STEPWISE SELF-ASSOCIATION OF SODIUM TAUROCHOLATE AND TAURODEOXYCHOLATE AS REVEALED BY CHROMATOGRAPHY

From frontal chromatograms and derivarites of sodium taurocholate (TC) and taurodeoxycholate (TDC) on Sephadex G-10 columns in 0.154 M sodium chloride, monomer concentrations (C-1), weight and number average aggregation numbers excluding and including the monomer contribution, critical micelle concentrations (cmc's), and minimum multimerization concentrations have been determined as a function of the total concentration (C) Since the micellization of TC occurs rather noncritically, the cmc values obtained from the same centroid volume data depend on the data treatment. The definition of cmc, that cmc is the total concentration at the inflection on the C-1 vs C curve, is applicable to TC and TDC as well as surfactants. The dimerization of TC and TDC has been proven by several pieces of chromatographic evidence, and their dimerization constants are determined independently from centroid volume and peak volume data. The stepwise aggregation constants are determined and are used to calculate micelle size distributions. Micelles of TC and TDC grow above the cmc more gradually than those of surfactants. This gradual growth is the main reason for inconsistencies in the literature of bile salt micellization. Since TDC is more hydrophobic than TC, the dimerization constant and aggregation numbers of TDC are larger than those of TC. This result supports the Small model for small micelles of bile salt.