Thermodynamic Study of Bile Salts Micellization

The aggregation process of sodium cholate, NaC, sodium deoxycholate, NaDC, sodium glycocholate, NaG, sodium glycodeoxycholate, NaDG, sodium taurocholate, NaT, and sodium taurodeoxycholate, NaDT in aqueous solution has been investigated at several temperatures, in the absence and in the presence of NaCl 0.15 mol kg(-1). Results show that both a decrease in the number of ring hydroxyls and an increase in the length of the side chain favor micellization, in the absence as well as in the presence of salt. These observations can be explained by considering that the hydrophobic effect is the driving force for the self-association process. This is in agreement with the Delta C-mic(p)o values, which point out that the micellization process leads to a diminution of the hydrophobic surface of the bile salt molecules exposed to water. The presence of NaCl 0.15 mol kg(-1) in the aqueous phase favors micellization by decreasing the CMC due to a diminution in the electrostatic repulsions within the micelles. However the presence of the background electrolyte has no substantial effect on either the micellar ionization degree or the enthalpy of micellization. The thermodynamic magnitudes indicate that the bile salts micellization is entropy driven.