A general treatment and classification of the solute adsorption isotherm part. II. Experimental interpretation

Abstract

The solute adsorption isotherm classification, examined theoretically in the previous paper, is illustrated by a number of further experiments which help to clarify some earlier obscurities and enable an isotherm to assist (a) in interpreting the adsorption mechanism and the solute orientation, (b) in identifying the presence of associated solute and of an impurity, and (c) in measuring the specific surface area of a solid. The production of the S-curve by either cooperative adsorption or by a competitive effect of an impurity in the solvent is illustrated and discussed. Thus p-nitrophenol is adsorbed from dry benzene by both silica and graphite. Traces of water in the solvent reduce the adsorption on silica by competition and change the curve from L- to S-shape, but have no effect on adsorption on graphite, where competition is negligible. The adsorption of aromatic hydrocarbons by alumina, which usually gives S-curves, is discussed. Cooperative adsorption can give the H-type of curve if the cooperating molecules move as a large unit from solvent to substrate, as in J-aggregate adsorption of cyanine dyes on silver halides. Examples of adsorption on inorganic and biological substrates giving C-type (linear) curves are quoted, illustrating the progressive penetration of micropores in the substrate suggested to be characteristic of this process. Data are given showing that maxima in isotherms for adsorption of high affinity solutes can be identified with completion of the monolayer. Surface tension measurements suggest that maxima may result from the presence of traces of a highly surface-active impurity.