X-ray, Electrolytic Conductance, and Dielectric Studies of Bile Salt Micellar Aggregates

Abstract

Previously, structural models, observed in fibers and crystals, were proposed for sodium deoxycholate (NaDC), glycodeoxycholate (NaGDC), taurodeoxycholate (NaTDC), and taurocholate (NaTC) micellar aggregates, and were verified in aqueous solutions by means of several techniques. Here we report the X-ray analysis of sodium glycocholate (NaGC) fibers, which indicates that NaGC micellar aggregates could be formed by dimers and octamers as in the case of NaTC. Moreover, we present electrolytic conductance and dielectric measurements on NaGDC, NaTC, and NaGC aqueous micellar solutions to verify our micellar aggregate models. Specific conductance values of 0.1 mol dm-3 NaDC, NaTDC, NaGDC, NaTC, and NaGC solutions containing NaCl at concentration ranging from 0 to 0.8 mol dm-3 practically do not depend on the particular bile salt. Comparison with NaCl values shows that bile salt contribution to conductance decreases by increasing NaCl concentration, is nearly zero around the concentration range 0.5−0.6 mol dm-3, and becomes negative at higher concentration. This behavior can be explained if Na+ ions strongly interact with bile salt anions and reinforce their interaction when micellar size increases. Even the inclusion of Na+ and Cl- ions, coming from NaCl, into micellar aggregates cannot be excluded, especially at high ionic strength. NaDC, NaTDC, NaGDC, NaTC, and NaGC present high values of the average electric dipole moment per monomer μ that can be justified by a remarkable hydration of their micellar aggregates. Reasonably, micellar aggregate composition and population change very slightly or do not change at all within the temperature range 15−45 °C, because μ is nearly constant in this interval. Results also suggest that Na+ ions are anchored to anions in dilute solution, thus forming ion pairs in the case of NaTC and NaGC, at least. Dihydroxy and trihydroxy bile salts are characterized by very similar cation−anion interaction strengths, even though their structures are different. The trend of μ, which moderately decreases by increasing bile salt concentration, agrees with our structural models and can be due to coexistence of two structures, at least.