Interaction between adsorbed substances of simple constitution, and insoluble monolayers

Abstract

Extensive studies of the effect of injecting soluble, capillary active substances underneath insoluble monolayers made in recent years (Hughes 1935; Schulman and Hughes 1935; Schulman and Rideal 1937; Schulman and Stenhagen 1938) have shown that many rather complex, and very easily adsorbed substances can either penetrate an insoluble monolayer, or in some cases displace the monolayer from the surface of the solution. Sodium cetyl sulphate is exceptionally active in penetrating monolayers. Quite recently Bilham (1938) has investigated the reason for the exceptionally low cohesion found by one of us (Adam 1930), with films of long-chain, insoluble amines on acetate buffer solutions of p H about 4; these are gaseous films with very much less lateral cohesion than films of the same amines on other acid solutions of equal or greater acidity. He has shown that the low cohesion is due to the adsorption of acetic acid at the surface, from the buffer solution. He also found that sodium ethyl sulphate has a similar destructive action on the cohesion of amine films. In the work here described, we study the effect of placing insoluble monolayers on the surface of solutions of simple, capillary active substances, principally butyl alcohol, butyric acid, and phenol. Equilibrium between the monolayer and the adsorbed solute appears to be reached very quickly in these cases; generally there is no change in the surface pressure or potential more than about 10 sec. after the area of the film is changed, and a minute always sufficed for the attainment of a final value for the surface pressure or potential, which did not change for an hour or so, provided, of course, that the film was not collapsing.