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Surface-active short-chain alkyl benzene sulfonates which are known as hydrotropes interact electrostatically as well as hydrophobically with cationic surfactants in the same manner as salicylate. The interaction in mixed aggregates of the three isomers of butyl benzene sulfonate (BBS-) and cetyl trimethylammonium bromide (CTAB) has been studied by surface tensiometry and rheological techniques. These mixed aggregates form rodlike micelles, giving rise to highly viscous solutions even at low concentrations. To study the interactions in the absence of electrolytes, CTA/BBS complexes were also obtained separately and characterized by similar techniques. The steric effect due to the structure of the butyl group in butyl benzene sulfonates on the interaction with CTAB has been emphasized during these investigations. The interactions are also characterized by infrared and ultraviolet absorption and nuclear magnetic resonance spectroscopy. Molecular modeling was carried out using an MM2 force field for molecular mechanics calculations for surfactant/hydrotrope pairs in the presence and absence of water. The PM3 semiempirical molecular quantum mechanical calculations were also carried out to calculate heats of formation of complexes. The steric hindrance due to the bulky tert-butyl group was found to be the maximum in tert-butyl benzene sulfonate (TBBS-), in comparison to n-butyl benzene sulfonate (NBBS-) and isobutyl benzene sulfonate (IBBS-).

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