Critical Behavior and Ensuing Phase Separations in Paraben-Solubilized Micellar Solutions of Ionic Surfactants

Aqueous solutions of cetyltrimethylammonium bromide (CTAB) show micellar growth in the presence of various organic and inorganic additives. The simultaneous presence of some of these additives exhibit synergistic effects on micellar growth in this system due to their diverse nature of influences on the critical packing parameter of CTAB micelles. In this manuscript, we report dynamic light scattering, small-angle neutron scattering and rheological studies on influences of the widely used preservative butyl paraben and NaCl on the growth and interaction of CTAB micelles. These additives show synergism in micellar growth and induce micellar attraction-driven phase separations of both upper consolute temperature and lower consolute temperature regimes in aqueous solutions of CTAB. Quite interestingly, such micellar attraction-driven phase separations are also induced in aqueous systems of other cationic and anionic surfactants in the simultaneous presence of a hydrophobic substance and NaCl. These systematic observations of micellar attraction-driven phase separation are the first of this kind in aqueous systems of ionic surfactants. The results also suggest that the use of parabens as preservatives in some ionic surfactantbased formulations can also help in tuning their rheological characteristics.