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surface tension,tendency of liquids to reduce their exposed surface to the smallest possible area. A drop of water, for example, tends to assume the shape of a sphere. The phenomenon is attributed to cohesion, the attractive forces acting between the molecules of the liquid (see adhesion and cohesionadhesion and cohesion,
attractive forces between material bodies. A distinction is usually made between an adhesive force, which acts to hold two separate bodies together (or to stick one body to another) and a cohesive force, which acts to hold together the like or unlike
..... Click the link for more information. ). The molecules within the liquid are attracted equally from all sides, but those near the surface experience unequal attractions and thus are drawn toward the center of the liquid mass by this net force. The surface then appears to act like an extremely thin membrane, and the small volume of water that makes up a drop assumes the shape of a sphere, held constant when an equilibrium between the internal pressure and that due to surface tension is reached. Because of surface tension, various small insects are able to skate across the surface of a pond, objects of greater density than water can be made to float, and molten lead when dropped into a cool liquid forms suddenly into shot. See capillaritycapillarity
or capillary action,
phenomenon in which the surface of a liquid is observed to be elevated or depressed where it comes into contact with a solid. For example, the surface of water in a clean drinking glass is seen to be slightly higher at the edges, where
..... Click the link for more information. .
an important thermodynamic characteristic of the interface of phases or bodies, defined as the energy required for the reverse isothermic formation of a unit of area of a given surface. In the case of a liquid interface, the surface tension may also be rightfully considered as the force acting on a unit of length of the surface contour and tending to contract the surface to a minimum for the given volumes of the phases. Both these definitions apply to mobile surfaces, although the first is preferred since it has a clearer physical sense. The surface tension at the boundary of two condensed phases is usually called the interphase tension.
The energy required to form a new surface is expended on overcoming the forces of intermolecular cohesion in transferring molecules of a substance from the bulk of the body to the surface layer. The resultant of the intermolecular forces in the surface layer is not equal to zero, as in the bulk of the body, and is directed toward the interior of the phase with the greater cohesion. Thus, the surface tension is a measure of the lack of compensation of intermolecular forces in the surface (interphase) layer. In other words, it is a measure of the excess free energy in the surface layer relative to the free energy in the volumes of the contiguous phases. In accordance with these definitions, surface tension is expressed in units of joules/m2 or newtons (N)/m (erg/cm2 or dynes/cm).
As a result of surface tension, in the absence of external forces a liquid adopts a spherical form corresponding to the minimum value of the surface and, consequently, to the smallest value of the free surface energy. Surface tension does not depend on the size and shape of the surface when the volumes of the phases are sufficiently great relative to the dimensions of the molecules. Surface tension decreases with increasing temperature as well as from the action of surfactants. Melts of metals have the greatest surface tension among liquids; for example, the surface tension of platinum at 2000°C is equal to 1,820 dynes/cm, and the surface tension of mercury at 20°C is 484 dynes/cm. The surface tension of molten salts is considerably less, ranging from several dozens of dynes/cm to 200-300 dynes/cm. The surface tension of water at 20°C is 72.8 dynes/cm, and the surface tension of most organic solvents ranges from 20 to 60 dynes/cm. The lowest surface tensions at room temperature, those of several fluoro-carbon liquids, are under 10 dynes/cm.
With multicomponent systems, the change in the surface tension in accordance with the Gibbs thermodynamic equation for adsorotion is
–d0 = Γ1dμ1 + Γ2dμ2 + ….
Here Γ1, Γ2, … are surface excesses of the components 1, 2, … —that is, the difference between the concentrations of the components in the surface layer and the volume of the solution or gas— and dμ1, dμ2, … are the changes in the chemical potentials of the corresponding components. (The minus sign indicates that the surface tension decreases with positive adsorption.) The surface pressure is determined by the difference in the surface tensions of the pure liquid and the liquid covered by the adsorption monolayer.
At mobile liquid-gas (vapor) or liquid-liquid boundaries, the surface tension may be measured directly by many methods. The usual methods are by the mass of a drop separating from the end of a vertical tube (a stalagmometer), by the maximum pressure required to reduce a gas bubble to a liquid, and by the shape of a drop or bubble lying on a flat surface.
It is difficult to determine by experiment the surface tensions of solids, since the molecules or atoms of solids are incapable of free displacement. An exception is the plastic flow of metals at temperatures close to the melting point. Because of the anisotropy of crystals, surface tensions on the different faces of a crystal vary. The concepts of surface tension and of free surface energy are not identical for solids. Defects in the space lattice, chiefly dislocations, edges, and apexes of crystals, as well as grain boundaries extending to the surface of polycrystalline bodies, contribute to the free surface energy. The surface tension of solids is usually determined indirectly from intermolecular and interatomic interactions. Many surface phenomena are determined by the magnitude and changes of surface tension, especially in disperse systems.
L. A. SHITS
In living organisms, the surface tension of the cell is one of the factors determining the shape of the entire cell and its components. It is low for cells with rigid or semirigid surfaces: such cells include those of Infusoria and many microorganisms and plant cells. For cells without a strong supramembranous structure (most animal cells, some protozoa, and the spheroplasts of bacteria), the surface tension largely determines the cell’s configuration; cells suspended within a liquid become almost spherical in shape. The shape of a cell attached to a substrate or to other cells depends primarily on such factors as contact structures and the cytoskeleton formed by the microtubules. It is assumed that local changes in surface tension are of importance in such phenomena as phagocytosis, pinocytosis, and gastrula-tion. The determination of the surface tension of the cell is a complex experimental problem; usually the surface tension of a cell does not exceed several dynes/cm (10–3 N/m).
A. G. MALENKOV
REFERENCEAdam, N. K. Fizika i khimiia poverkhnostei. Moscow-Leningrad, 1947.
(Translated from English.) Surface and Colloid Science, vol. 1. Edited by E. Matijevic. New York, 1969.