SOLUBILIZATION OF CHOLESTEROL IN 2 BINARY MIXED MICELLES OF BILE-SALT AND NONIONIC SURFACTANT

The saturated amounts of solubilized cholesterol (C-ch) in mixed micelles of sodium cholate (NaC) and octaoxyethylene glycol mono n-decyl ether (C(10)E(8)) and of sodium deoxycholate (NaDC) and C(10)E(8) were measured by enzyme assay at 25, 29, 33 and 37 degrees C. The C-ch values in both systems increase with the total surfactant concentration. Because the mixed micelles for both systems tend to form C(10)E(8)-rich micelles near the critical micellar concentration (CMC) of the mixed system, the curves of cholesterol solubility approached the C-ch curve for C(10)E(8) alone near the CMC. The tendency of C-ch to decrease in both systems with increasing mole fractions of bile salts resembled that of the mean aggregation number of micelles. Thermodynamic analyses of cholesterol solubilization showed that the free energy of solubilization, if considered as the transfer of cholesterol from solid state to micellar environ ment, increased with increasing mole fraction of bile salt. The enthalpy of cholesterol solubilization (Delta H-S-->M) decreased with the mole fraction of bile salts and showed break points around the mole fraction of 0.75 for the NaC-C(10)E(8) system and at 0.60 for the NaDC-C(10)E(8), system, respectively. These phenomena resemble earlier hydrophobicity data for mixed micelles by fluorescence measurements. Furthermore, C-ch values for the NaDC-C(10)E(8) system were larger than those for the NaC-C(10)E(8) system because of the structural differences at the 7 alpha hydroxyl group between NaC and NaDC. This fact was confirmed by thermodynamic calculations.