The aggregation patterns of sodium taurochenodeoxycholate (TCDC) and taurou
rsodeoxycholate (TUDC) in an isotonic sodium chloride solution are investig
ated by frontal chromatography on Sephadex G-10 columns at 298.2 K and are
quantitatively analyzed on the basis of stepwise aggregation models. The mi
cellar aggregation numbers, critical micelle concentrations (cmc's), stepwi
se aggregation constants, and micelle size distributions of TCDC and TUDC a
re determined and compared to those of sodium taurocholate and taurodeoxych
olate. The logarithms of gel-water partition coefficients, dimerization con
stants, and reciprocal cmc values for the four bile salts exhibit linearity
with their hydrophobic molecular surface areas, and the slopes are close t
o that of the logarithms of reciprocal cmc values against the hydrophobic m
olecular surface areas of lecithins. This fact shows that the gel adsorptio
n, dimerization, and micellization of bile salts are driven mainly by hydro
phobic interactions and that the Small model for the micellization of bile
salts is better than models based on hydrophilic interactions. The micelle
of TUDC grows less cooperatively than that of TCDC, an epimer of TUDC. This
result suggests that the 7 beta-hydroxyl group of TUDC inhibits micellar g
rowth because of increased intermolecular mismatching contact with the hydr
ophobic surface and decreased intermolecular hydrogen bonds in the micelle.