In the early stages of the development of the doctrine of colloidal systems, it was believed that molecular kinetic properties are inherent only in true solutions. Long-term studies have proved that these properties are inherent in colloidal solutions. It was established that there are no qualitative differences between them, but only quantitative ones, which depend mainly on the size and shape of colloidal particles (micelles). Therefore, the discovery of the Brownian motion in this sense was of great importance.
For the first time (in 1827), the Brownian movement was investigated by the English botanist Robert Brown. Observing under an ultramicroscope for plant pollen suspended in a drop of water, the scientist discovered that microscopic particles of flower pollen are randomly (randomly) moving continuously. Brownian motion is a random, zigzag or chaotic movement of microparticles. Numerous studies have established that the chaotic motion of molecules is determined by the size of the particles, the temperature and viscosity of the dispersion medium. Moreover, the nature of the substance practically does not affect their movement.
Brownian motion and modern molecular kinetic theory of liquids
Frenkel suggested that when one molecule is displaced, a rearrangement of nearby molecules takes place, with each of them striving to occupy the previous position, which is most advantageous in energy terms.
As a result of abrupt and continuous movement of molecules, a self-diffusion process occurs. Microparticles dissolved in a liquid (dispersion phase) move approximately the same as solvent molecules (dispersion medium). Due to the continuous chaotic movement, they are actively moving and do not remain in any place.
Brownian motion of particles of colloids and suspensions arises as a result of the thermal motion of particles of the environment and their chaotic impacts on this molecule. As a result of such impacts, microparticles randomly move in space (dispersion medium). These movements are obtained as a result of shock during a certain research time (in one second a certain molecule can experience up to 1020 strokes). Given that small molecules receive an unequal number of strokes from different sides, they move in different directions. With a microparticle diameter of more than five micrometers, Brownian motion is practically not observed. The increase in size and molecular weight compensates for the shock. Therefore, particles with a high molecular weight (up to five micrometers) perform only vibrational rotations.
Brownian motion and diffusion
As a result 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.
Osmotic pressure is determined by the presence of micelles. Due to the large size of the molecules and low concentrations, their pressure is very low. Of course, part of the analyzed pressure in colloidal solutions largely depends on the presence of impurities of various electrolytes. So, high molecular weight solutions - polysaccharides, rubber, proteins - at 10-12 percent concentration have significant osmotic pressure. Thanks to special instruments (osmometers), the osmotic pressure of blood plasma was determined, which averages about 25 mmHg. It is proved that this pressure is directly proportional to the concentration of dissolved substances in both colloidal and true solutions.