Surface tension: general information and biological significance

Between the molecules of the fluid there are cohesive forces, different for the surface and deep (internal) layers. Molecules on the surface layer get one-sided attraction from the deep layers, and in the deep layers, the forces of attraction between the molecules are mutually balanced. Molecules of the surface layer are attracted by air and vapor molecules, their resultant force is directed downward, and the surface layer of molecules is drawn, as it were, by deep layers. The surface tension in turn depends on the nature of the liquid and the boundary media, the surrounding temperature, the presence of impurities in the fluids, and so on.

To increase the surface of the liquid, it is necessary to do some work directed against the forces of surface tension. The size of the surface tension depends on the size of the droplets, the meniscus, the size of the gas bubbles passing through the liquid, the wettability and non-wetting of the solid surface by the liquid,

The surface tension force also depends on the vapor pressure above the liquid: the greater the vapor, the smaller the internal pressure of the liquid. If the liquid is bordered by air, then the molecules of its surface layer are almost not subjected to gravitational forces on the side of the gaseous phase. In cases where the vapor pressure above the liquid increases, its surface tension will be less. Thus, the molecules of the surface layer are not balanced, and therefore the system at the phase interface always has a certain surplus of free energy. Under all other identical conditions, this surplus is greater than a larger surface. According to the second law of thermodynamics, the system tends to reduce internal energy. Therefore, the process of reducing the interface is a process independent. That is why the liquid, which no forces act on, forms the shape of a sphere, since such a shape has a smallest surface for a certain volume.

The value of the surface tension is affected by the hydrocarbon radical of the substance. So, if we take several carboxylic acids of the same homologous series (HCOOH, CH3COOH, C2H5COOH, C3H7COOH), their surface tension decreases as the carbon chain lengthens, by 3.2 times for each CH2 group.

The presence of impurities of various substances in liquids affects the magnitude of the surface tension. Many substances, mainly organic, lower the surface tension. Such compounds are called surface-active (proteins, bile acids, soaps, alcohols, aldehydes, esters, ketones, tannides, etc.). Substances whose presence does not significantly affect surface tension are called surface-inactive (fructose, glucose, starch, etc.). Reduction of surface tension is of great importance in the processes of absorption of liquid nutrients by the epithelium of the intestinal mucosa. Thus, fats and other lipids enter the food canal in the form of drops. These drops are emulsified by bile in the small intestine and after that become accessible for hydrolytic cleavage by lipase and other lipolytic enzymes. Adding surfactants to the liquids increases their ability to wet the surface to be treated. When using insecticides, soaps are often added, which allows them to make good contact with the surface of the body of insects and have an insecticidal effect.

Surface tension of a liquid: methods of its determination

There are a number of methods for determining the surface tension: stalagmometric, the height of the rise of the liquid in the capillary, the greatest pressure in the bubbles, and the ring detachment.

Most often, a stalagmometric method is used, based on the use of a special instrument - a stalagmometer. It is a capillary tube with the top and bottom marks and extension. The stalagmometer gathers water to the mark, after which it is released and the number of drops is counted. After that, the same thing is done with the liquid being tested. Under the action of surface tension forces, the outflowing liquid and water take the form of globular droplets.

Surface tension in the life of organisms

Surface tension is one of the factors determining the shape of the cell and its parts. A cell is an elementary particle of living matter, of which the human, animal and plant organism consists. For cells in which a strong and hard surface (plants, microorganisms), the value of surface tension is small. Most of the cells in the animal's body have a shape that is close to spherical. Low surface tension allows cells to be easily isolated from each other. The shape of cells attached to other cells or to the substrate depends on other factors-their cytoskeleton, formed by a complex of membranes, contact structures, etc. Separate cell types (eg, leukocytes and ovules) become loose from the action of surface tension and lose their activity . Local changes in surface tension play an important role in the processes of perception and transmission of nerve impulses, in phagocytosis, pinocytosis, gastrulation, amoeboid movement and, especially, in the permeability of cell membranes.