Critical-Type Behavior and Micellar Growth Observed in an Ionic Surfactant/Salt System

The cationic surfactant dodecyl ammonium chloride (DAC) undergoes a one- to two-fluid phase transition with decreasing temperature T at constant [NaCl]. In the case of [DAC]= 0.025M, there is phase separation in the region T ≳ 16°C provided that [NaCl] ≳ 0.23M. Below [NaCl] ∼ 0.23M the apparent micellar hydrodynamic radius Rh aPP, determined by dynamic light scattering from the diffusion coefficient D, increases with decreasing T. This behavior is typical of a variety of ionic micellar systems in the presence of added salt and has been ascribed to micellar growth (e.g. SDS + NaCl). However, with DAC + NaCl for [NaCl] > 0.23M, there may be an additional mechanism which causes Rh aPP to increase greatly with decreasing T -- i.e., critical-type fluctuations at the approach to a phase boundary. This behavior is qualitatively similar to that observed for nonionic micelles and micro- emulsions. In these systems the decrease in D (i.e. increase in Rh app) upon the approach to a cloud point or phase boundary is believed to be due to long-range correlated diffusion of micelles/droplets of basically unchanging size. Hence, the diffusivity results for DAC + NaCl at [NaCl] > 0.23M may be due to either micellar growth or critical-type behavior, or a combination of the two.