of transition valency (iridium, ruthenium, cobalt, tungsten, magnesium, rhodium), metal phthalocyanines, dipyridine compounds, fullerenes with the addition of alkali metal anions, electrically conductive polymers with a conjugated chain of double bonds (polypyrrole, polyaniline, polythiophenes, polyfurans).
The main areas of application of electrochromic materials are: fashionable clothing that changes color; camouflage that completely matches the color of the environment (morning, afternoon, twilight, night); devices that measure current strength by color intensity.
Solvatochromia– reversible color change when replacing the solvent (polar to non-polar and vice versa). The mechanism of solvatochromy is the difference in solvation energy of the ground and excited states in different solvents. Depending on the nature of the solvents being replaced, bathochromic or hypsochromic shifts occur in the absorption spectra and, accordingly, a change in color shade
Most solvatochromes are metal complex compounds.
Mechanochromia– manifests itself in the presence of deformation loads (pressure, tension, friction). This is most clearly evident in the case of colored polymers, the main chain of which is a long chain of conjugated double π bonds. For them to exhibit mechanochromia, the combined action of mechanical impulses, heating and changes in the pH of the environment is often required.
For example, polydiacetylenes, when cooled without mechanical loads, have a blue color (λ ~ 640 nm), in a stressed state at 45 ° C, the material soaked in acetone becomes red (λ ~ 540 nm). By chemically modifying mechanochromic polymers, it is possible to change the color spectrum under mechanical loads.
By carrying out graft polymerization of polydiacetylene with polyurethane, an elastomeric polymer is obtained, which can be used in various fields to assess mechanical stress by color change, as well as in fashionable “stretch” clothing made from fibers of this structure. In places of bends (knees, elbows, pelvis) coloring will appear.
The most striking examples of the use of chromium in practice at present
Photochromia. Coloristic effects: change or appearance of color when irradiated with UV rays: fabrics, shoes, jewelry, cosmetics, toys, furniture; protection of banknotes, documents, brands, camouflage, actinometers, dosimeters, windows, sunglass lenses, facades made of glass and other materials, optical memory, photo switches, filters, shorthand.
Thermochromia. Temperature measurement (thermometers), indicator packaging of food products, document protection, liquid crystal thermochromic systems for decorating various materials, cosmetics, skin temperature measurement.
Chromia in fashionable clothes. Microcapsules with photochromic dyes (spiropyran derivatives) are introduced into printing ink and applied to the fabric using printing technology. When illuminated by sunlight (contains near UV ~ 350–400 nm), a reversible color appears (blue - dark blue).
The Japanese company Tory Ind Inc has developed a technology for the production of thermochromic fabrics using a microencapsulated mixture of 4 thermochromic pigments. In the temperature range –40 – +80°С (thermal sensitivity step ~ 5°С) the color changes, covering almost the entire color spectrum (64 shades). This technology is used for winter sportswear, fashionable women's clothing, and for window curtains.
An interesting technology is proposed for combining conductive yarn dyed with thermochromic dyes (incorporation of metal threads). Applying a weak current causes the yarn to heat up and color it. If fabric with conductive threads is printed with thermochromic dyes, then by changing the weave and current strength, you can not only develop and change the color, but also create a variety of patterns. Mollusks are capable of such a change in pattern with the help of chromatophores (organelles containing mechanochromic pigments). Such fabrics can and are used for camouflage; the color and pattern change to suit the type of surrounding area (desert, forest, field) and time of day. Using this principle, a flexible display is made on a textile basis, which is mounted on outerwear. When a low current is applied to such a display (for example, from a battery), animation can be shown.
Clothes made from stretch (elastomer) fibers dyed with mechanochromic dyes look very impressive. Places of clothing with greater extensibility (knees, elbows, pelvis) have a different color from other parts of clothing.
Chrome dyes make it possible to produce camouflage textiles and clothing. If textiles are printed with a mixture of conventional textile and photochromic dyes, camouflage can be achieved in any lighting conditions and environmental conditions.
Chameleon camouflage fabrics can be produced by printing with electrochromic dyes. By applying a weak current, you can achieve complete fusion of color and pattern with the environment.
The problem of protecting banknotes, business papers, and the fight against counterfeit products is successfully solved with the help of chromium dyes and pigments and, above all, photo- and thermochromic ones. The application of colorless chromium substances to the material allows them to be detected under UV illumination or heating.
Further prospects for the use of chromium dyes (substances)
Along with the use of chromium (thermo-, photo-, electro-, mechanical) dyes in the creation of fashionable clothing and shoes with interesting color effects, their use for technical purposes is expanding: optics, photonics, computer science, detection of harmful substances.
When using chromium dyes on textiles, the following problems arise:
- high price;
- problems of fixing and ensuring the permanent effect under the operating conditions of the product (washing, dry cleaning, light fastness);
- limited number of color reversibility cycles;
- toxicity.
The advantage that attracts the phenomenon of chromium is the ability to give materials and products special properties (functionality) that cannot be imparted to them by any other means.
- A.N.Terenin. "Photonics of Dye Molecules and Related Organic Compounds". - Leningrad: Science, 1967. - 616 p.
- V.A.Barachevsky, G.I.Lashkov, V.A.Tsekhomsky. "Photochronism and its applications." Moscow, “Chemistry”, 1977 - 280 p.
- H. Meier. Die Photochemie der organischen Farbstoffe; Springer. Verlag: Berlin-GBttingen-Heidelberg, 1964; p. 471.
- G.E. Krichevsky. Photochemical transformations of dyes and light stabilization of colored materials. – M.: Chemistry, 1986. – 248 p.
- G.E.Krichevsky, J.Gombkete. Lightfastness of dyed textiles. M., Light Industry, 1975 - 168 p.
- Yu.A. Ershov, G.E. Krichevsky, Advances in Chemistry, v. 43, 1974, 537 p.
- U.A. Ershov, G.E. Krichevsky. Text.Res.J., 1975, v.45, p.187–199.
- G.E. Krichevsky. ZhVKhO named after D.I. Mendeleev, 1976, v. 21, no. 1, p. 72–82.
- Photochemistry of dyed and pigmented polymers / ed. by N. S. Allen, J. F. McKellar. Applied Science Publishers Ltd, London, 1980, p. 284.
- G.E. Krichevsky. Chemical technology of textile materials. T.2 (Coloring). M., Moscow State University, 2001, 540 p.
- G.E. Krichevsky. Explanatory dictionary of terms (textiles and chemistry). M., Moscow State University, 2005, 296 p.
- G.E. Krichevsky. Structural coloring. “Chemistry and Life”, 2010, No. 11, p. 13–15.
- G.E. Krichevsky. The man who created a colorful tomorrow. "Chemistry and Life", 2007, p. 44–47.
- Research methods in textile chemistry. Ed. G.E. Krichevsky. M.: Legprombytizdat, 1993 – 401 p.
- G.E. Krichevsky. Chemical, nano-, biotechnologies in the production of fibers, textiles and clothing. M., Moscow State University, 2011, 600 pp., in press.
Panteleev Pavel Alexandrovich
The work provides explanations for the appearance of color in various compounds, and also examines the properties of chameleon substances.
Download:
Preview:
Chemistry of color. Chameleon substances
Section: natural science
Completed by: Panteleev Pavel Nikolaevich,
Student 11 "A" class
Secondary school No. 1148
them. F. M. Dostoevsky
Teacher: Karmatskaya Lyubov Aleksandrovna
1. Introduction. Page 2
2. Nature of color:
2.1. organic substances; Page 3
2.2. inorganic substances. Page 4
3. The impact of the environment on color. Page 5
4. Chameleon substances. Page 7
5. Experimental part:
5.1. Transition of chromate to dichromate and vice versa; Page 8
5.2. Oxidizing properties of chromium (VI) salts; Page 9
5.3. Oxidation of ethanol with a chromium mixture. Page 10
6. Photochromism. Page 10
7. Conclusions. Page 13
8.List of sources used. Page 14
1. Introduction.
At first glance, it may seem difficult to explain the nature of color. Why do substances have different colors? How does color come into being?
Interestingly, in the depths of the ocean there live creatures in whose bodies blue blood flows. One of these representatives is holothurians. Moreover, the blood of fish caught in the sea is red, like the blood of many other large creatures.
What determines the color of various substances?
First of all, color depends not only on how the substance is colored, but also on how it is illuminated. After all, in the dark everything seems black. Color is also determined by the chemical structures that predominate in the substance: for example, the color of plant leaves is not only green, but also blue, purple, etc. This is explained by the fact that in such plants, in addition to chlorophyll, which gives the green color, other compounds predominate.
Blue blood in sea cucumbers is explained by the fact that their pigment, which provides the color of blood, contains vanadium instead of iron. It is its compounds that give the blue color to the liquid contained in holothurians. In the depths where they live, the oxygen content in the water is very low and they have to adapt to these conditions, so compounds have arisen in the organisms that are completely different from those of the inhabitants of the air environment.
But we have not yet answered the questions posed above. In this work we will try to give complete, detailed answers to them. To do this, a number of studies should be carried out.
The purpose of this work will be to explain the appearance of color in various compounds, as well as to explore the properties of chameleon substances.
Tasks are set in accordance with the goal
In general, color is the result of the interaction of light with the molecules of a substance. This result is explained by several processes:
* interaction of magnetic vibrations of a light beam with molecules of matter;
* selective absorption of certain light waves by molecules with different structures;
* exposure to rays reflected or passed through a substance on the retina of the eye or on an optical device.
The basis for explaining color is the state of the electrons in the molecule: their mobility, their ability to move from one energy level to another, to move from one atom to another.
Color is associated with the mobility of electrons in a molecule of a substance and with the possibility of electrons moving to still free levels when absorbing the energy of a light quantum (elementary particle of light radiation).
Color arises as a result of the interaction of light quanta with electrons in the molecules of a substance. However, due to the fact that the state of electrons in atoms of metals and non-metals, organic and inorganic compounds is different, the mechanism for the appearance of color in substances is also different.
2.1 Color of organic compounds.
In organic substancesHaving color (and not all of them have this property), the molecules are similar in structure: they are, as a rule, large, consisting of dozens of atoms. For the appearance of color in this case, it is not the electrons of individual atoms that matter, but the state of the system of electrons of the entire molecule.
Ordinary sunlight is a stream of electromagnetic waves. A light wave is characterized by its length - the distance between adjacent peaks or two adjacent troughs. It is measured in nanometers (nm). The shorter the wave, the greater its energy, and vice versa.
The color of a substance depends on which waves (rays) of visible light it absorbs. If sunlight is not absorbed by a substance at all, but is reflected and scattered, the substance will appear white (colorless). If a substance absorbs all rays, then it appears black.
The process of absorption or reflection of certain rays of light is associated with the structural features of the molecule of a substance. The absorption of light flux is always associated with the transfer of energy to the electrons of the molecule of the substance. If a molecule contains s electrons (forming a spherical cloud), then to excite them and transfer them to another energy level requires a lot of energy. Therefore, compounds that have s electrons always appear colorless. At the same time, p-electrons (forming a figure eight cloud) are easily excited, since the connection they make is less strong. Such electrons are found in molecules that have conjugated double bonds. The longer the conjugation chain, the more p-electrons and the less energy is required to excite them. If the energy of visible light waves (wavelengths from 400 to 760 nm) is sufficient to excite electrons, the color that we see appears. The rays spent on exciting the molecule will be absorbed by it, and the unabsorbed ones will be perceived by us as the color of the substance.
2.2 Color of inorganic substances.
In inorganic substancescolor is due to electronic transitions and charge transfer from an atom of one element to an atom of another. The outer electron shell of the element plays a decisive role here.
As in organic substances, the appearance of color here is associated with the absorption and reflection of light.
In general, the color of a substance is the sum of the reflected waves (or those passing through the substance without delay). In this case, the color of a substance means that certain quanta are absorbed by it from the entire range of wavelengths of visible light. In molecules of colored substances, the energy levels of electrons are located close to each other. For example, substances: hydrogen, fluorine, nitrogen - seem colorless to us. This is due to the fact that visible light quanta are not absorbed by them, since they cannot transfer electrons to a higher level. That is, ultraviolet rays pass through these substances, which are not perceived by the human eye, therefore the substances themselves have no color for us. In colored substances, for example, chlorine, bromine, iodine, the electronic levels are located closer to each other, so the light quanta in them are able to transfer electrons from one state to another.
Experience. Effect of metal ion on the color of compounds.
Instruments and reagents: four test tubes, water, salts of iron (II), cobalt (II), nickel (II), copper (II).
Performing the experiment. Pour 20-30 ml of water into test tubes, add 0.2 g of iron, cobalt, nickel and copper salts and mix until dissolved. The color of the iron solution turned yellow, cobalt - pink, nickel - green, and copper - blue.
Conclusion: As is known from chemistry, the structure of these compounds is the same, but they have a different number of d-electrons: iron - 6, cobalt - 7, nickel - 8, copper - 9. This number affects the color of the compounds. That's why the difference in color is visible.
3. The impact of the environment on color.
The ions in solution are surrounded by a shell of solvent. A layer of such molecules immediately adjacent to an ion is calledsolvation shell.
In solutions, ions can affect not only each other, but also the solvent molecules surrounding them, and those, in turn, affect the ions. When dissolved and as a result of solvation, color appears in a previously colorless ion. Replacing water with ammonia deepens the color. Ammonia molecules are more easily deformed and the color intensity increases.
Now Let us compare the color intensity of copper compounds.
Experiment No. 3.1. Comparison of color intensity of copper compounds.
Instruments and reagents: four test tubes, 1% CuSO solution 4, water, HCl, ammonia solution NH 3, 10% solution of potassium hexacyanoferrate(II).
Performing the experiment. Place 4 ml of CuSO in one test tube 4 and 30 ml H 2 O, in the other two - 3 ml CuSO 4 and 40 ml H 2 O. Add 15 ml of concentrated HCl to the first test tube - a yellow-green color appears, to the second - 5 ml of a 25% ammonia solution - a blue color appears, to the third - 2 ml of a 10% solution of potassium hexacyanoferrate(II) - we see a red color. brown sediment. Add CuSO solution to the last test tube 4 and leave it for control.
2+ + 4Cl - ⇌ 2- + 6H 2 O
2+ + 4NH 3 ⇌ 2+ + 6H 2 O
2 2 + 4- ⇌ Cu 2 + 12 H 2 O
Conclusion: When reducing the amount of reagent (substance involved in a chemical reaction), necessary for the formation of the compound, the color intensity increases. When new copper compounds are formed, charge transfer and color change occur.
4. Chameleon substances.
The concept of "chameleon" is known primarily as a biological, zoological term meaninga reptile that has the ability to change the color of its skin upon irritation, a change in the color of the environment, etc.
However, “chameleons” can also be found in chemistry. So what's the connection?
Let's turn to the chemical concept:
Chameleon substances are substances that change their color in chemical reactions and indicate changes in the environment under study. Let’s highlight the general thing – change in color (color). This is what connects these concepts. Chameleon substances have been known since ancient times. Old manuals on chemical analysis recommend using a “chameleon solution” to determine the sodium sulfite content of sodium sulfite in samples of unknown composition. 2 SO 3 , hydrogen peroxide H 2 O 2 or oxalic acid H 2 C 2 O 4 . “Chameleon solution” is a solution of potassium permanganate KMnO 4
, which during chemical reactions, depending on the environment, changes its color in different ways. For example, in an acidic environment, a bright purple solution of potassium permanganate becomes discolored due to the fact that from the permanganate ion MnO 4
-
a cation is formed, i.e.positively charged ion Mn 2+ ; in a strongly alkaline environment from bright purple MnO 4
- produces green manganate ion MnO 4
2-
. And in a neutral, slightly acidic or slightly alkaline environment, the final product of the reaction will be an insoluble black-brown precipitate of manganese dioxide MnO 2
.
We add that due to its oxidizing properties,those. the ability to donate electrons or take them from atoms of other elements,and visual changes in color in chemical reactions, potassium permanganate has found wide application in chemical analysis.
This means that in this case, the “chameleon solution” (potassium permanganate) is used as an indicator, i.e.a substance indicating the presence of a chemical reaction or change that has occurred in the test environment.
There are other substances called "chameleons". We will consider substances containing the chromium element Cr.
Potassium chromate - inorganic compound, metal saltpotassium And chromic acid with the formula K 2 CrO 4 , yellow crystals, soluble in water.
Potassium bichromate (potassium dichromate, potassium chromium) - K 2 Cr 2 O 7 . Inorganic compound, orange crystals, soluble in water. Highly toxic.
5. Experimental part.
Experiment No. 5.1. The transition of chromate to dichromate and back.
Instruments and reagents: potassium chromate solution K 2 СrO 4 , potassium bichromate solution K 2 Cr 2 O 7 , sulfuric acid, sodium hydroxide.
Performing the experiment. We add sulfuric acid to a solution of potassium chromate; as a result, the color of the solution changes from yellow to orange.
2K 2 CrO 4 + H 2 SO 4 = K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O
I add alkali to the potassium dichromate solution, as a result the color of the solution changes from orange to yellow.
K 2 Cr 2 O 7 + 4NaOH = 2Na 2 CrO 4 + 2KOH + H 2 O
Conclusion: In an acidic environment, chromates are unstable, the yellow ion turns into a Cr ion 2 O 7 2- orange, and in an alkaline environment the reaction proceeds in the opposite direction:
2 Cr 2 O 4 2- + 2H + acidic medium - alkaline medium Cr 2 O 7 2- + H 2 O.
Oxidizing properties of chromium (VI) salts.
Instruments and reagents: potassium dichromate solution K 2 Cr 2 O 7 , sodium sulfite solution Na 2 SO 3 , sulfuric acid H 2SO4.
Performing the experiment. To solution K 2 Cr 2 O 7 acidified with sulfuric acid, add Na solution 2 SO 3. We observe a color change: the orange solution turned green-blue.
Conclusion: In an acidic environment, chromium is reduced by sodium sulfite from chromium (VI) to chromium (III): K 2 Cr 2 O 7 + 3Na 2 SO 3 + 4H 2 SO 4 = K 2 SO 4 + Cr 2 (SO 4 ) 3 + 3Na 2 SO 4 + 4H 2 O.
Experiment No. 5.4. Oxidation of ethanol with a chromium mixture.
Instruments and reagents: 5% solution of potassium dichromate K 2 Cr 2 O 7 , 20% sulfuric acid solution H 2 SO 4 , ethyl alcohol (ethanol).
Performing the experiment: To 2 ml of a 5% solution of potassium dichromate add 1 ml of a 20% solution of sulfuric acid and 0.5 ml of ethanol. We observe a strong darkening of the solution. Dilute the solution with water to better see its shade. We obtain a yellow-green solution.
TO 2 Cr 2 O 7 + 3C 2 H 5 OH+ H 2 SO 4 → 3CH 3 -COH + Cr 2 O 3 + K 2 SO 4 + 4H 2 O
Conclusion: In an acidic environment, ethyl alcohol is oxidized by potassium dichromate. This produces an aldehyde. This experiment shows the interaction of chemical chameleons with organic substances.
Experiment 5.4. clearly illustrates the principle by which indicators work to detect alcohol in the body. The principle is based on the specific enzymatic oxidation of ethanol, accompanied by the formation of hydrogen peroxide (H 2 O 2 ), causing the formation of a colored chromogen,those. an organic substance containing a chromophore group (a chemical group consisting of carbon, oxygen, and nitrogen atoms).
Thus, these indicators visually (on a color scale) show the alcohol content in a person’s saliva. They are used in medical institutions when establishing facts of alcohol consumption and alcohol intoxication. The scope of application of indicators is any situation where it is necessary to establish the fact of alcohol consumption: conducting pre-trip inspections of vehicle drivers, identifying drunk drivers on roads by traffic police, use for emergency diagnostics, as a means of self-control, etc.
6. Photochromism.
Let's get acquainted with an interesting phenomenon, where a change in the color of substances also occurs, photochromism.
Today, glasses with chameleon lenses are unlikely to surprise anyone. But the history of the discovery of unusual substances that change their color depending on the light is very interesting. In 1881, the English chemist Phipson received a letter from his friend Thomas Griffith in which he described his unusual observations. Griffith wrote that the front door of the post office, located opposite his windows, changes its color throughout the day - darkens when the sun is at its zenith, and brightens at dusk. Curious about the message, Phipson examined lithopone, the paint used to paint the post office door. His friend's observation was confirmed. Phipson was unable to explain the cause of the phenomenon. However, many researchers have become seriously interested in the reversible color reaction. And at the beginning of the twentieth century, they managed to synthesize several organic substances called “photochromes,” that is, “photosensitive paints.” Since Phipson's time, scientists have learned a lot about photochromes -substances that change color when exposed to light.
Photochromism, or tenebrescence, is the phenomenon of a reversible change in the color of a substance under the influence of visible light and ultraviolet radiation.
Exposure to light causes in a photochromic substance, atomic rearrangements, changes in the population of electronic levels. In parallel with the color change, the substance can change its refractive index, solubility, reactivity, electrical conductivity, and other chemical and physical characteristics. Photochromism is inherent in a limited number of organic and inorganic, natural and synthetic compounds.
There are chemical and physical photochromism:
- chemical photochromism: intramolecular and intermolecular reversible photochemical reactions (tautomerization (reversible isomerism), dissociation (cleavage), cis-trans isomerization, etc.);
- physical photochromism: the result of the transition of atoms or molecules into different states. The color change in this case is due to a change in the population of the electronic levels. Such photochromism is observed when a substance is exposed to only powerful light fluxes.
Photochromes in nature:
- Mineral tugtupit capable of changing color from white or pale pink to bright pink.
Photochromic materials
The following types of photochromic materials exist: liquid solutions and polymer films (high molecular weight compounds), containing photochromic organic compounds, glasses with silver halide microcrystals evenly distributed throughout their volume (compounds of silver with halogens), photolysis ( decay by light) which is caused by photochromism; crystals of alkali and alkaline earth metal halides, activated by various additives (for example, CaF 2/La,Ce; SrTiO 3 /Ni,Mo).
These materials are used as light filters of variable optical density (i.e., they regulate the flow of light) in means of protecting eyes and devices from light radiation, in laser technology, etc.
Photochromic lenses
Photochromic lens exposed to light, partially covered with paper. A second level of color is visible between the light and dark parts, as photochromic molecules are located on both surfaces of the lens polycarbonate and others plastics . Photochromic lenses usually darken in the presence of ultraviolet light and lighten in its absence in less than a minute, but the complete transition from one state to another occurs in 5 to 15 minutes.
Conclusions.
So, the color of different compounds depends on:
*from the interaction of light with molecules of matter;
*in organic substances, color arises as a result of the excitation of electrons of the element and their transition to other levels. The state of the electron system of the entire large molecule is important;
*in inorganic substances, color is due to electronic transitions and charge transfer from an atom of one element to an atom of another. The outer electron shell of the element plays a major role;
*the color of the compound is influenced by the external environment;
*The number of electrons in a compound plays an important role.
List of sources used
1. Artemenko A. I. “Organic chemistry and man” (theoretical foundations, in-depth course). Moscow, “Enlightenment”, 2000.
2. Fadeev G. N. “Chemistry and color” (a book for extracurricular reading). Moscow, “Enlightenment”, 1977.
Determination of color factors. What is color from a chemical point of view? It is impossible to consider the chemical essence of color without knowledge of the physical properties of visible light. We owe it to the great English physicist I. Newton that he explained the phenomenon of the decomposition of white color into a set of rays of the color spectrum. Each wavelength corresponds to a certain energy that these waves carry. The color of any substance is determined by the wavelength whose energy predominates in a given radiation. The color of the sky depends on how much sunlight reaches our eyes. Rays with a short wavelength (blue) are reflected from air molecules and scattered. Our eye perceives them and determines the color of the sky - blue, cyan (Table 3.).
The same thing happens in the case of colored substances. If a substance reflects rays of a certain wavelength, then it is colored. If the energy of light waves across the entire spectrum is absorbed or reflected equally, the substance appears black or white. The human eye contains an optical system: the lens and the vitreous body. The retina of the eye contains light-sensitive elements: cones and rods. Thanks to cones we distinguish colors.
Table 3. Color of substances having one absorption band in the visible part of the spectrum
Thus, what we call color is the result of two physical and chemical phenomena: the interaction of light with the molecules of a substance and the effect of waves coming from the substance on the retina of the eyes. So, first factor formation of color - light.
Consider examples of the following, second factor– structure of substances.
Metals have a crystalline structure; they have an ordered structure of atoms and electrons. Color is related to electron mobility. When metals are illuminated, reflection dominates and their color depends on the wavelength they reflect. White shine is due to the uniform reflection of almost the entire set of visible rays. This is the color of aluminum and zinc. Gold has a reddish-yellow color because it absorbs blue, indigo and violet rays. Copper also has a reddish color. Magnesium powder is black, which means this substance absorbs the entire spectrum of rays.
Next, third the factor in the appearance of color is the ionic state of substances. The color also depends on the environment around the colored particles. Cations and anions in solution are surrounded by a shell of solvent, which affects the ions.
Factors influencing the color change of chemicals. When conducting a simple experiment with the addition of the following substances to a solution of beet juice (raspberry color): acetic acid; alkali or water solution, as a result, you can observe a change in the color of the beetroot solution. In the first case, the acidic environment leads to a change in the color of the beetroot solution to purple, in the second experiment, the alkaline environment changes the color of the solution to blue, and the addition of water (neutral environment) does not cause a color change.
Chemists know the indicator for determining an alkaline environment - phenolphthalein. It changes the color of alkali solutions to crimson. A historical fact is associated with the color change of the iron ion when surrounded by potassium thiocyanate. In 1720, political opponents of Peter I from the clergy organized a “miracle” in one of the St. Petersburg cathedrals - the icon of the Mother of God began to shed tears, which was commented on as a sign of her disapproval of Peter’s reforms. Peter I carefully examined the icon and noticed something suspicious: he found small holes in the eyes of the icon. He also found the source of the tears: it was a sponge soaked in a solution of iron thiocyanate, which has a blood-red color. The weight pressed evenly on the sponge, squeezing drops through the hole in the icon. “This is the source of wonderful tears,” said the sovereign.
Chemicals are part of the nature that surrounds us on all sides. Animal blood and leaf greens contain similar structures, but blood contains iron ions - Fe, and plants - Mg. This provides the colors: red and green. By the way, the saying “blue blood” is true for deep-sea animals whose blood contains vanadium instead of iron. Also, algae that grow in places where there is little oxygen are blue.
Plants with chlorophyll are able to form organomagnesium substances and use light energy. The color of photosynthetic plants is green.
Blood hemoglobin, which contains iron, serves to transport oxygen in the body. Hemoglobin with oxygen gives the blood a bright red color, but without oxygen it gives the blood a dark color.
It is necessary to draw the following conclusions regarding the physicochemical nature of color:
The first factor in the formation of color is light;
The second factor is the chemical structure of substances;
The third factor in the appearance of color is the ionic state of chemicals; color depends on the environment around the colored particles.
4.2. Chemistry of dyes .
Color harmony is one of the components of the art of design. The most ancient paints were coal, chalk, clay, cinnabar and some salts, such as copper acetate (verdigris). Paints and dyes are used by artists, decorators and textile workers.
The use of the first coloring substances - inorganic pigments - began in the Stone Age. Primitive people used colored natural minerals to paint their bodies, various household items and clothing. Beautiful drawings in caves have survived to this day, outliving their creators for hundreds of centuries. It is colored minerals, together with noble metals, that have always been symbols of the power and wealth of people. With the development of mankind, the need for dyes only grew.
Back in the 10th century. BC, on the bottom of the Mediterranean Sea near the city of Tyre (ancient Phenicia), needle snails were caught. Slaves dived into the sea for these snails day after day. Other slaves squeezed them out, ground them with salt and subjected them to further processing, which consisted of many operations. The extracted substance was at first white or pale yellow, but under the influence of air and sunlight gradually became lemon yellow, then green, and finally acquired a magnificent violet-red color. Received purple for several centuries it was the most valuable of all dyes. He was then a symbol of power - the right to wear purple-dyed robes was the privilege of rulers and the nobles closest to them. Dyeing just one square meter of fabric with dye obtained in this way was very expensive. After all, to obtain one gram of purple, 10,000 snails had to be processed!
The backbreaking labor of the slaves of Tyre is not the only example of this kind in history. In a few hundred years indigo– a violet-blue dye extracted from the plant Indigofera tinctiria, became one of the major sources of profit for the British East India Company. The ships of the East India Company annually delivered from 6 to 9 million kilograms of this valuable dye to all parts of the world. They used to paint sails, now they use it to paint jeans.
Nowadays, the production of modern, cheap and at the same time bright dyes of all colors and shades no longer requires the backbreaking labor of slaves or the population of the colonies. They, including purple and indigo, are produced in chemical plants. However, purple and indigo have lost their former glory. They have been replaced by more light-resistant synthetic dyes, a wide selection of which we have today.
The path to current successes was opened thanks to the work of many chemists. In 1826, 1840 and 1841, Unferdorben, Fritzsche and Zinin independently obtained aniline from indigo. In 1834, Runge discovered aniline in coal tar, in the same year he discovered phenol and, a little later, the first dye from coal tar - rosolic acid, giving the color purple.
In 1856, 18-year-old chemist Perkin, working during the holidays in his home laboratory, unexpectedly received a bright reddish-violet dye in an unsuccessful attempt to synthesize quinine - mauvais. Together with his father and brother, Perkin founded the company and a year later organized the production of mauvais on a factory scale. Thus, Perkin laid the foundation for the creation of the aniline dye industry.
In 1868, Grebe and Liebermann revealed the secret alizarin– a red dye extracted from the roots of madder. Syntheses followed eosin and other phthalein dyes by Bayer and Caro and deciphering the structure of anthracene dyes by E. Fisher and O. Fisher. By the end of the 19th century. These achievements culminated in the introduction into industry of indigo synthesis using the method developed by Heimann and other chemists.
The great merit of German chemists in the development of the paint and varnish industry. Already in 1911, German firms exported 22,000 tons of synthetic indigo. By simultaneously releasing 1,500 tons of cheap synthetic alizarin, they almost completely replaced natural alizarin, which led to a sharp reduction in madder cultivation.
Why do substances illuminated by white light acquire one color or another? The fact is that passing through the dye, light is absorbed by its molecules. The structure of the dye molecules is such that light is selectively absorbed. The dye molecule “selects” the rays that are characteristic only of it and that make up white light—spectral lines. Losing some of the colors, the incident beam is colored by the so-called complementary colors (green - red, yellow - violet, blue - orange). For example, the loss of red color will lead to green coloring.
What does the absorption spectrum of a substance depend on? We have before us the formula of a dye with a relatively simple structure: Its exact chemical name is sodium n,n"-dimethylaminoazobenzenesulfonate. This substance is used as an indicator, it was called otherwise - methyl orange. This dye, however, is not suitable for dyeing, since when acid is added, the yellow color turns red. It is no coincidence that organic dyes have a complex structure. Research by many chemists has established a connection between the color of a compound and its structure. The backbone, or core, of the dye molecule is usually formed by a ring structure. Color carriers - chromophores - must be attached to it. These are always unsaturated groups:
CH=CH – ethylene group;
C=O – carbonyl group (oxo group, keto group);
N=N – azo group;
N=O – nitroso group;
NO2 – nitro group.
The nucleus and chromophore groups together form a colored system - a chromogen. In most cases, the presence of only one chromophore does not yet give color. For example, in the orange molecule b-carotene– carrot dye – contains 11 double bonds. In addition, the color depends on exactly how the chromophores are located and connected to each other. To enhance the color, deepen its shade and achieve greater color fastness, additional groups - auxochromes - must be attached to the chromophore core. These include, first of all, the hydroxyl group OH and the amino group NH2, which not only affect the coloring, but also, due to their acidic or basic nature, increase the affinity of the dye for the fiber. Modern electronic color theory considers color as the result of the interaction of the electron cloud of a dye molecule with light. The absorption spectrum of the molecule depends on its parameters, which are determined by the presence of chromophore and auxochromophore groups.
Phosphors. Conventional dyes scatter the absorbed light in the form of infrared radiation invisible to the human eye. However, there are molecules capable of emitting rays of visible color after being excited by external energy, returning back to an unexcited state. These are phosphors. The energy required for their glow can be chemical (“phosphors”), mechanical (“triboluminophores”), electrical (“electroluminophores”) or light (“photoluminophores”), as well as under the influence of radiation.
Phosphorescent phosphors exist in nature. Glow can occur due to the slow oxidation of a substance in air (for example, white phosphorus, luciferin in some insects, microbes, fungi, fish). Such substances do not glow without access to an oxidizing agent (air oxygen). Some substances can glow when rubbed or shaken (for example, crystalline chelidonine, some sulfides activated with manganese, etc.). This glow is called triboluminescence. Substances that glow in the presence of radiation or invisible X-ray rays are used to make permanently glowing compounds. For example, paraffin is used as a radioactive substance, in the molecules of which some of the atoms of ordinary hydrogen (protium) are replaced by atoms of super-heavy radioactive hydrogen (tritium). Due to the presence of radioactive elements in their composition, such visible light sources are hazardous to health. Electroluminescent phosphors are widely used in lighting technology.
However, it is inorganic or organic photoluminophores that are used as phosphor dyes. Depending on the time the excitation of their molecules is maintained, phosphors can glow in the dark with an excitation time of several hours (many such luminous toys are sold), or at short times the phosphors simply turn into a characteristic color. Of particular interest are such phosphors that actively absorb UV radiation. Clothes dyed with such phosphors “burn” brightly in the sun. The red clothes of the Ministry of Emergency Situations employees are visible for many kilometers even in the fog. Phosphor paints are used for road signs and advertisements, rescue boats. But there are also unexpected ways to use such phosphors.
UV protection. There are many cosmetics available on the market that protect people from harmful UV radiation, such as sunscreens. The main active components of these products are UV absorbers - the same phosphors that absorb harmful hard radiation.
But it’s not just the human body that needs to be protected from ultraviolet radiation. UV absorbers - light stabilizers - are widely used to protect polymers. An example is Tinuwin. In the unexcited state, a stable hydrogen bond is formed between the hydrogen of the hydroxyl group and the nitrogen atom closest to it. Its stability is due to the formation of a stable hexagon. Absorption of a quantum of UV radiation is sufficient to destroy this ring. When it is restored, energy is emitted, but this is no longer harmful ultraviolet radiation, but safe infrared radiation. (The surface of all metal objects is destroyed under the influence of the environment. Their protection is most effective with colored pigments: aluminum powder, zinc dust, red lead, chromium oxide).
Optical brighteners. Each of you must have noticed that at the disco, when the special lights are turned on, people’s white shirts and blouses begin to glow brightly blue. A sheet of white paper will shine even brighter. This means that special phosphors - optical brighteners - have been added to the fabric of your clothing and paper. Their action is similar to the action of ordinary “blue”, which was previously added to the water during washing to whiten clothes. Today, for the purpose of bleaching, substances are added to washing powders that give the fabric a bluish fluorescence.
The blue color complementary to yellow “kills” the yellowness of the fabric. The phosphor does the same thing by converting UV radiation into blue radiation. At the same time, it protects the material from ultraviolet radiation.
Phosphor for greenhouse film. Conventional greenhouse polyethylene film is already outdated (by the way, the “greenhouse effect” is due to the fact that UV and visible rays pass through the polyethylene layer almost without loss, and for thermal infrared rays from the soil surface, polyethylene is opaque). New photoconversion films have appeared that glow red in the sun. It is emitted by a special phosphor synthesized on the basis of europium oxide, which converts green, blue and UV radiation into red. Of course, it’s very beautiful, it’s not about beauty.
A plant at the initial stage of development requires a large amount of red color to grow green mass (leaves). The phosphor serves precisely this purpose. It has a complex structure that provides stepwise conversion of UV radiation into the required red color. Therefore, the amount of red color in the light falling on plant leaves increases several times, which leads to an increase in the yield of greenhouse crops. True, when the time comes for the fruit to ripen, such a film should be replaced with a blue one. On the contrary, it absorbs red rays. The leaves stop growing, and all the plant’s energy is directed to the growth of fruits.
Lost River. Fluorescence is clearly visible even when 1 g of radomin 6G is dissolved in 100,000 liters of water. The ability of phosphors to be extremely easily detected in negligible concentrations is used to determine the direction of underground water flows. An example is the solution to the issue of the “disappearance” of the Danube. In the upper reaches of this river, near the Immedingen railway station, most of the Danube water is lost in loose limestone rocks. To establish the direction of water movement, in 1877, 10 kg of fluorescein was poured into the Danube near this station. After 60 hours, one of the exposed posts detected distinct fluorescence in a small river. Nowadays, this property of phosphors has proven to be very useful in environmental inspections of leaks and effluents from hazardous industries. Let's not forget about the phosphor printing protection system for documents and, finally, banknotes.
Quantum dots. Nanoparticles of phosphors (quantum dots), absorbed by microorganisms with nutrient media, make it possible to trace their movement and development in a living organism. The selective uptake of such particles by malignant cells is already being used to diagnose cancer and other diseases in the early stages.
In addition to those described above, there are many interesting dyes. For example, photochromic dyes have been developed that change color with increasing doses of UV radiation, increasing temperature, and exposure to an electric field. There are dyes that color films differently in reflected and transmitted light. A long article could be written about interference coloring with multilayer pearlescent pigments, about holographic coloring, about the use of liquid crystal structures, about digital printing and much more.
Despite the fact that the basic rules for creating chromophore molecules are known, the discovery of a new dye even today is sometimes caused by a happy accident. The technology of dyes is chemistry, physiology, and art.
5. Basic patterns of color perception: