Solute partitioning into chain molecule interphases: Monolayers, bilayer membranes, and micelles

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Abstract
A mean field lattice theory is presented for the mixing of small molecule solutes within interfacial phases of chain molecules, and for their partitioning from an external solvent. Interphases include monolayers, bilayers, vesicles, cylindrical and spherical micelles, and stationary phases of grafted chains used in reversed phase liquid chromatography. Transfer depends on the entropy of mixing; the configurations of the chains; the contact interactions among solute and chains, treated here in the regular solution approximation; and the volumetric expansion of curved interphases. The theory predicts stable gradients of solute concentration at equilibrium due to the variation of chain organization with depth. For example, alkyl solutes should prefer mid-bilayer, since the surfactant chains are most disordered there; this prediction agrees with recent small-angle neutron scattering experiments. Increasing the surface density of the interphase chains should lead to solute expulsion. Most solutes should prefer the outer regions of spherical micellar cores due to the greater volume available there, even though densities are predicted to be highest at the core center.