Determination of the aggregation constants of bile salts by capillary electrophoresis

Capillary electrophoresis has been investigated as a novel experimental method for determination of the aggregation constants of surfactants. The tendency of sodium cholate and sodium taurodeoxycholate to associate was studied in phosphate buffers of pH 8.0 and pH 7.0, respectively. Stepwise aggregation equilibria of bile salt monomers has been described in terms of mass-balance equations. The Offord equation was used to model the electrophoretic mobility of the bile salt associates, and the experimental mobility values could be fitted to the model. Interestingly, only even-membered aggregates-dimers and tetramers-besides the monomers were proposed from the results of the curve-fitting for both bile salts. The aggregation constants calculated were (in molar units): cholate log K-A2 = 1.37, log K-A4 = 4.98; taurodeoxycholate log K-A2 = 1.68, log K-A4 = 6.46. From these values, more pronounced aggregation of taurodeoxycholate starting at lower concentrations has been deduced, supporting the back-to-back association model of bile salts.