Water adsorption isotherms of lipids.

Hydration of bilayer lipids is a fundamental property of biological membranes. The available database of lipid hydration isotherms is fitted over the entire range of water activities by using a statistical mechanical approach that is an extension of the common Brunauer-Emmett-Teller model, to include differential energies of association for water molecules beyond the first strongly bound layer. Three-parameter fits are obtained that can be used to represent the experimental isotherms to a good degree of accuracy over the complete range of water-binding activities. Fits are also made in terms of the hydration pressure and correlation length of water ordering, by using the polarization theory of lipid hydration. The relationship of the latter approach to measurements of hydration forces between lipid bilayers is discussed.