Physical mechanisms of the interaction between lipid membranes in the aqueous environment

•Classical density functional theory explains basic features of the hydration repulsion.•The overlap of ordered interfacial water layers constitutes one mechanism for repulsion.•The choice of order parameters and boundary conditions is crucial in this description.•Several alternative mechanisms are discussed as contributions to the total membrane–membrane interaction.

We review various mechanisms underlying the interaction between biological membranes in their aqueous environment. In the first part we describe van der Waals interactions, derive generally-applicable density functional expressions for solute-mediated interactions with a focus on electrostatic interactions across aqueous electrolytes, and discuss interaction mechanisms involving the overlap of structured interfacial water layers. We then turn our attention to the long-debated hydration repulsion and review its quantitative investigation using classical molecular dynamics simulations.