All surfaces in water experience at short separations hydration repulsion or hydrophobic attraction, depending on the surface polarity. These interactions dominate over the more long-ranged electrostatic and van der Waals interactions and are ubiquitous in biological and colloidal systems. Despite their importance for all scenarios where the surface separation is in the nanometer range, the origin of these hydration interactions is still unclear. Using atomistic solvent-explicit molecular dynamics simulations, we analyze the interaction free energies of charge-neutral model surfaces with different elastic and water-binding properties. The surface polarity is shown to be the most important parameter that not only determines the hydration properties and thereby the water contact angle of a single surface, but also the surface–surface interaction and whether two surfaces attract or repel. Elastic properties of the surfaces are less important. Based on surface contact angles and surface–surface binding affinities, we construct a universal interaction diagram featuring three different interaction regimes: hydration repulsion, dry adhesion, and cavitation-induced attraction, and for intermediate surface polarities, dry adhesion. Based on scaling arguments and perturbation theory, we establish simple combination rules that predict the interaction behavior for combinations of dissimilar surfaces.

Hydration repulsion dominates the interaction between polar surfaces in water at nanometer separations and ultimately prevents the sticking together of biological matter. Although confirmed by a multitude of experimental methods for various systems, its mechanism remained unclear. A simulation technique is introduced that yields accurate pressures between solvated surfaces at prescribed water chemical potential and is applied to a stack of phospholipid bilayers. Experimental pressure data are quantitatively reproduced and the simulations unveil a rich microscopic picture: Direct membrane–membrane interactions are attractive but overwhelmed by repulsive indirect water contributions. Below about 17 water molecules per lipid, this indirect repulsion is of an energetic nature and due to desorption of hydration water; for larger hydration it is entropic and suggested to involve water depolarization. This antagonistic nature and the presence of various compensating contributions indicate that the hydration repulsion is less universal than previously assumed and rather involves finely tuned surface-water interactions.

A model of the outer membrane of Gram-negative bacteria was created by the deposition of a monolayer of purified rough mutant lipopolysaccharides at an air/water interface. The density profiles of monovalent (K ⁺ ) and divalent (Ca ²⁺ ) cations normal to the lipopolysaccharides (LPS) monolayers were investigated using grazing-incidence X-ray fluorescence. In the absence of Ca ²⁺ , a K ⁺ concentration peak was found in the negatively charged LPS headgroup region. With the addition of CaCl ₂ , Ca ²⁺ ions almost completely displaced K ⁺ ions from the headgroup region. By integrating the experimentally reconstructed excess ion density profiles, we obtained an accurate measurement of the effective charge density of LPS monolayers. The experimental findings were compared to the results of Monte Carlo simulations based on a coarse-grained minimal model of LPS molecules and showed excellent agreement.

Membrane systems that naturally occur as densely packed membrane stacks contain high amounts of glycolipids whose saccharide headgroups display multiple small electric dipoles in the form of hydroxyl groups. Experimentally, the hydration repulsion between glycolipid membranes is of much shorter range than that between zwitterionic phospholipids whose headgroups are dominated by a single large dipole. Using solvent-explicit molecular dynamics simulations, here we reproduce the experimentally observed, different pressure-versus-distance curves of phospholipid and glycolipid membrane stacks and show that the water uptake into the latter is solely driven by the hydrogen bond balance involved in non-ideal water/sugar mixing. Water structuring effects and lipid configurational perturbations, responsible for the longer-range repulsion between phospholipid membranes, are inoperative for the glycolipids. Our results explain the tight cohesion between glycolipid membranes at their swelling limit, which we here determine by neutron diffraction, and their unique interaction characteristics, which are essential for the biogenesis of photosynthetic membranes.