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surface layer[′sər·fəs ‚lā·ər]
a thin layer substance near the contact surface of two phases, bodies, or mediums whose characteristics differ from those of the substances in the bulk phases. The characteristics typical of surface layers result from the accumulation of excess free energy in these layers and from the layers’ structure and composition. Surface layers at the boundary between condensed phases are often called interphase layers.
The thickness of surface layers depends on the difference in the densities of the phases, on the intensity and type of the intermolecular interactions in the boundary zone, and on the temperature, pressure, chemical potentials, and other thermodynamic parameters of the system. The surface layer is sometimes no thicker than a monomolecular layer, and at other times reaches dozens and hundreds of molecular diameters in thickness. Thus, the surface layers of liquids close to the critical temperature for combining may have a thickness of 1,000 Å (100 nanometers) and more. Surface layers formed by molecules or ions of an adsorbed substance are called adsorption layers. The composition and properties of surface layers become particularly altered upon adsorption of surfactants.
Adsorptive, chemisorptive, or chemical action on the surface layer of a solid may lead to its lyophilization or lyophobization, may reduce its strength, or may improve its mechanical properties. The state of the surface layer of various construction and radiotechnological materials strongly affects their operational and technical characteristics. Many types of surface phenomena are related to the properties of surface layers.
L. A. SHITS