Micelle formation of sodium ursodeoxycholate and solubilization into the micelle

The micellization of sodium ursodeoxycholate (NaUDC) was studied at 298.2 K by aqueous solubility at different pH values. Using an association model of ursodeoxycholate anion without sodium counterion, the aggregation number (n) of the ursodeoxycholate micelle was evaluated and found to increase with the total concentration, indicating that the mass action model worked quite well. The n value at 40 mmol dm−3 was found equal to 5. The electrode potential measurement for sodium ion was made in order to confirm the low counterion binding to micelle. The solubilization of n-alkylbenzenes (benzene, ethylbenzene, n-butylbenzene, n-hexylbenzene) and polycyclic aromatic compounds (naphthalene, anthracene, pyrene) into the aqueous micellar solution of sodium ursodeoxycholate was performed. Solubilizate concentrations at equilibrium were determined spectrophotometrically at 298.2 K. The first stepwise association constants (K1) between a solubilizate monomer and a vacant micelle were evaluated from the equilibrium concentrations and found to increase with hydrophobicity of the solubilizate molecules. From the Gibbs energy changes for solubilization at the different mean aggregation numbers and from molecular structures of the solubilizates, the function of sodium ursodeoxycholate micelle as solubilizer was discussed and compared with the data from conventional aliphatic micelles and from sodium cholate micelle.