The micellization of sodium cholate (NaC) at 293.2, 298.2 303.2 308.2, and 313.2 K by cholate anion concentration was studied over the pH range from 6.0 to 7.2. Using a stepwise association model of cholate anions without bound sodium counterions, the aggregation number ((n) over bar) of the cholate micelles was evaluated and found to increase with the total concentration, indicating that the stepwise association model is applicable. The n values go up and down with increasing temperature: 17 at 298.2 and 12 at 313.2 K and at 60 mM of the sodium cholate. The fluorescence of pyrene was measured in sodium cholate solution to determine the critical micelle concentration (CMC), indicating a narrow concentration range for CMC. A sodium-ion-specific electrode was used to determine a relatively low degree of counterion binding to micelles, supporting the validity of the present association model of cholate anions. The aggregation numbers evaluated at a constant ionic strength of 0.15 and at lower but variable ionic strengths were similar except for higher cholate concentrations. (C) 2003 Elsevier Science (USA). All rights reserved.