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Simulation studies of the friction between layers of dialkyl surfactants have been performed using a non-equilibrium molecular dynamics method. The model layers at a temperature of 298K and a normal pressure of 210MPa are sheared at a relative velocity of 1 ms-I. The friction coefficient has been studied as a function of the molecular geometry by simulating bilayers of CloCls (asymmetrical) and ClsCls and CloClo (symmetrical) surfactants. In all cases the hydrocarbon chains are attached to a positively charged dimethylammonjum head-group which interacts with a negatively charged surface. At a head-group area of 50A2 per molecule, the friction between the layers of asymmetrical surfactants is greater than that between layers of symmetrical surfactants at approximately the same normal pressure. At 77 A* the friction between the ClsClo layers remains higher than that of the ClsCls layers but is now lower than that of the CloClo, where the surface structure is highly disordered and the two layers are separated by only 15.8A. The friction between the layers correlates well with the amount of layer overlap as defined by the common area under the chain density profiles. These observation, which are in broad aggreement with the experimental measurement on similar dichain surfactants are rationalised in terms of the translational, orientational and confonnational structures of the layers.

BACKGROUND: The exact mechanisms that acetamide and glycerol interact with cell membrane remains a matter of debate. OBJECTIVE: To investigate the microscopic interactions of acetamide and glycerol with phospholipid bilayers at various temperatures. MATERIALS AND METHODS: Molecular dynamics simulations of a hydrated dipalmitoyl-phosphatidylcholine (DPPC) bilayer in the presence of glycerol and acetamide were performed. The system contains 128 lipids and about 700 cryoprotectant molecules, and simulations extended to 15 ns. RESULT: When compared to glycerol, acetamide shows a stronger affinity with water rather than the lipid bilayer. CONCLUSION: The knowledge of the mixing dynamics of present system helps to develop better cryoprotective formulas and to propose more optimal cooling/warming protocols.