Simulations Of Amphiphilic Molecular Membrane And Development Of Thermodynamic Scaling Monte Carlo Method

Liquid film is composed of surfactants for which its unique architectures result in their self-assembly into the ordered structures at mesoscopic scale. The stabilization of liquid film is crucially related to the role of surfactants. We used Lattice Monte Carlo (LMC) simulation method to study the aggregation behaviors of surfactants in the liquid film.Phospholipid molecules, which are an important component of cellular membrane, are of amphiphilic structure.In our research,phospholipid molecules are dipalmitoyl phosphatidyl choline, (DPPC), have been successfully used to prepare molecular ordered LB film experimentally. In this work, we used molecular dynamics method (MD) to investigate surface fluctuations of lipid bilayer consisting of phospholipid molecules.For computer simulations on fluid phase equilibrium calculations, an important issue is to determine the unlike pair interaction parameter between unlike components, which is at present determined by a trial-and-test method or by laborious iterative calculations. In this work we did some attempts to solve the problem.The main results and conclusions are summarized as follows.1. LMC method was used to study the adsorption morphologies and aggregation of surfactants in the liquid film on a vapor-liquid interface, focusing on morphology transition near the critical micellar concentration (CMC). Moreover, in this work, we found with LMC calculations that the adsorption amount and states of surfactants fluctuate strongly near CMC, forming two alternating relatively stable states, which is a bistable system.2. We used molecular dynamics (MD) simulation with Gromacs software and Marrink coarse grained model for the phospholipid molecules (DPPC). We studied the influence of compression constants, lateral pressure and simulated box size on the distribution of lipid molecules of the bilayer.3. In this work, basing on the thermodynamic scaling method put forward and developed by Graham, Valleau and Kiyohara etc, we propose a new method to calculate the fluid phase equilibria for a range of unlike pair interaction parameters in a single simulation run, which makes the determination of unlike parameter much easily. In this work, we also present a method to optimize the weight in thermodynamics scaling Monte Carlo methods. The method, which is based on the perturbation theory, requires only a single simulation run on a reference state. Our simulation results indicate that the method converges quickly and improves greatly the calculation accuracy.