Prominent properties of surface chemistry

There are many properties of matter which are related to their surface. These surface properties become prominent when the material is present in finely divided form or if its surface is rough. Under these conditions, surface area of the material increases enormously and large amount of the substance is present at the surface.

If we consider a liquid or a solid substance then we find that a molecule which is present in the bulk of the substance is being attracted uniformly from all sides by the neighbouring molecules and hence there is no net force acting on the molecule and there are no unbalanced forces of the molecule. On the other hand, if we consider a molecule that is present near the surface, we find that such a molecule is not bonded to other molecules from all sides but has some unutilized valencies because there are no neighbouring molecules above the surface to which it can be bonded. As a result of these unbalanced forces the molecules present at the solid or liquid surfaces tend to satisfy their residual forces by attracting and retaining on their surface, the molecules of other species when brought in contact with them.

This results in increasing the concentration of the substance near the surface compared with the bulk phases. A substance which has different concentration at the surface than in the bulk phases is said to adsorbed and the existence of a substance at the surface in a different concentration than in the adjoining bulk phases is adsorption. The material upon whose surface the adsorption takes place is called an adsorbent while the molecular species that get adsorbed are called adsorbate. The process of removal of an adsorbed substance from the surfaces on which it is adsorbed is called desorption. It is reverse of adsorption and can be brought about by heating or by reducing the pressure.

Adsorption is different from absorption. In absorption, the molecules of a substance are uniformly distributed in the bulk of the other, whereas in adsorption molecules of one substance are present in higher concentration at the surface of the other substance. It may be noted that in certain cases absorption and adsorption both take place simultaneously and are non-distinguishable. In such cases the word sorption is used to describe the phenomenon. It is simply the difference in concentration of the substance that creates a difference in all the three types. Also the force of adsorption are by virtue of the unbalanced forces or unutilized valencies that are present at the surface. These forces are likely to increase when a solid piece is pulverized to a powder or broken into new surfaces.

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