Chemical Processes where Size and Shape are Important

Abstract

It has long been known that molecules may become concentrated, i.e. adsorbed, at the surfaces of some solids. This is of practical value, for instance, when charcoal is used to remove traces of coloured impurities from solutions or to adsorb vapours in domestic cooker-hoods and in gas masks. It might be expected that the amount adsorbed for a given weight of adsorbing material would depend on the partial pressure or concentration of the adsorbed species and on its nature, thus:

$$x = K{P^n}$$

where x is the amount adsorbed for unit mass of adsorbent, P is the partial pressure of the adsorbed material (or concentration if it is in solution) and K and n are constants. This expression, known as the Freundlich adsorption isotherm, was originally arrived at empirically and has been found to fit many experimental results. It can be written:

$$\log x = \log K + n\log P$$

so the graph of log x against log P should be a straight line with a slope of n and an intercept of −logK/n when log x = 0. There should be no upper limit to the amount which can be adsorbed and as the partial pressure is increased it should eventually reach the saturated vapour pressure and condensation will occur.