CRITICAL-BEHAVIOR OF IONIC MICELLAR SYSTEMS AT DIFFERENT SALT CONCENTRATIONS

The coexistence curves of the system dodecylammonium chloride+water+KCl have been measured at different salt concentrations. The results can be described with the usual Ising 3-D value for the critical exponent beta=0.325. The analysis of the diameter indicates that the correct order parameter is defined in terms of an effective concentration calculated according to Eq. (4). Both the order parameter and the diameter of the coexistence curve point out that the range of validity of simple scaling decreases with the KCl concentration, i.e., as the system approaches a critical end point. The critical line, and an estimation of the Krafft temperatures have allowed us to estimate the position of the critical end point within the T-w-[KCl] space, w indicating the weight fraction of the surfactant. The pressure dependence of the critical temperature has been measured and found to be independent of the salt content for the present range of concentrations.