Surface Phenomena

Surface Phenomena

 

the special properties of surface layers, that is, the thin layers of a substance at the boundary of contiguous bodies, mediums, or phases. These properties result from the excess free energy of the surface layer and from the special features of the layer’s structure and composition.

Surface phenomena may be purely physical in nature, or they may be accompanied by chemical transformations; they occur at liquid (highly mobile) and solid interphase boundaries. Surface phenomena related to surface tension and resulting from the deformation liquid boundary surfaces are also called capillary phenomena. Such phenomena include capillary absorption of liquids into porous bodies, capillary condensation, and the establishment of the equilibrium shape of drops, gas bubbles, and menisci. The properties of a contact surface of two solids or of a solid with a liquid or gaseous medium are determined by such effects as adhesion, wetting, and friction.

The molecular nature and properties of a surface may be radically altered as a result of the formation of surface monomolecular layers or phase (polymolecular) films. Such changes often result from such physical processes as adsorption, surface diffusion, and the spreading of liquids or from the chemical interaction of components of the contiguous phases. Any modification of the surface, or interphase, layer usually leads to an increase or decrease in molecular interaction between the contiguous phases. Physical or chemical transformations in surface layers strongly affect the nature and rate of such heterogeneous phenomena as corrosive, catalytic, and membrane processes.

Surface phenomena also affect the typically volumetric properties of bodies. Thus, a decrease in the free surface energy of solids by an actively adsorptive medium results in a decrease in the strength of these bodies (Rebinder effect). Such surface phenomena as electroadhesive and electrocapillary phenomena and electrode processes, which result from the presence of electric charges in the surface layer, form a special group. Physical or chemical changes in the surface layer of a conductor or semiconductor significantly affect the electron work function. They also affect such surface phenomena in semiconductors as surface states, surface conductivity, and surface recombination; these influences are reflected in the operational characteristics of such semiconductor instruments as solar batteries and photodiodes.

Surface phenomena are found in any heterogeneous system consisting of two or several phases. In essence, the entire physical world, from cosmic bodies to submicroscopic formations, is heterogeneous. Only systems in limited volumes of space may be regarded as homogeneous. Thus, the role of surface phenomena in natural and technological processes is great. Surface phenomena are especially important in disperse (microhetero-geneous) systems, in which the interphase surface is most highly developed. In fact, the conditions facilitating the appearance and prolonged existence of such systems are related to surface phenomena.

The major problems of colloid chemistry reduce to surface phenomena in disperse systems. All the processes that lead to changes in the size of the particles of the highly dispersed phase, including coagulation, coalescence, peptization, and emulsifica-tion, are due to the interaction of Brownian motion and surface phenomena. In coarsely disperse and macroheterogeneous systems, a primary role is played by the tension between surface forces and external mechanical actions. Surface phenomena, by affecting the magnitude of the free surface energy and the structure of the surface layer, control the origin and growth of new-phase particles in supersaturated vapors, solutions, and melts. Surface phenomena also control the interaction of colloidal particles in the formation of various types of disperse structures. Surfactants, which alter the structure and properties of interphase surfaces as a result of adsorption, often fundamentally affect the extent and tendency of processes caused by surface phenomena.

The foundation of the modern thermodynamics of surface phenomena was laid by the American physical chemist J. Gibbs. The Soviet scientists P. A. Rebinder, A. N. Frumkin, B. V. Deriagin, and A. V. Dumanskii developed theoretical concepts of the nature and molecular mechanism of surface phenomena that are of great practical importance.

The utilization of surface phenomena in industry permits the improvement of existing technological processes. Surface phenomena often determine methods for extending the durability of important structural and construction materials; they also determine the efficiency of mining and concentrating minerals and the quality and properties of products of the chemical, textile, food-processing, and pharmaceutical industries. Surface phenomena have great importance in metallurgy and in the production of ceramics, metal ceramics, and such polymer materials as plastics, rubbers, paints, and varnishes. Such surface phenomena as lubrication, abrasion, contact interaction, and structural changes in polycrystalline and composite materials, as well as electrical and electrochemical processes and phenomena occurring on the surfaces of solids, are important in technology.

A knowledge of surface phenomena in biology makes it possible to control biological processes with the aim of increasing agricultural productivity and developing the microbiological industry and the potentialities of medicine and veterinary medicine.

L. A. SHITS

In biology, surface phenomena are important primarily in relation to the cellular, subcellular, and molecular levels of organization of living systems. Various biological membranes separate the cell from its environment and secure its mi-croheterogeneity. Vital processes take place on the membranes of cells and of such intracellular organelles as mitochondria and plastids. These processes include the reception of such exogenous and endogenous biologically active substances as hormones, mediators, antigens and pheromones; enzymatic catalysis (many enzymes exist within membranes, forming mul-tienzymatic catalytic aggregates); the transformation of chemical energy into osmotic action; and oxidative phosphorylation, that is, the linking of oxidation processes with the accumulation of energy in macroergic compounds.

The special properties of surface interactions are responsible for the aggregation of cells and their attachment to living and nonliving substrata; this includes the formation of thrombi when the walls of vessels are injured, as well as the sorption of viruses on cells. The functioning of such important enzymatic systems as the aggregate of respiratory enzymes is an example of heterogeneous catalysis. The adsorption of corresponding physiologically active substances on surfaces is the basis for the identification of innate and foreign macromolecules as well as the basis of narcosis and of the transfer of nervous impulses. As a whole, surface phenomena in living systems differ from those in nonliving nature in their far greater chemical specificity and their mutual compatibility over time and in space. For example, the reception of a hormone on the surface of a cell leads to a conformational transition of a series of membrane components. This causes a change in the membrane’s permeability and heterocatalytic activity and in turn leads to numerous physicochemical and biochemical changes in the cell. These changes in the aggregate determine the cell’s reaction to various influences.

With the progress of evolution, surface phenomena have played an increasingly important role in vital activity. Thus glycolysis, the most ancient mechanism for providing cells with energy, is effected by enzymes in the cytoplasm that are only partially attached to structures of the endoplasmic network. The later and more efficient means of obtaining energy— respiration — is achieved by means of heterocatalytic systems. In unicellular organisms, nourishment occurs by means of the ingestion of entire macromolecules and their subsequent breakdown within the cell. In higher organisms, an important role is played by parietal (contact) digestion, in which enzymatic hydrolysis of the food macromolecules occurs on the external surface of the cell and is coordinated with the subsequent transport of the breakdown products to the cell.

A. G. MALENKOV

REFERENCE

Moelwyn-Hughes, E. A. Fizicheskaia khimiia, book 2. Page 807. Moscow, 1962. (Translated from English.)
Kurs fizicheskoi khimii, 2nd ed., vol. 1. Edited by la. I. Gerasimov. Moscow-Leningrad, 1969.
Uspekhi kolloidnoi khimii. Edited by P. A. Rebinder and G. I. Fuks. Moscow, 1973.
Gibbs, J. W. Termodinamicheskie raboty. Moscow-Leningrad, 1958. (Translated from English.)
Rusanov, A. I. Fazovye ravnovesiia i poverkhostnye iavleniia. Leningrad, 1967.
Mezhfazovaia granitsa gaz— tverdoe telo. Moscow, 1970. (Translated from English.)
Deriagin, B. V., N. A. Krotova, and V. P. Smilga. Adgeziia tverdykh tel. Moscow, 1973.
Zimon, A. D. Adgeziia zhidkosti i smachivanie. Moscow, 1974.
Semenchenko, V. K. Poverkhnostnye iavleniia v metallakh i splavakh. Moscow, 1957.
Recent Progress in Surface Science, vols. 1–5. Edited by J. F. Danielli [et al.] New York-London, 1964–72.
Vasil’ev, Iu. M., and A. G. Malenkov. Kletochnaiapoverkhnost’i reaktsii kletok. Leningrad, 1968.
Pasynskii, A. G. Biofizicheskaia khimiia, 2nd ed. Moscow, 1968.
Surface Phenomena in Chemistry and Biology. London, 1958.
Surface Chemistry of Biological Systems. New York-London, 1970.
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