Molecular interaction in monolayers. I. Complex formation

Abstract

With the aid of a compensated Langmuir balance an investigation of the interaction between insoluble monolayers and soluble surface-active compounds has been undertaken by studying monolayer penetration at constant pressure and constant area, and by mixed monolayer compression. The studies have been limited to cases where the interaction is known to be strong. Compression force-area curves of cholesterol spread on solutions of sodium cetyl sulfate and digitonin, and of eicosylamine spread on sodium cetyl sulfate have shown that 1:1 association mixtures of the soluble and insoluble components are stable up to the collapse point of the mixed film. It is suggested that the film which collapses is, in fact, the 1:1 complex. Monolayer penetration experiments at constant surface area on the above systems have yielded results consistent with the earlier hypothesis that under these conditions 1:1 complexes are formed. Experiments at constant surface pressure on single and mixed films have provided further evidence in support of the existence of complexes between cholesterol and digitonin (or saponin); it is shown, however, that such an interpretation cannot be drawn in the experiments involving sodium cetyl sulfate as penetrant. A mechanism has been suggested in outline to explain the characteristic shape of these expansion curves.