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Surface Chemistry

Surface chemistry is the study of chemical phenomena that occurs at the interface of two phases, usually between a gas and a solid or between a liquid and a solid. It can be roughly defined as the study of chemical reactions at interfaces. One of the important aspects of surface chemistry study is to determine whether a molecule attaches itself to a surface by chemisorption or by physisorption. Surface chemistry is of particular importance to the field of heterogeneous catalysis. It is the surface chemistry which deals with Colloidal states which finds a lot of uses.

It is a well known fact that the molecular forces at the surface of a liquid are in a state of strain, unbalanced or unsaturation. The same is true of the surface of a solid, where molecules or ions in the surface of a crystal do not have all their forces satisfied by union with other particles. As a result of these residual forces, the surface of a solid or liquid tends to attract and retain molecules when it is brought in contact with a gas or a solution. As the molecules are thus taken up at the surface and do not  go into the interior or bulk, their concentration is more at the surface than in the bulk or interior of the  solid (or liquid), as the case may be. This change of concentration at a surface brought about by the action of surface is known as adsorption

An ordinary solution is composed of solvent and solute. The particles of solute in an ordinary solution are usually either normal molecules or ions and their size is generally between 1 and 10 Å or 0.1 – 1mm. There are various systems, where the particles are considerably larger and may in fact range from upper limit for ordinary solutions upto several microns in size. Systems which consist of media with dissolved or dispersed particles ranging approximately 1mm to several microns in size are called Colloids. Colloidal particles are smaller than coarse, filterable particles but larger than atoms and small molecules.

A colloidal system consists of two phase, viz.

  • Dispersed Phase: The phase which is dispersed or scattered through the dispersion medium is called Dispersed phase or discontinuous phase.

  • Dispersion Medium: The phase in which the scattering is done is called the dispersion medium or continuous medium.

Each of the two phases constituting a colloidal system may be a gas, a liquid or a solid. In milk, for example, the fat globules are dispersed in water. Hence fat globules form a dispersed phase and water is the dispersion medium. The term sol is applied to the dispersion of solid in liquid, solid or gaseous medium. The dispersion of a solid (dispersed phase) in a liquid (dispersion medium) is called colloidal solution. The dispersion of a solid (dispersed phase) in a gas (dispersion medium) is called solid aero sol. When a liquid is dispersed in another liquid, the resulting system is called an emulsion. When colloidal system becomes fairly rigid, it is refereed to a gel.

We will discuss the surface chemistry and its  various  related  concepts under following sub topics 

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