Between the molecules of the liquid there are forces of adhesion, different surface and deep (inner) layers. The molecules on the surface layer, get a one-way attraction from the deep layers and in deep layers of the forces of attraction between molecules are mutually balanced. The molecules of the surface layer are attracted by the molecules of air and steam is less, the resultant force is directed downwards, and the surface layer of molecules would be drawn into the underlying layers. Surface tension in turn depends on the nature of the fluid and boundary environments, ambient temperature, presence of liquids, impurities, etc.
To increase the liquid surface, should do some work against forces of surface tension. From the values of the coefficient of surface tension depends on droplet size, the meniscus, the size of the gas bubbles passing through the liquid, the wettability and nesmeshivaemost liquid to a solid surface, etc.
The Force of surface tension also depends on the vapor pressure above the liquid: the more steam, the less the magnitude of the internal pressure of the liquid. If the liquid is bounded with the air, the molecules of the surface layer is almost not subject to the forces of gravity from the gaseous phase. In those cases, when the vapor pressure above the liquid increases, its surface tension will be smaller. Thus, the molecules of the surface layer is not balanced, and therefore the system on the interface always has a certain excess of free energy. Under all other equal conditions the surplus is greater than a large surface. According to the second law of thermodynamics, the system tends to reduce the internal energy. Therefore, the process of reducing the surface section is process independent. Therefore, the liquid on which no forces are applied, forms a sphere because this form when a certain volume has the smallest surface.
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On the surface tension affects the magnitude of the hydrocarbon radical substances. So, if you take a few carboxylic acids of the same homologous series (HCOOH,CH3COOH, С2Н5СООН, С3Н7СООН), the surface tension decreases as you lengthen the carbon chain – 3.2 times for each CH2 group.
The Presence of impurities of various substances in liquids affect the surface tension. Many substances, mainly organic, to lower the surface tension. Such compounds are called surfactants (proteins, bile acids, Soaps, alcohols, aldehydes, esters, ketones, hannidy, etc.). Substances whose presence does not significantly affect the 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 to the epithelium of the mucous membrane of the intestines. So, fats and other lipids in the food channel come in the form of drops. These droplets are emulsified by bile in the small intestine and then becomes available for hydrolytic cleavage by the lipase and other lipolytic enzymes. Add to liquids surfactants increases their ability to moisten the surface to be treated. When using insecticides is often added to soap, which allows them to contact with the surface of the body of insects and to provide insecticidal activity.
The Surface tension of liquids: methods of its determination,
There are a number of methods for the determination of surface tension: stalagmometric, the height of rise of liquid in the capillary tube, the highest pressure in the bubbles for separation of a ring.
Most Often used stalagmometric method based on the use of a particular device — stalagmometer. It is a capillary tube with upper and lower labels and extension. In stalagmometer typed water to the mark, and then it comes and count the number of drops. Then the same thing is done with a researched liquid. Under the action of surface tension forces resulting liquid and water take the form of spherical drops.
The Surface tension in the life of organisms
The Surface tension is one of the factors that determine the shape of the cell and its parts. Cage - an elementary particle of living matter that makes up the human body, animals and plants. For cells in which a solid and rigid surface (plants, microorganisms), the value of surface tension is small. Most of the cells of the animal body has a shape close to spherical. Low surface tension allows the cells easily isolated from one another. The shape of the cells attached to other cells or to the substrate, depends on other factors — their cytoskeleton, formed by complex membranes, contact structures, etc. in certain types of cells (e.g., leukocytes and ova) during the liberation from the action of surface tension take the form of a ball and lose activity. Local changes of surface tension plays an important role in the processes of perception and transmission of nerve impulses, phagocytosis, pinocytosis, gastrulation, amoeboid movement, and especially in the permeability of cell membranes.
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