Revisiting the self-aggregation behavior of cetyltrimethylammonium bromide in aqueous sodium salt solution with varied anions

Abstract

The study of self-aggregation of cetyltrimethylammonium bromide (CTAB) and its associated physicochemical features in aqueous sodium salt medium with varied anions has been presented. Anions (F−, Cl−, Br−, PO43−, salicylate−, oxalate2− and citrate3−) of sodium salts interacted with CTA+ to produce two kinds of assemblies of different sizes with varied extents of bound counter anions. The enthalpies of micellization of CTAB with and without salt were different. Israelachivili's parameter of packing of monomers amphiphile in micelle, intrinsic viscosity and static light scattering (SLS) methods supported spherical CTAB micelle at CMC (critical micelle concentration) whereas SANS (small angle neutron scattering) supported prolate micelle in solution. The micelle aggregation number (N) determined by the static fluorescence quenching and SANS methods at [CTAB]>CMC where processed to get N at CMC. The literature reports of N by SLS, and time resolved fluorescence quenching methods have been presented and compared. The CMC and enthalpy of micellization obeyed the Hofmeister series of the salt anions in the reverse order. The CMC has been found to fairly correlate with hydrodynamic radius and polarizability of the added counter anions. An elaborate and planned evaluation of different micelle parameters of CTAB in aqueous and salt media, and their analysis and comparison was hardly done in the past.