Hydration repulsion acts between all sufficiently polar surfaces in water at small separations and prevents dry adhesion up to kilobar pressures. Yet it remained unclear whether this ubiquitous force depends on surface structure or is a sole water property. We demonstrate that previous deviations among different experimental measurements of hydration pressures in phospholipid bilayer stacks disappear when plotting data consistently as a function of repeat distance or membrane surface distance. The resulting pressure versus distance curves agree quantitatively with our atomistic simulation results and exhibit different decay lengths in the ordered gel and the disordered fluid states. This suggests that hydration forces are not caused by water ordering effects alone. Splitting the simulated total pressure into membrane–membrane and water-mediated parts shows that these contributions are opposite in sign and of similar magnitude, thus they are equally important. The resulting net hydration pressure between membranes is what remains from the near-cancellation of these ambivalent contributions.
