HYDRATION VS PROTRUSION FORCES BETWEEN LIPID BILAYERS

For separation of the order of 1 nm, lipid bilayers exhibit a strong repulsive interaction which could arise from direct hydration forces or from thermally excited protrusions. A simple model for the competition between these two forces is introduced. In this model, collective protrusion modes consisting of the correlated displacements of several molecules are included. The model is studied by Monte Carlo simulations and functional renormalization. Two different regimes are found in which the repulsion is dominated by protrusion and by hydration forces, respectively. Both regimes should be accessible to experiments by changing the temperature or the lipid-solvent composition.