Multiplicity of States and Internal Structure of Supported Lipid Bilayers

Lipid bilayers attached to solid surfaces play important role in biomimetic materials and devices and serve as model systems for studies of interactions of cell membranes with particles and biomolecules. Despite active experimental and theoretical studies, the interactions of lipid membranes with solid substrates are still poorly understood. In this work, we explore, using atomistic molecular dynamics simulations, equilibrium and stability of phospholipid DMPC membrane supported on hydroxylated amorphous silica. We reveal two distinct types of thermodynamically stable states, characterized by the different width of the water layer between the membrane and the substrate. In α-states, the membrane is closely attached with the lipid head groups interacting directly with surface hydroxyls, however due to the molecular level roughness of amorphous silica surface, there exists an inhomogeneous water layer trapped between the substrate and the membrane. In β-states, the membrane is separated from the silica surface by a water film of ~2.5 nm in thickness. The thermodynamic equilibrium is quantified in terms of the disjoining pressure isotherm as a function of membrane-substrate separation, which has a double sigmoidal shape with two minima and one maximum, which correspond to the limits of stability of α and β states. The thermodynamic properties and bilayer structure are compared with experimental findings and simulation results for relevant systems.