Preferential affinity of the components of liquid mixtures at a rigid non-polar surface: enthalpic and entropic driving forces.

The modulation of the properties of lipid membranes by polyhydroxylated cosolutes such as sugars is a phenomenon of considerable biological, technological and medicinal relevance. A few years ago, we proposed the sugar-like mechanism--binding driven by the release of water molecules--as an attempt to rationalize the preferential affinity of carbohydrate molecules compared to water molecules for the surface of lipid bilayers, which is presumably related to the bioprotective action of these compounds. The goal herein is to gain a better understanding of the driving force underlying this mechanism, in terms of specific interactions or effects, as well as in terms of the energy-entropy partitioning. This is done in the simplest possible context of an apolar rigid-wall model representing the membrane, and mixtures of closely related and possibly artificial species in solution, namely monomers or dimers of Lennard-Jones particles, water with physical or reduced charges, and hydroxymethyl groups. The results indicate that although the sugar-like mechanism seems phenomenologically reasonable, the main driving force underlying this mechanism is not the entropy gain upon releasing water molecules into the bulk, as originally suggested, but rather the hydrophobic effect. Note that the latter effect is a generic concept and may in principle involve both a solvent release and an interaction component, depending on the solute considered.