COMPUTER-SIMULATION OF LIQUID/LIQUID INTERFACES .2. SURFACE-TENSION AREA DEPENDENCE OF A BILAYER AND MONOLAYER

A constant normal pressure-surface tension algorithm for molecular dynamics simulation, developed in the preceding paper, was used to laterally expand and compress the surface area of a dipalmitoylphosphatidylcholine (DPPC) lipid bilayer. Then, from simulations carried out at constant normal pressure and surface area, values of the surface tension and other thermodynamic variables such as the internal energy and system volume were determined at four different values of the surface area per lipid, 60.0, 65.1, 68.1, and 72.1 Angstrom(2). The surface tension shows dramatic variations with area, going from 6 to 60 dyn/cm at areas per molecule of 65.1 and 68.1 Angstrom(2), respectively. An approximate thermodynamic analysis indicates that an area of 68.1 Angstrom(2)/lipid is the closest of the four to the free energy minimum for this system, in agreement with experimental measurements. The effect of surface area changes on the calculated deuterium order parameters, which can be compared with those obtained from nuclear magnetic resonance experiments, is found to be quite large. Additionally, simulations of lipid monolayers were performed at the same surface areas and, though the dependence of the surface tension with area shows qualitative agreement with experiment, the simulation results are more sensitive to area changes than is observed experimentally. The variation in surface tension with area is much greater for the bilayer than the monolayer, suggesting that monolayers are a good model of bilayers only in a narrow range of surface areas.