Rheological characterization of mixtures of cetyl trimethylammonium bromide and sodium butyl benzene sulfonate in aqueous solutions

Abstract

The rheological behavior of mixed aqueous solutions of cetyl trimethylammonium bromide (CTAB) and anionic hydrotropes, such as sodium salts of n-butyl benzene sulfonate (Na-NBBS), iso-butyl benzene sulfonate (Na-IBBS) and tert-butyl benzene sulfonate (Na-TBBS), has been studied over a range of the hydrotrope concentration at different temperatures. The zero shear viscosity measurements for the mixed systems show a bimodal curve with a first viscosity maximum at a Na-BBS/CTAB ratio of approximately 0.3 and other ones at about 2.5. The three isomers of butyl benzene sulfonate (BBS) differ in their effects on the viscosity of the solutions. The difference in the viscosities of the mixed CTAB–hydrotrope solutions is more prominent at higher values of Na-BBS/CTAB ratio. Since the viscosity depends on the microstructure generated by the CTAB-BBS mixtures in solutions, the size and shape of the mixed micelles varies with the structure of the butyl group. The linear NBBS induces higher changes even at lower concentrations. The CTAB-hydrotrope solutions in the region of the first viscosity maxima exhibit Maxwellian fluid behavior where the mixed system is highly viscoelastic and the micelles are entangled. In the region of second viscosity maxima, however, the mixed solutions are non-viscoelastic with much lower values of storage and loss modulii. The living polymer formation by linear growth of CTAB micelles, on addition of hydrotropes, to very large aggregation number is evidenced from intrinsic viscosity and rheological properties of solutions.