Abstract:
Knowledge of reactions at solid/liquid and solid/gas interfaces is of great importance in the study of all adsorption phenomena. Techniques that enable a study of molecules (liquid or gaseous) adsorbed onto a surface may be divided into two categories: (a) those that upset the equilibrium between molecules in the gaseous (or liquid) phase above the solid surface and molecules actually adsorbed onto it, and (b) those that do not. Those techniques that do not disturb this equilibrium will give results that would be expected to have greater reliability than those obtained from techniques that upset this equilibrium (for example by heating or by affecting one component of the equilibrium by titration, precipitation etc.) In an endeavour to study the properties of water adsorbed onto various substances such as clay, wool and textile fibres without affecting the equilibrium the technique of isotopic exchange has been developed. Essentially the procedure is to take a closed adsorber system in equilibrium with a gas (or liquid), part of which is in the sensitive region of a geiger counter, and to add a very small amount of radioactively labelled gas (or liquid) to the system. The adsorber is placed in the bottom of a geiger counter out of the sensitive volume and a known fraction of gas (or liquid) is in the sensitive volume. As the system is at equilibrium there is continuous exchange between the adsorbed molecules on the sample and the molecules in the gaseous (or liquid) state. Thus, when a very small amount, by weight, of the radioactively labelled gas (or liquid) is added to the system, exchange will take place with the non-radioactive molecules adsorbed on the surface of the material under study. Thus radioactivity will be removed from the sensitive volume of the geiger counter and adsorbed onto the surface of the material, and so the specific activity (count rate), as measured with the geiger counter, will drop. The advantage of this technique is that the equilibrium between the adsorbed molecules and the free gas (or liquid) is not disturbed. The actual amount of radioactive material added is so minute that there is no effective change in the concentration of the free gas (or liquid).
