Quaternary salts as solubilization site modifiers of organic compounds in anionic micellar solutions

The effects of the addition of different salts (quaternary bromides or NaBr) on the viscosity (measured under Newtonian flow conditions) of sodium dodecylbenzenesulfonate (SDBS) micellar solutions were studied at 30°C. Tetra-n-butylammonium bromide (Bu₄NBr) was found most effective in increasing the viscosity (due to possible change in micellar shape) at fairly low SDBS concentration (50 mM). The effectiveness of Bu₄NBr may be due to the presence of four butyl chains and to the positive charge on its counterion (Bu₄N⁺) which can subsequently interact with anionic SDBS micelles electrostatically as well as hydrophobically; owing to solubility/steric problems, other salts were ineffective. Organic additives such as cyclohexylamine and cyclohexanol had marginal effects on viscosity when added to 50 mM SDBS solutions having no Bu₄NBr. However, in the presence of Bu₄NBr, the effect was dependent on salt concentration and the nature of the additive. The overall effect is discussed on the basis of change in the solubilization site of the additive in the presence of Bu₄NBr.