Monolayers formed by mixtures of anionic and cationic surface-active agents

Abstract

A study has been made of the monolayers formed by salts of the type alkyltrimethylammonium alkyl sulphate at the air/solution interface. Insoluble monolayers were examined on a Langmuir trough. Although the substrate or the particular molecular species involved has some effect, maximum interaction invariably occurs at a 1 : 1 ratio of long chain cation to anion. The composition of the surface layers of the soluble members has been measured by radiotracer techniques. When the long chain cation and anion have the same chain length an equimolar surface composition is obtained. However, if the ions have different chain lengths, the longer chain ion is preferentially adsorbed to give an asymmetric surface layer, the asymmetry being enhanced by the addition of inorganic electrolyte. It is concluded that the adsorption promoted by the elimination of alkyl chain/water interface may be modified considerably by the electrical interactions in the plane of the head groups.