Equilibrium dialysis studies on aqueous taurocholate-lecithin solutions: further validation of the method

Since the 1980 publication by Mazer, Benedek and Carey on the concept of the simple micelle-mixed micelle coexistence phenomenon in the bile salt-lecithin systems, it became apparent that a reliable method was needed for measuring the coexisting species concentrations. We recently published a method based on membrane dialysis equilibrium, and the purpose of this report is to review and to validate this previous work. In the present studies, two dialysis membranes with molecular weight cut-off of 8,000 and 12,000 to 14,000 Da, respectively, and two sets of solution volume conditions were investigated with the taurocholate-lecithin system. Coexistence data were obtained over a wide range of lecithin concentrations and the results were found to be in good agreement with those from the previous studies. The bile salt/lecithin molar ratios for the mixed micelles were also deduced from the data. The principal conclusion of this study is that although there is a moderate amount of variability (10% to 15%), the method is both satisfactory and useful in studying bile salt-lecithin equilibria and dynamics.