Hydration force and the interfacial structure of the polar surface

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Abstract
Solvation, in particular the hydration force, for simple surfaces is believed to decay approximately exponentially on the length scale of a few tenths of a nanometre. For phospholipid membranes its range varies, however, with the surface at least as strongly as with the solvent characteristics. One possible reason for this is fine interfacial structure, most notably, the finite interfacial thickness. From a generalized non-local electrostatic model of hydration discussed in this work, the spatial decay of the interfacial hydration is concluded to be sensitive to the volume distribution and to the transverse fluctuations of the surface polar residues. If the effective interfacial width significantly exceeds the solvent–structure decay length, the interfacial width and the interfacial polarity distribution may ultimately become more important for the solvation range than the solvent characteristics themselves. For any relatively thick, for example biological, interface such effects are prone to dominate the hydration force and act as a messenger of the interfacial structure. Approaching or interacting macromolecules or membranes, consequently, can sense and pick up mutual surface patterns throughout the intermediate solvent, provided that the surface polarity profile has a ‘tail’ longer than the solvent–structure decay length. This may be one of the reasons for the biological significance of the hydration force. On the one hand, a rationale is thus proposed for the extraction of detailed structural information from the macroscopic hydration-force data. On the other hand, one possible reason is identified for the non-linear dependence of the maximal repulsion of hydration as a function of the overall surface hydrophilicity.

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