Between the liquid molecules there are adhesion forces that are different for the surface and deep (inner) layers. Molecules located on the surface layer receive one-sided attraction from the side of 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 the molecules of air and steam less, their resultant force is directed downward, and the surface layer of molecules is as if drawn into the deep layers. Surface tension, in turn, depends on the nature of the fluid and the boundary media, the ambient temperature, the presence of impurities in the fluids, etc.
To increase the surface of the liquid, a certain work should be done directed against the forces of surface tension. The size of the droplets, the meniscus, the size of the bubbles of gas passing through the liquid, the wettability and non-wettability of the surface of a solid body, etc.
The force of surface tension also depends on the vapor pressure above the liquid: the more steam, the lower the magnitude of the internal pressure of the liquid. If the fluid is bordered by air, then the molecules of its surface layer are almost not subjected to attractive forces from 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 excess of free energy. Under all other identical conditions, this surplus is greater the larger the surface. According to the second law of thermodynamics, the system seeks to reduce internal energy. Therefore, the process of reducing the interface is an independent process. That is why a liquid, on which no forces act, forms the shape of a ball, since such a shape with a certain volume has the smallest surface.
The magnitude of the surface tension is also affected by the magnitude of the hydrocarbon radical of the substance. So, if we take several carboxylic acids of the same homologous series (HCOOH, CH3COOH, C2H5COOH, C3H7COOH), then 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 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.). The decrease in surface tension is of great importance in the processes of absorption of liquid nutrients by the epithelium of the intestinal mucosa. So, fats and other lipids enter the food channel in the form of drops. These drops are emulsified by bile in the small intestine and then become available for hydrolytic cleavage by lipase and other lipolytic enzymes. Adding surfactants to liquids increases their ability to wet the treated surface. When using insecticides, soaps are often added, which allows them to contact well with the surface of the body of insects and have an insecticidal effect.
The surface tension of a liquid: methods for its determination
There are a number of methods for determining the surface tension: stalagmometric, by the height of the liquid in the capillary, the highest pressure in the bubbles, by the separation of the ring.
Most often, the stalagmometric method is used, based on the use of a special device - a stalagmometer. It is a capillary tube with upper and lower marks and expansion. In the stalagmometer, water is drawn up to the mark, after which it is released and the number of drops is counted. Then the same thing is done with the test fluid. Under the action of surface tension forces, the flowing liquid and water take the form of spherical drops.
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 that makes up the human, animal, and plant organism. For cells with a strong and rigid surface (plants, microorganisms), the value of surface tension is small. Most of the cells in an animal’s body are nearly spherical. Low surface tension allows cells to isolate easily from one another. The shape of the cells attached to other cells or to the substrate depends on other factors - their cytoskeleton formed by a complex of membranes, contact structures, etc. Certain types of cells (for example, white blood cells and eggs), when released from the surface tension, acquire the shape of a ball and lose 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.