Determinants of fatty acid and alcohol monomer activities in mixed micellar solutions.

The determinants of monomer activities of lipids dissolved in micellar bile salt solutions have been studied using polyethylene discs as the organic phase of a partitioning system. The studies show that fatty acids and alcohols interact with micelles as a partitioning system so that the monomer activity is determined by micelle volume and the lipid's partition coefficient as well as mass of lipid in the solution. Influence of the partition coefficient is seen in the dependence of monomer activity on chain length, unsaturation and carboxyl or alcohol polar groups. Dependence on chain length is equivalent to an incremental free energy of approximately -700 cal. mol(-1) per methylene group. Substitution of an alcohol group for the carboxyl group at pH 7.4 decreases monomer activity by a factor of 900. Expansion of taurodeoxycholate micelles with 5mM monooleoylglycerol slightly decreases monomer activity whereas solutions of lipids in taurocholate have relatively greater monomer activities, demonstrating the influence of volume of the micelle organic phase. With constant micelle structure, monomer activity was linearly dependent on lipid mass in the system as predicted by partitioning theory. Addition of low concentration of lecithin, lysolecithin, or monoacylglycerol to the solutions had only small effects on the monomer activities consistent with the small change in total micelle organic phase. Data provided allow calculation of monomer activities of fatty acids and alcohols in many complex micellar solutions. Such data are important for evaluating such processes as intestinal absorption and gallstone formation and dissolution.