Specific Alkali Cation Effects in the Transition from Micelles to Vesicles through Salt Addition

Abstract

The transition of ionic micelles to vesicles with added salts is explored in this paper. The catanionic surfactant soln. was comprised of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) with an excess of SDS. The micellar size increased with concn. for all salts. No anion specificity was found, probably because of the excess of SDS. However, when the cation of the added salt was varied, large differences were obsd. in the hydrodynamic radii of the aggregates. A classification of the cations according to their ability to increase the measured hydrodynamic radii follows a Hofmeister series. The change in aggregate size can be explained by modified counterion binding and dehydration of the surfactant headgroups.