MOLECULAR-STRUCTURE OF THIN SURFACTANT FILMS AT THE LIQUID-VAPOR INTERFACE

A weighted-density functional method is developed to describe the molecular structure of a binary mixture of monoatomic and polyatomic liquids with high density gradients, using an atom-atom intermolecular interaction potential. Thin surfactant films on the melt surface near the triple point of subphase liquid are considered. We present some results for different values of surfactant adsorption with models of stiff-chain and freely jointed molecules. Surfactant molecules composed of three subunits are discussed. It is shown, that in films containing molecules with one hydrophilic group a monolayer-bilayer transition can be realized. In films composed of certain molecules with two hydrophilic groups a liquid-expanded-liquid-condensed phase transition is observed. It can be continuous or discontinuous dependent upon the chemical structure and flexibility of chain molecules. This transition is accompanied by a crucial change in conformational and orientational state of surfactant molecules, and/or by a change in the amount of solvent molecules in the surfactant film. In some cases a quasi-smectic ordering of the subphase liquid takes place in the region adjacent to the surfactant film.