Bulk and interfacial properties of a cationic micellar system near the critical point

The behaviour of the cationic micellar system [dodecylammonium chloride + water + KCl] has been studied at two different salt concentrations. Detailed measurements of density, of vapour-liquid surface tension, and of liquid-liquid interfacial tension have been performed. The temperature dependence of the density, and the use of extended scaling equations have allowed us to determine the binodal and pseudospinodal curves. The diameter of the coexistence curve and the order parameter can be described with the 3D-Ising critical exponents. The temperature dependence of the effective critical exponent Beff, obtained from the density measurements, follows the same master curve previously found using the composition of the coexisting phases for many binary mixtures. The behaviour of the vapour-liquid surface tension data for both the homogeneous and for the two-phase system, is compatible with Widom's equations. In particular the three curves have a common tangent at the critical point. In spite that the concentrations of the mixtures studied are above the critical micelle concentration, the difference of the surface tension of the two coexisting phases is well above the experimental uncertainty, and increases with |Tc − T|. The liquid-liquid interfacial tension data has been described accurately with Ising critical exponents. However, the combination of the critical amplitudes obtained from the surface and interfacial tension, and from light scattering are not in agreement with the universal values calculated from theory. This leads to the conclusion that Antonow's rule does not hold for this micellar system. The analysis of the vapour-liquid surface tension in the one-phase region at T ≈ Tc leads to the conclusion that the system's behaviour corresponds to the weak field approach.