Membrane/water partition of oligo(ethylene oxide) dodecyl ethers and its relevance for solubilization

Mixed aqueous dispersions containing palmitoyloleoylphosphatidylcholine and oligo(ethylene oxide) dodecyl ethers C12EOn with n = 2-8 have been investigated. The aggregates composition has been determined as a function of the aqueous detergent concentration using Laurdan fluorescence spectroscopy. The partition of the detergent between the membranes and the aqueous phase has been analyzed on the basis of the regular solution model. Solubilization has been analyzed in terms of the limiting detergent fraction in the membrane, the minimal detergent fraction in micelles and the critical aqueous detergent concentration using thermodynamical coexistence conditions for the aqueous, bilayer and micellar pseudo-phases. A thermodynamic criterion for solubilization is presented. The standard chemical potential differences of the transfer of the detergents from water to the bilayer have been found to follow the empirical relation delta u o(n = 2-8) = -37.6 + 0.9 n kJ/mol. Thus, a similar conformation and localization of the oxyethylene units within the membrane are suggested.