COMPUTER-SIMULATION OF A WATER MEMBRANE INTERFACE

In an effort to understand the origin of the hydration force, we performed a molecular dynamics computer simulation on a system composed of water molecules embedded between surfaces of dilauroylphosphatidylethanolamine (DLPE) molecules. The simulations show that the orientational polarization of water displays an oscillatory decay and its behavior is independent of thermal motion of the head groups of the membrane molecules. Since the Landau-type theory proposed by Marcelja and Radic predicts a monotonic decay for the order parameter, we conclude that the orientational polarization is not a good order parameter. Instead, we proposed a new quantity to measure the order parameter, "hydrogen bond deficiency", and calculate its behavior for the simulations. It agrees well with the theoretical predictions.