HYDRATION OR STERIC FORCES BETWEEN AMPHIPHILIC SURFACES

The strongly repulsive short-range force between amphiphilic surfaces has, since Langmuir, been associated with how these surfaces modify the local water structure. We consider the results of both old and new experiments and conclude that these so-called “hydration” forces are not due to water structure. Instead, they originate from the entropic (osmotic) repulsion of molecular groups that are thermally excited to protrude from these fluid-like surfaces. Genuine hydration effects play a minor role, mainly in determining the hydrated size of the protruding groups. Our conclusions resolve many inconsistencies and observations that were not reconcilable with the hydration model. © 1990, American Chemical Society. All rights reserved.