Characterization of interaction between butylbenzene sulfonates and cetyl pyridinium chloride in a mixed aggregate system

Surface active short chain alkyl benzene sulfonates, which are known as hydrotropes, interact electrostatically as well as hydrophobically with cationic surfactants. The mixtures of butylbenzene sulfonate (BBS-) with cetyl pyridinium chloride (CPyCl), as shown by surface tensiometry, rheological techniques, and IR and NMR spectroscopy form rodlike micelles or vesicles. These microstructures give rise to highly viscous solutions even at very low concentrations in the millimolar region. The CPy+/BBS- complexes also show similar behavior in the absence of electrolytes. The steric hindrance due to the butyl group to the interaction with CPyCl is at a maximum with tert-butyl benzene sulfonate (TBBS-) as compared to n-butyl benzene sulfonate (NBBS-) and iso-butyl benzene sulfonate (IBBS-). The molecular interactions between surfactant/hydrotrope pairs are simulated using the MM2 force field in the presence and absence of water. The presence of water reduces the electrostatic interaction by solvation and reinforces the hydrophobic interactions. The stability of the surfactant/hydrotrope complexes was examined in the presence and absence of water. The CPy+/BBS- interactions are significantly stronger as compared with the cetyltrimethylammonium-butyl benzene sulfonate (CTA(+)/BBS-) systems.