Role of the surfactant headgroup on the counterion specificity in the micelle-to-vesicle transition through salt addition

Abstract

A transition from micelles to vesicles is reported when salts are added to a catanionic micellar soln. composed of Na dodecylcarboxylate (SL) and dodecyltrimethylammonium bromide (DTAB), with an excess of SL. The counterion binding and increase in aggregate size was monitored by mass spectrometry, rheol. and dynamic light scattering measurements, whereas the vesicles were characterized by freeze-fracture and cryo-transmission microscopy expts. The effect of counterions on the formation of vesicles was studied and compared to a previously studied catanionic system with a sulfate head group, SDS/DTAB. As in the latter case, no anion specificity was found, while large differences in the hydrodynamic radii of the formed objects were obsd., when the cation of the added salt was varied. A classification of the cations could be made according to their ability to increase the measured hydrodynamic radii. If the sulfate headgroup of the anionic surfactant is replaced by a carboxylic group, the order of the ions is reversed, i.e. it follows the reversed Hofmeister series. Different morphologies are obsd. as the ionic strength of the system is increased. The aggregates are analogous to those found in the SDS/DTAB system.