CATANIONIC AMPHIPHILIC LAYERS - A MONTE-CARLO SIMULATION STUDY OF SURFACE FORCES

Monte Carlo simulations have been performed to study the interaction b etween catanionic amphiphilic surfaces in an aqueous solution. Experim entally these systems show a phase behavior similar to phospholipid sy stems, where a lamellar phase is in equilibrium with almost pure water , and the swelling of the lamellar phase cannot be explained by the DL VO theory. It has recently been proposed that the short-range repulsiv e force between bilayers has its origin in the confinements of the pro trusional motions of the amphiphilic molecules as a second bilayer app roach. A model has been developed which allows for the amphiphiles to move in a direction perpendicular to the hydrocarbon-water interface a nd for the correlations between these motions. The result shows that t he protrusional degree of freedom extends the range of the repulsive f orce to about 1 nm; it is decaying exponentially with a decay length c onsistent with the protrusion force model of Israelachvili and Wenners trom. The structure of the surface and its dependence on the interacti ons between the headgroups of the amphiphiles were also investigated. The results indicate rough surfaces with varying headgroup protrusions , the degree of protrusion being dependent on the electrostatic intera ctions within the surface and of the size of the polar headgroups. Whe n the surface consists of negatively charged amphiphiles, such as for example an SDS-water system, a more diffuse surface results compared t o the catanionic lamellar system. Increasing the size of the headgroup s also gives rise to larger protrusions. The nonhomogeneity in relativ e permittivity across the hydrocarbon-water interface has also been ta ken into account within a simple model, and it is found that the intro duction of a dielectric discontinuity results in less diffuse surfaces . The simulated forces are used to calculate the phase equilibria in c atanionic systems, and we find a good quantitative agreement.