PHASE-SEPARATION OF MICELLAR SOLUTIONS AT EXTREME DILUTION - A STUDY OF POLYSTYRENE-B-POLY(SODIUM ACRYLATE) MICELLES IN AQUEOUS NACL SOLUTIONS

Micellar solutions of polystyrene-b-poly(sodium acrylate) copolymers i n aqueous NaCl were studied by static light scattering (SLS). It was f ound that micellar solutions of the copolymer, at concentrations of Na Cl at, or above, 2.0 mol dm(-3), became turbid on dilution at constant salt concentration and at constant temperature. Turbidity arose from highly dilute solutions (typically at a concentration three orders of magnitude lower than the overlap concentration of the micelle, C), bu t at concentrations above the expected critical micellization concentr ations (c.m.c.s). The observed turbidity was attributed to the phase s eparation of the micellar phase. A systematic investigation of the pha se separation phenomenon was performed. The effects of various paramet ers on the solution behavior of the micellar solutions were studied, i ncluding the effect of the concentration of NaCl, the effect of temper ature, and the effect of the length of the hydrophilic, corona-forming poly(sodium acrylate) block. Phase separation was attributed to the p resence of a very large excess of NaCl in the dilute micellar solution s. It was proposed that phase separation arose because of the reduced hydration of the polyion, the decreased electrostatic repulsion betwee n the micelles, and the increase in the amount of ion binding, which o ccur in highly dilute salt solutions. (C) 1996 John Wiley & Sons, Inc.