AGGREGATION BEHAVIOR OF SODIUM CHOLATE IN AQUEOUS-SOLUTION

We analyze the freezing point depression, DELTAT/k, and pNa measurements for aqueous solutions of sodium cholate (NaC). The results show the existence of a break point at 0.0197 m in the plot of DELTAT/k vs molality. Below this point (more exactly 0.0160 m) NaC behaves as a 1:1 strong electrolyte. Above it, in the range of concentrations studied, NaC forms aggregates with an aggregation number equal to 3.09 +/- 0.06. The fraction of bound counterions is also deduced, ranging from 0.03 to 0.16 (average value O.077). These results indicate that less than one Na+ counterion is bound per aggregate. The value for the equilibrium constant for the formation of the trimer is K(3) = (3.4 +/- 0.9) X 10(3), from which a value of w0 = -2.6k(B)T is obtained for the reversible transfer of a free surfactant ion, together with the associated counterions, from the bulk solution to the aggregate. The effect of the addition of NaCl is also studied. The results show that the aggregation number increases slowly with NaCl concentration. This result is corroborated by viscosity measurements.