Brownian motion: general information.

In the early stages of the development of the theory of colloidal systems, it was believed that the molecular-kinetic properties are inherent only in true solutions. Long-term studies have shown that these properties are inherent in colloidal solutions. It is established that there are no qualitative differences between them, but there are only quantitative differences, which depend mainly on the size and shape of the colloidal particles (micelles). Therefore, the discovery of the Brownian movement in this sense was of great importance.

For the first time (in 1827) Brownian movement was investigated by the English botanist Robert Brown. Observing an ultramicroscope behind the flower pollen of plants suspended in a drop of water, the scientist discovered that the microscopic particles of the pollen are randomly (chaotically) and continuously moving. Brownian motion is a disorderly, zigzag or chaotic movement of microparticles. Numerous studies have established that the chaotic motion of molecules is caused by the particle size, temperature, and viscosity of the dispersion medium. In this case, the nature of the substance has practically no effect on their movement.

Brownian motion and the modern molecular-kinetic theory of liquids

Frenkel suggested that when one molecule is displaced, a rearrangement of nearby molecules takes place, each of which tends to occupy the former position, which is the most profitable in terms of energy.

As a result of a discontinuous and continuous movement of molecules, a self-diffusion process occurs. Dissolved in the liquid microparticles (dispersion phase), the motion is about the same as the molecules of the solvent (dispersion medium). Because of the continuous chaotic movement, they actively move and do not remain in any place.

The Brownian motion of particles of colloids and suspensions arises from the thermal motion of environmental particles and their chaotic impacts on the given molecule. As a result of such impacts, microparticles randomly move in space (dispersion medium). These movements are obtained as a result of the impact of impacts for a certain time of investigation (in one second a certain molecule can experience up to 1,020 strokes). Considering the fact that molecules of small sizes receive an unequal number of impacts from different sides, they move in different directions. With a diameter of microparticles more than five micrometers, Brownian motion is practically not observed. Increasing the size and molecular weight of them compensates for the impact. Therefore, particles with a large molecular mass (up to five micrometers) perform only vibrational rotations.

Brownian motion and diffusion

As a result of the action of Brownian, as well as thermal motion, the concentration of molecules is equalized throughout the volume of the solution. Diffusion can occur in colloidal and true solutions.

The osmotic pressure is due to the presence of micelles. Due to the large size of the molecules and insignificant concentrations, their pressure is very low. Of course, part of the analyzed pressure in colloidal solutions depends to a large extent on the presence of impurities of various electrolytes. So, high-molecular solutions - polysaccharides, rubber, proteins - have a significant osmotic pressure at 10-12% concentration. Due to special devices (osmometers), the osmotic pressure of blood plasma was determined, which on average is about 25 mmHg. It is proved that this pressure is directly proportional to the concentration of dissolved substances in both colloidal and true solutions.