In terms of optical properties, colloidal solutions differ significantly from true solutions of low molecular weight substances, as well as from coarsely dispersed systems. The most characteristic optical properties of colloidal disperse systems are opalescence, the Faraday-Tyndall effect and color. All these phenomena are caused by the scattering and absorption of light by colloidal particles.

Depending on the wavelength of visible light and the relative sizes of particles of the dispersed phase, light scattering takes on a different character. If the particle size exceeds the wavelength of light, then the light from them is reflected according to the laws of geometric optics. In this case, part of the light radiation can penetrate into the particles, experience refraction, internal reflection and absorption.

If the particle size is smaller than the half-wavelength of the incident light, diffraction scattering of light is observed; the light seems to bypass (bend around) the particles encountered along the way. In this case, partial scattering occurs in the form of waves diverging in all directions. As a result of light scattering, each particle is a source of new, less intense waves, i.e., it is as if self-luminescence of each particle occurs. The phenomenon of light scattering by tiny particles is called opalescence. It is characteristic primarily of sols (liquid and solid) and is observed only in reflected light, i.e., from the side or against a dark background. This phenomenon is expressed in the appearance of some turbidity of the sol and in a change (“overflow”) of its color compared to the color in transmitted light. Color in reflected light, as a rule, is shifted towards a higher frequency of the visible part of the spectrum. Thus, white sols (sol of silver chloride, rosin, etc.) become opalescent with a bluish color.

Faraday-Tyndall effect. Diffraction scattering of light was first noticed by M. V. Lomonosov. Later, in 1857, this phenomenon was observed by Faraday in gold sols. The phenomenon of diffraction (opalescence) for liquid and gaseous media was studied in most detail by Tyndall (1868).

If you take one glass with a solution of sodium chloride and the other with egg white hydrosol, it is difficult to determine which is the colloidal solution and which is the true one, since both liquids appear colorless and transparent (Fig. 6.5). However, these solutions can be easily distinguished by performing the following experiment. We will put a light-proof case with a hole on the light source (table lamp), in front of which we will place a lens in order to obtain a narrower and brighter beam of light. If we place both glasses in the path of the light beam, in the glass with sol we will see a light path (cone), while in the glass with sodium chloride the beam is almost invisible. After the scientists who first observed this phenomenon, the luminous cone in the liquid was called the Faraday-Tyndall cone (or effect). This effect is characteristic of all colloidal solutions.

The appearance of the Faraday-Tyndall cone is explained by the phenomenon of light scattering by colloidal particles 0.1-0.001 microns in size.

The wavelength of the visible part of the spectrum is 0.76-0.38 microns, so each colloidal particle scatters the light incident on it. It is visible in the Faraday-Tyndall cone, when the line of sight is directed at an angle to the beam passing through the sol. Thus, The Faraday-Tyndall effect is a phenomenon identical to opalescence, and differs from the latter only in the type of colloidal state, i.e., microheterogeneity of the system.

The theory of light scattering by colloidal disperse systems was developed by Rayleigh in 1871. It establishes the dependence of the intensity (amount of energy) of scattered light (I) during opalescence and in the Faraday-Tyndall cone on external and internal factors. Mathematically, this dependence is expressed in the form of a formula called the Rayleigh formula:

6.1

where I is the intensity of scattered light in the direction perpendicular to the beam of incident light; K is a constant depending on the refractive indices of the dispersion medium and the dispersed phase; n is the number of particles per unit volume of sol; λ is the wavelength of the incident light; V is the volume of each particle.

From formula (6.1) it follows that light scattering (I) is proportional to the particle concentration, the square of the particle volume (or for spherical particles - the sixth power of their radius) and inversely proportional to the fourth power of the wavelength of the incident light. Thus, the scattering of short waves occurs relatively more intensely. Therefore, colorless sols appear reddish in transmitted light, and blue in diffuse light.

Coloring colloidal solutions. As a result of selective absorption of light (absorption) in combination with diffraction, one or another color of the colloidal solution is formed. Experience shows that most colloidal (especially metallic) solutions are brightly colored in a wide variety of colors, ranging from white to completely black, with all shades of the color spectrum. Thus, As 2 S 3 sols are bright yellow, Sb 2 S 3 - orange, Fe(OH) 3 - reddish brown, gold - bright red, etc.

The same sol has a different color depending on whether it is viewed in transmitted or reflected light. Sols of the same substance, depending on the method of preparation, can acquire different colors - the phenomenon of polychromy (multicolor). The color of the sols in this case depends on the degree of particle dispersion. Thus, coarsely dispersed gold sols are blue in color, those with a greater degree of dispersion are violet, and highly dispersed ones are bright red. It is interesting to note that the color of a metal in its non-dispersed state has nothing in common with its color in a colloidal state.

It should be noted that the color intensity of sols is tens (or even hundreds) times greater than that of molecular solutions. Thus, the yellow color of the As 2 S 3 sol in a layer 1 cm thick is clearly visible at a mass concentration of 10 -3 g/l, and the red color of the gold sol is noticeable even at a concentration of 10 -5 g/l.

The beautiful and bright color of many precious and semi-precious stones (rubies, emeralds, topazes, sapphires) is due to the content in them of negligible (not detectable even on the best analytical balances) amounts of impurities of heavy metals and their oxides, which are in a colloidal state. Thus, to artificially obtain bright ruby ​​glass, used for car, bicycle and other lamps, it is enough to add only 0.1 kg of colloidal gold per 1000 kg of glass mass.

One glass with a solution of sodium chloride, and the other with egg white hydrosol; it is difficult to determine which is the colloidal solution and which is the true one, since both liquids are colorless and transparent in appearance (Fig. 85). However, these solutions can be easily distinguished by performing the following experiment. Let's put a light-proof case with a hole on (the table lamp), in front of which we'll put a lens in front of it in order to obtain a narrower and brighter beam of light. If we place both glasses in the path of the light beam, in the glass with sol we will see a light path (cone), while in the glass with sodium chloride the beam is almost invisible. After the scientists who first observed this phenomenon, the luminous cone in the liquid was called the Faraday-Tyndall cone (or effect). This effect is characteristic of all colloidal solutions.

Thus, the Faraday-Tyndall effect is a phenomenon identical to opalescence, and differs from the latter only in the type of colloidal state, i.e., microheterogeneity of the system.

In VMC solutions, the Faraday-Tyndall effect is not clearly detected due to the fact that the refractive index of solvated particles of the solute n differs little from the refractive index of the solvent Po, therefore the difference n - o-O, and the intensity of light scattering by VMC solutions is insignificant (see Chap. VII, 91). For the same reason, macromolecules cannot be detected under an ultramicroscope.

All the optical properties of highly dispersed systems, of which we will consider here color, opalescence, the Faraday-Tyndall effect and phenomena observed through an ultramicroscope, are interesting primarily because, as illustrated very schematically in Fig. 2, their intensity is maximum in the colloidal region of dispersion. This feature is due to the fact that the light wavelength of the visible part of the spectrum (760-400 mmk) exceeds the particle size of highly dispersed systems (200-2 mmk). The intensity of expression of these properties depends on the difference in the densities of the substances of the dispersed phase d and the dispersion medium o and with the difference in their refractive indices n and n. The greater the differences d-and n-n, the more sharply the optical properties are expressed. This explains the fact that optical properties are generally incomparably more pronounced in sols (especially metallic ones) than in solutions of high molecular weight compounds. For this reason, our further description of optical properties will concern almost exclusively sols.

OPALESCENCE AND THE FARADAY-TYNDALL EFFECT

It was found that when a beam of light is passed through pure water and other pure liquids and through clean (i.e., devoid of droplets and crystals of water and dust) air, and solutions with a low molecular weight solute, the Faraday-Tyndall effect is not observed, just as it is not observed in them and opalescence. Such media are called optically empty. Consequently, the Faraday-Tyndall effect was an important means for detecting the colloidal state, i.e., the microheterogeneity of a system.

Faraday - Tyndall, and the phenomenon itself is the Faraday - Tyndall effect.

The phenomenon of light scattering by tiny particles lies in

In turbid environments, violet and blue light are scattered the most, while orange and red light are scattered the least.

The Tyndall effect was discovered as a result of a scientist's study of the interaction of light rays with various media. He found that when light rays pass through a medium containing a suspension of tiny solid particles - for example, dusty or smoky air, colloidal solutions, cloudy glass - the scattering effect decreases as the spectral color of the beam changes from violet-blue to yellow-red part of the spectrum. If, however, white light, such as sunlight, which contains the full color spectrum, is passed through a turbid medium, then the light in the blue part of the spectrum will be partially scattered, while the intensity of the green-yellow-red part of the light will remain almost the same. Therefore, if we look at scattered light after it has passed through a cloudy medium away from the light source, it will appear bluer than the original light. If we look at a light source along the scattering line, that is, through a turbid medium, the source will seem redder to us than it actually is. This is why haze from forest fires, for example, appears bluish-violet to us.

The Tyndall effect occurs when scattering on suspended particles whose dimensions exceed the dimensions of atoms by tens of times. When suspension particles are enlarged to sizes on the order of 1/20 of the light wavelength (from approximately 25 nm and above), scattering becomes polychrome, that is, the light begins to scatter evenly over the entire visible range of colors from violet to red. As a result, the Tyndall effect disappears. This is why dense fog or cumulus clouds appear white to us - they consist of a dense suspension of water dust with particle diameters ranging from microns to millimeters, which is well above the Tyndall scattering threshold.

You might think that the sky appears blue to us due to the Tyndall effect, but this is not so. In the absence of clouds or smoke, the sky turns blue due to the scattering of “daylight” by air molecules. This type of scattering is called Rayleigh scattering(in honor of Sir Rayleigh; cm. Rayleigh criterion). In Rayleigh scattering, blue and cyan light is scattered even more than in the Tyndall effect: for example, blue light with a wavelength of 400 nm is scattered in clean air nine times more strongly than red light with a wavelength of 700 nm. This is why the sky appears blue to us - sunlight is scattered across the entire spectral range, but in the blue part of the spectrum it is almost an order of magnitude stronger than in the red. Ultraviolet rays that cause sun tanning are scattered even more strongly. That is why the tan is distributed fairly evenly over the body, covering even those areas of the skin that are not exposed to direct sunlight.

John Tyndall, 1820-93

Irish physicist and engineer. Born in Leighlin Bridge, County Carlow. After graduating from high school, he worked as a topographer and surveyor in military organizations and in the construction of railways. At the same time he graduated from the Mechanical Institute in Preston. Dismissed from the military geodetic service for protesting against poor working conditions. He taught at Queenwood College (Hampshire), while continuing his self-education. In 1848-51. attended lectures at the Universities of Marburg and Berlin. Returning to England, he became a teacher and then a professor at the Royal Institution in London. The scientist's main works are devoted to magnetism, acoustics, absorption of thermal radiation by gases and vapors, light scattering in turbid media . He studied the structure and movement of glaciers in the Alps.

Tyndall was extremely passionate about the idea of ​​popularizing science. He regularly gave public lectures, often in the form of free lectures for everyone: for workers in factory yards during lunch breaks, Christmas lectures for children at the Royal Institution. Tyndall's fame as a popularizer also reached the other side of the Atlantic - the entire print run of the American edition of his book Fragments of Science Science, 1871) was sold out in one day. He died an absurd death in 1893: while preparing dinner, the scientist’s wife (who outlived him by 47 years) mistakenly used one of the chemical reagents stored in the kitchen instead of table salt.

In turbid environments, violet and blue light are scattered the most, while orange and red light are scattered the least.

The Tyndall effect was discovered as a result of a scientist's study of the interaction of light rays with various media. He found that when light rays pass through a medium containing a suspension of tiny solid particles - for example, dusty or smoky air, colloidal solutions, cloudy glass - the scattering effect decreases as the spectral color of the beam changes from violet-blue to yellow-red part of the spectrum. If, however, white light, such as sunlight, which contains the full color spectrum, is passed through a turbid medium, then the light in the blue part of the spectrum will be partially scattered, while the intensity of the green-yellow-red part of the light will remain almost the same. Therefore, if we look at scattered light after it has passed through a cloudy medium away from the light source, it will appear bluer than the original light. If we look at a light source along the scattering line, that is, through a turbid medium, the source will seem redder to us than it actually is. This is why haze from forest fires, for example, appears bluish-violet to us.

The Tyndall effect occurs when scattering on suspended particles whose dimensions exceed the dimensions of atoms by tens of times. When suspension particles are enlarged to sizes on the order of 1/20 of the light wavelength (from approximately 25 nm and above), scattering becomes polychrome, that is, the light begins to scatter evenly over the entire visible range of colors from violet to red. As a result, the Tyndall effect disappears. This is why dense fog or cumulus clouds appear white to us - they consist of a dense suspension of water dust with particle diameters ranging from microns to millimeters, which is well above the Tyndall scattering threshold.

You might think that the sky appears blue to us due to the Tyndall effect, but this is not so. In the absence of clouds or smoke, the sky turns blue due to the scattering of “daylight” by air molecules. This type of scattering is called Rayleigh scattering(after Sir Rayleigh; see Rayleigh criterion). In Rayleigh scattering, blue and cyan light is scattered even more than in the Tyndall effect: for example, blue light with a wavelength of 400 nm is scattered in clean air nine times more strongly than red light with a wavelength of 700 nm. This is why the sky appears blue to us - sunlight is scattered across the entire spectral range, but in the blue part of the spectrum it is almost an order of magnitude stronger than in the red. Ultraviolet rays that cause sun tanning are scattered even more strongly. That is why the tan is distributed fairly evenly over the body, covering even those areas of the skin that are not exposed to direct sunlight.

• … He began to think what was what.
• Apparently, the light is afraid of torment.
• So the flour is perfect
• So that the wave diffracts!
• All kinds of dust, and suspension, and turbidity
• A beam of light can collapse...
• From "Ode to Tyndall" (E.Nickelsparg)

Element "AIR"

An apple fell on Newton, the Chinese admired the drops on lotus flowers, and John Tyndall, probably walking through the forest, noticed a cone of light. Fairy tale? Maybe. But it is in honor of the last hero that one of the most beautiful effects of our world is named - the Tyndall effect. Why is it beautiful - judge for yourself!

This is an optical effect that occurs when a light beam passes through an optically inhomogeneous medium. Typically observed as a luminous cone visible against a dark background. What is an optically inhomogeneous medium? In this case, dust or smoke, which is formed by colloidal particles that form aerosols. The size of the particles does not matter, because even nanoparticles in the atmosphere, be it particles of sea salt or volcanic dust, can cause such a beautiful spectacle. Studying light, Tyndall is rightfully the founder of fiber-optic communications, which have already become vital in our everyday life, which in the modern world has been improved to the nanolevel.

Element "WATER"

Take a look at the solutions shown in the figure. Outwardly, they appear almost identical: colorless and transparent. However, there is one “but”: the laser beam passes unhindered through the right glass, but is strongly scattered in the left glass, leaving a red trace. What's the secret?

In the right glass there is ordinary water, but in the left one there is a colloidal solution of silver. Unlike an ordinary or, as chemists say, a “true” solution, a colloidal solution does not contain molecules or ions of a dissolved substance, but its smallest particles. However, even the smallest nanoparticles can scatter light. This is the Tyndall effect.

What should the particle size be for their solution to be called “colloidal”? In various textbooks, it is suggested that particles whose size ranges from 1 nm to 100 nm, from 1 nm to 200 nm, from 1 nm to 1 micron are considered colloidal. However, the classification of sizes, like any other, is very conditional. The Tyndall effect in liquid media is used, for example, to assess the quality of wine. To assess the clarity of wines, a glass of wine is tilted slightly and placed between the light source and the eye, but not in line. The degree of transparency is determined not by the passage of rays through the wine, but by their reflection from suspended particles even of nanometer size! (Tyndall effect). To characterize the degree of transparency, a verbal scale is used, which includes such definitions as “light opal”, “opalescent”, “dull, with significant opalescence”. A number of optical methods for determining the size, shape and concentration of colloidal particles are based on the Tyndall effect.

“Although nanocolloidal particles are so small that they cannot be observed with an optical microscope, their content in a platinum-silver colloidal solution has been proven by using a laser beam directed into the colloidal solution and observing the Tyndall effect, i.e. scattering of light and bright radiance of the light beam,” from the annotation of Noadada cosmetics (Japan).

Element "EARTH"

The concept of “opalescence” is also directly related to John Tyndall. OPAL is a precious stone, from the play of light of which the term comes opalescence, denoting a special type of radiation scattering characteristic only of this crystal.

This is how Pliny described the opal: “The fire of opal is similar to the fire of a carbuncle, only softer and more gentle, while it glows purple like an amethyst and the green of the sea like emerald; everything merges together into unimaginable, sparkling splendor. The unimaginable charm and beauty of the stone earned it from many the name “paideros” - “love of a youth”. It is second only to emerald.”

Opal contains spherical silica particles with a diameter of 150-450 nanometers, which, in turn, are composed of small globules with a diameter of 50-100 nanometers, arranged in concentric layers or randomly. They form a fairly ordered packing (pseudocrystalline structure of opal). The spheres act as a three-dimensional diffraction grating, causing a characteristic scattering of light - opalescence. Thus, opal is a natural photonic crystal. The opal cluster superlattice served as a prototype for the creation of artificial photonic crystals. For example, in one of the very first works on the synthesis of photonic crystals, carried out at the Physico-Technical Institute (St. Petersburg) and Moscow State University in 1996, a technology was created for producing optically perfect synthetic opals based on microscopic spheres of silicon dioxide. The technology made it possible to vary the parameters of synthetic opals: sphere diameter, porosity, refractive index.

In opal, the lattices formed by closely packed spheres of silicon dioxide contain voids, occupying up to 25% of the total volume of the crystal, which can be filled with substances of a different type. The change in the optical properties of opals when filling voids with water was already known to scientists of the ancient world: a very rare variety of opal - hydrophane (hydrophane), in Old Russian - water light, becomes transparent when immersed in water. In modern developments, this property of a photonic crystal is used to create a light switch - an optical transistor.

Element "FIRE"

Possessing a rare talent as a lecturer and an extraordinary skill as an experimenter, Tyndall brought the “SPARK” of knowledge to the masses. Tyndall created an era with his popular lectures on physics, and may justly be considered the father of the modern popular lecture. His lectures were for the first time accompanied by brilliant and varied experiments, which are now included in the basic course of physics; all subsequent popularizers of physics followed in Tyndall's footsteps. He wrote: “In order to see the picture as a whole, its creator needs to distance himself from it, and in order to evaluate the general scientific achievements of any era, it is advisable to take the point of view of the subsequent one.” I would like to end with a poem I wrote on the topic of light and life:

• Walk on the edge of a knife
• Standing on the tip of a needle
• Where macro power is not important
• Compared to the power of the wave.
• Where gravity is weak
• If you are light as a charge,
• Only variable fields
• They will launch you like a missile.
• Interference lights
• They burn with the northern lights.
• And like spring streams
• The charges are quick and in a hurry.
• Perhaps this world of wonders
• Not visible to my eye,
• But he is the basis of all substances,
• Which means I live in it!