MEMBRANE WATER PARTITION OF OLIGO(ETHYLENE OXIDE) DODECYL ETHERS AND ITS RELEVANCE FOR SOLUBILIZATION

Mixed aqueous dispersions containing palmitoyloleoylphosphatidylcholin e and oligo(ethylene oxide) dodecyl ethers C(12)EO(n) with n = 2-8 hav e been investigated. The aggregates composition has been determined as a function of the aqueous detergent concentration using Laurdan fluor escence spectroscopy. The partition of the detergent between the membr anes and the aqueous phase has been analyzed on the basis of the regul ar solution model. Solubilization has been analyzed in terms of the li miting detergent fraction in the membrane, the minimal detergent fract ion in micelles and the critical aqueous detergent concentration using thermodynamical coexistence conditions for the aqueous, bilayer and m icellar pseudo-phases. A thermodynamic criterion for solubilization is presented. The standard chemical potential differences of the transfe r of the detergents from water to the bilayer have been found to follo w the empirical relation Delta mu degrees(n = 2-8)= -37.6 + 0.9n kJ/mo l. Thus, a similar conformation and localization of the oxyethylene un its within the membrane are suggested.