Critical behavior of a cationic-surfactant–water–salt system near and far from the Krafft temperature

The cationic micellar system docecylammonium chloride +water+KCl has been studied by light scattering near the critical point for different concentrations of salt. For the lowest value of salt concentration, the critical point is rather close to the Krafft line, and therefore there is a critical mixture in equilibrium with two noncritical phases: vapor and pure solid surfactant. Both the correlation length and osmotic susceptibility data for the critical mixtures can be accurately described by the usual three-dimensional Ising exponents for all values of the concentration of KCl (${\mathit{c}}_{\mathrm{KCl}}$). This is in agreement with theoretical predictions that indicate that approaching a critical end point does not affect the critical exponents. Two off-critical mixtures have also been studied, and their behavior has been found to be compatible with the linear model equation of state.