Using solution X-ray scattering, we determined the structure and interactions of saturated (DLPS) and unsaturated (DOPS) charged lipids under osmotic stress. The measured pressure–distance curves were fit to the theoretical free energy of charged liquid membranes that included numerical solutions of the Poisson–Boltzmann theory, modified to account for the mixing entropy of lipid molecules with adsorbed and dissociated counterions, the nonelectrostatic interactions between  the counterions and the lipid membrane surface, and the ineteractions between the counterions near the surface. The numerical solutions that fit the data showed that at a critical osmotic pressure, there is a sharp drop in the gap between bilayers owing to massive adsorption of the counterions onto the membrane. The adsorption increases with the osmolyte concentration, and is accompanied by a lateral phase separation into domains of neutral (counterion adsorbing) and charged (non-adsorbing) lipid molecules (Soft Matter 2013). The transition is analogous to the gas-liquid first-order phase transition when the pressure increases. 

The effect holds for pure water, salt solutions, and solutions of ionic liquids.