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 dyn amics simulation, developed in the preceding paper, was used to latera lly expand and compress the surface area of a dipalmitoylphosphatidylc holine (DPPC) lipid bilayer. Then, from simulations carried out at con stant normal pressure and surface area, values of the surface tension and other thermodynamic variables such as the internal energy and syst em 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 tensio n shows dramatic variations with area, going from 6 to 60 dyn/cm at ar eas per molecule of 65.1 and 68.1 Angstrom(2), respectively. An approx imate 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 su rface area changes on the calculated deuterium order parameters, which can be compared with those obtained from nuclear magnetic resonance e xperiments, is found to be quite large. Additionally, simulations of l ipid monolayers were performed at the same surface areas and, though t he dependence of the surface tension with area shows qualitative agree ment with experiment, the simulation results are more sensitive to are a changes than is observed experimentally. The variation in surface te nsion with area is much greater for the bilayer than the monolayer, su ggesting that monolayers are a good model of bilayers only in a narrow range of surface areas.