Surface Chemistry is the study of all physico-chemical changes occurring at the interfaces of different substances. such as gases, liquids and solids. The surface of separation or the contact between two immiscible phases is called interface.
When solids are allowed to remain in contact with a gas. a film of gas molecules accumulates on the surface. The molecules or ions at the surface of a solid have unsaturated or residual forces. Solid and liquid surfaces therefore have a tendency to satisfy their residual forces by attracting onto and retaining on their surfaces, molecules of gases or other substances with which they come in contact. This results in higher concentration of molecules on the surface of a solid or liquid than in the bulk of the medium. The phenomenon is known as adsorption.
The substance on which the adsorption takes place is known as the adsorbent and the substance which adsorbed is known as adsorbate. The phenomenon of adsoiption should not be confused with absorption which refers m one material passing into the bulk of another. For example water is absorbed by a sponge and water vapours arc adsorbed by anhydrous calcium chloride. Similarly, the process in which both absorption and adsorption take place simultaneously is known as sorption. For example: dyeing of cotton fibers.
All solids adsorb gases to some measurable extent. The amount of adsorbed gas as a rule decreases with increase of temperature at a constant pressure and increases with a fhll in temperature. Hence, according to Le Chatelier Braun principle, adsorption of a gas by a solid is accompanied by evolution of heat which is known as the heat of adsorption. The extent of adsorption increases with increase of pressure.