Elastic properties of surfactant monolayers at liquid-liquid interfaces: Amolecular dynamics study

Using a simple molecular model based on the Lennard-Jones potential, we sys tematically study the elastic properties of liquid-liquid interfaces contai ning surfactant molecules by means of extensive and large-scale molecular d ynamics simulations. The main elastic constants of the interface, correspon ding to the interfacial tension and the mean bending modulus are determined from the analyses of the long-wavelength behavior of the structure factor of the capillary waves. We found that the interfacial tension decreases wit h increasing surfactant interfacial coverage and/or surfactant chain length . However, we found that the corresponding change in the bending rigidity i s nonmonotonic. Specifically, we found that the bending rigidity decreases with increasing surfactant interfacial coverage for small surfactant interf ace coverages, but then it increases as the surfactant interface coverage i s further increased. Using a Gaussian theory on an interfacial Ginzburg-Lan dau model of surfactants, we find that the initial decrease of the bending rigidity is attributed to coupling between fluctuations of the surfactant o rientation field to those in the interfacial height. (C) 2000 American Inst itute of Physics. [S0021-9606(00)70717-3].