Optical properties of colloids. Tyndall effect. Opalescence, light scattering

Both physics and chemistry study a phenomenon that helps explain why certain particles are visible at certain times. This phenomenon is known as the Tyndall Effect. This physical phenomenon was studied by Irish scientist John Tyndall in 1869. Since then, these studies have found numerous applications in the fields of physics and chemistry. And the fact is that he studies some particles that are not visible to the naked eye. However, because they can reflect or refract light, they become invisible in certain situations.

In this article, we will tell you everything you need to know about the Tyndall effect and its importance to physics in chemistry.

History of the study

This phenomenon got its name in honor of the Irish physicist D. Tyndall, who first documented it in the mid-19th century. The scientist conducted experiments with colloidal solutions. At the same time, he established an interesting feature of the behavior of a directed beam of light when passing through a solution containing microscopic substances. The scattering had clearly defined boundaries and was shaped like a cone with its apex at the point where the light entered. Increased visibility was achieved with a dark background.

Diagram of the Tyndall effect

Publications: Other (categorizer)

… 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 comes the term opalescence , denoting a special type of radiation dispersion characteristic only of this crystal.

This is how Pliny described opal: “The fire of opal is like the fire of a carbuncle, only softer and more tender, 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 type 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:

Physical side of the process

This behavior of light rays is quite typical when passing through substances (liquid or solid) containing impurities. Visible opalescence is caused by the scattering and polarization of light waves when they are reflected from particles inside. The latter have a size tens of times greater than the atoms of the substance.

John Tyndall established critical boundaries and derived dependency. According to his research, if the size of a colloidal particle is 1/20 of an atom, polychrome scattering of light with bluish or red tints appears. With a further increase in the size of microparticles, the effect disappears.

Conditions for the effect to occur

The Tyndall cone is visible to the naked eye, especially against a dark background. A similar phenomenon can be observed in some minerals under the following conditions:

Availability of a directional light source;
Transparent structure;
Jewelry processing;
Residence of colloidal particles in the crystal.

The Tyndall effect observed in this case may not be clear enough, as in experiments with solutions, however, the deflection and scattering of the rays is noticeable without special magnifying or optical instruments.

Thus, the conical divergence of white light rays can be observed when illuminating a mineral containing microparticles:

Chroma;
Selena;
Sodium;
Calcium;
Phosphorus;
Silicon;
Manganese;
Copper.

Optical properties of colloids. Tyndall effect. Opalescence, light scattering

ELECTROKINETIC PROPERTIES OF COLLOIDS

Electrokinetic phenomena are divided into two groups: direct and reverse. Direct ones include those electrokinetic phenomena that arise under the influence of an external electric field (electrophoresis and electroosmosis). Electrokinetic phenomena are called inverse, in which, during the mechanical movement of one phase relative to another, an electrical potential arises (percolation potential and sedimentation potential).

Electrophoresis and electroosmosis were discovered by F. Reuss (1808). He discovered that if two glass tubes are immersed in wet clay, filled with water and electrodes placed in them, then when a direct current is passed, clay particles move towards one of the electrodes.

This phenomenon of movement of dispersed phase particles in a constant electric field was called electrophoresis.

In another experiment, the middle part of a U-shaped tube containing water was filled with crushed quartz, an electrode was placed in each elbow of the tube and a direct current was passed. After some time, a rise in the water level was observed in the knee where the negative electrode was located, and a decrease in the other. After turning off the electric current, the water levels in the tube elbows were equalized.

This phenomenon of movement of a dispersion medium relative to a stationary dispersed phase in a constant electric field is called electroosmosis.

Later, Quincke (1859) discovered a phenomenon inverse to electroosmosis, called percolation potential. It consists in the fact that when fluid flows under pressure through a porous diaphragm, a potential difference arises. Clay, sand, wood, and graphite were tested as diaphragm materials.

A phenomenon inverse to electrophoresis, called sedimentation potential, was discovered by Dorn (1878). When particles of a quartz suspension settled under the influence of gravity, a potential difference arose between levels of different heights in the vessel.

All electrokinetic phenomena are based on the presence of a double electrical layer at the boundary of the solid and liquid phases.

https://junk.wen.ru/o_6de5f3db9bd506fc.html

18. The special optical properties of colloidal solutions are due to their main features: dispersity and heterogeneity. The optical properties of disperse systems are largely influenced by the size and shape of the particles. The passage of light through a colloidal solution is accompanied by such phenomena as absorption, reflection, refraction and scattering of light. The predominance of any of these phenomena is determined by the relationship between the particle size of the dispersed phase and the wavelength of the incident light. In coarse systems, reflection of light from the surface of particles is mainly observed. In colloidal solutions, the particle sizes are comparable to the wavelength of visible light, which determines the scattering of light due to the diffraction of light waves.

Light scattering in colloidal solutions manifests itself in the form of opalescence - a matte glow (usually bluish tints), which is clearly visible against a dark background when the sol is illuminated from the side. The cause of opalescence is the scattering of light on colloidal particles due to diffraction. Opalescence is associated with a phenomenon characteristic of colloidal systems - the Tyndall effect: when a beam of light is passed through a colloidal solution from directions perpendicular to the beam, the formation of a luminous cone is observed in the solution.

Tyndall effect, Tyndall scattering - an optical effect, the scattering of light when a light beam passes through an optically inhomogeneous medium. Typically observed as a luminous cone (Tyndall cone) visible against a dark background.

Characteristic of solutions of colloidal systems (for example, metal sols, diluted latexes, tobacco smoke), in which the particles and their environment differ in refractive index. A number of optical methods for determining the size, shape and concentration of colloidal particles and macromolecules are based on the Tyndall effect .

19. Sols are poorly soluble substances (salts of calcium, magnesium, cholesterol, etc.) existing in the form of lyophobic colloidal solutions.

Newtonian fluid is a viscous fluid that obeys Newton’s law of viscous friction in its flow, that is, the tangential stress and velocity gradient in such a fluid are linearly dependent. The proportionality between these quantities is known as viscosity.

Newtonian fluid continues to flow even if the external forces are very small, as long as they are not strictly zero. For a Newtonian fluid, viscosity, by definition, depends only on temperature and pressure (as well as on the chemical composition, if the fluid is not pure), and does not depend on the forces acting on it. A typical Newtonian fluid is water.

A non-Newtonian fluid is a fluid in which its viscosity depends on the velocity gradient. Typically, such liquids are highly heterogeneous and consist of large molecules that form complex spatial structures.

The simplest obvious household example is a mixture of starch with a small amount of water. The faster the external influence on the macromolecules of the binder suspended in the liquid occurs, the higher its viscosity.

Tandal effect stones

The Tyndall cone is observed in refined stones that have the ability to exhibit opalescence:

Opal;
Amethyst;
Quartz;
Amber.

from left to right amber, quartz, amethyst

It should be noted that depending on the geometric features of the surface of the particles, they reflect the rays differently. If their thickness is less than or equal to the wavelength of white light, the observer will be able to see a rainbow glow. Solid blue, appears when illuminated by bute opal.

opal butte