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

A weighted-density functional method is developed to describe the mole cular structure of a binary mixture of monoatomic and polyatomic liqui ds with high density gradients, using an atom-atom intermolecular inte raction potential. Thin surfactant films on the melt surface near the triple point of subphase liquid are considered. We present some result s for different values of surfactant adsorption with models of stiff-c hain and freely jointed molecules. Surfactant molecules composed of th ree subunits are discussed. It is shown, that in films containing mole cules 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 observe d. It can be continuous or discontinuous dependent upon the chemical s tructure and flexibility of chain molecules. This transition is accomp anied by a crucial change in conformational and orientational state of surfactant molecules, and/or by a change in the amount of solvent mol ecules in the surfactant film. In some cases a quasi-smectic ordering of the subphase liquid takes place in the region adjacent to the surfa ctant film.