Micellar properties of dihydroxy and trihydroxy bile salts: Effects of counterion and temperature

Abstract

The shift of the wavelength of maximum absorption (λ-max) of a dye, Rhodamine 6G, was used to determine the critical micelle concentration (CMC) of two bile salts, sodium taurocholate (NaTC), a trihydroxy bile salt, and sodium taurodeoxycholate (NaTDC), a dihydroxy bile salt. The micellar properties of these salts and equimolar mixtures of both were studied in water and with added counterion (NaCl) over a temperature range of 10–80°C. The CMC range of the bile salts was: NaTC 0.6–6.0 m M, NaTDC 0.8–3.3 m M and NaTC NaTDC mixture 0.9–5.6 m M. As predicted on molecular grounds both salts have both nonionic and anionic surfactant properties. The hydroxyl groups clustered on one side of the bile salt nucleus prevent precipitation of dye bile salt complexes at low bile salt concentrations. Thus, bile salts behave as nonionic detergents with dyes of opposite charge and allow the first break point in a plot of the λ-max shift vs. log concentration of bile salt to be taken as an estimate of the CMC. The CMC of both salts and mixtures first fell slightly (about 10% decrease) and then increased with increases of temperature. Added counterion (NaCl) had little effect on the CMC of NaTC until very high concentrations (1–3 M). The CMC of NaTDC was decreased slightly by added NaCl. An assessment of micellar charge, determined from the slope of the log CMC vs. log NaCl plot showed that the micelles are highly charged and bind little or no counterions in water. The CMC values obtained in this study, when compared with values in the literature, were found to be somewhat lower. Finally, the standard thermodynamic functions of micellization were calculated on the assumption that micelles and monomers can be treated mathematically as a mass action equilibrium. The free energy changes (Δ F) were negative and varied little with temperature. The difference in ΔF between NaTC and NaTDC on micellization was −310 cal/mole and is approximately equivalent to half the ΔF value of micelle formation when a classical alkyl detergent has its hydrocarbon chain lengthened by one methylene group. At low temperatures micelle formation in bile salt solutions is an entropy-directed process while above 60°C a negative enthalpy change appears to be the predominant driving force.