PHASE-BEHAVIOR AND METASTABILITY IN A NONIONIC SURFACTANT WATER-ELECTROLYTE SYSTEM

We use static and dynamic light scattering to investigate the concentration fluctuations occurring in the single to binary phase transition of the two analogous systems C12E5/H2O and C12E6/H2O/NaCl (10 wt %). The effect of surfactant concentration and temperature on the osmotic compressibility, static correlation length, and diffusion coefficient is investigated. Both systems fall into the same universality class as simple binary fluids. For concentrations differing from the critical concentration, the suppression of critical fluctuations results in finite correlation lengths and compressibilities. Measurements at the critical concentration reveal a true divergence of these quantities with correlation lengths exceeding the instrumental resolution (i.e. >5000 Å). Measurements of the diffusion coefficient vs momentum transfer follow the predictions of simple mode-mode coupling theory for concentrations less than or equal to the critical concentration but Kawasaki mode-mode fluctuations are suppressed significantly above the critical concentration. The formation of large nonmicellar entities in the two-phase region leads to an asymmetry in the width of the critical region about the critical point. © 1990 American Chemical Society.