Solubilization-Site-Dependent Micellar Morphology: Effect of Organic Additives and Quaternary Ammonium Bromides

Here we report systematic viscosity measurements under Newtonian flow conditions for sodium dodecyl sulfate (SDS) micellar solutions in the presence of aliphatic hydrocarbons ( n -hexane, n -heptane, n -octane), toluene, and 1-alkanols (1-heptanol, 1-octanol) at 30 °C. The influence of 0.1 M quaternary ammonium bromides (R 4 NBr; R = H, CH 3, C 2 H 5, n -C 3 H 7, n -C 4 H 9, n -C 5 H 11 ) was seen on relative viscosity (η r ) vs additive concentration profiles. Lower members of R 4 NBr (R = H, CH 3, C 2 H 5 ) have marginal influence on viscosity profiles and hence on micellar morphology. This may be due to the less pronounced effect on association structures as well as on solubilization sites (and on solubilizate content at a particular site) of added organics. As the alkyl part of R 4 NBr was increased (R ≥ n -C 3 H 7 ), the effect, which depended upon the nature of the additive (i.e., its hydrophobic/polarity character), was significant. This is possibly due to salts affecting the partitioning content of organic additives at different micellar solubilization sites with concomitant changes in micellar morphology, as well as viscosity of the micellar solutions. The effect of tetra- n -butylammonium bromide (R = n -C 4 H 9 ) concentration was seen to substantiate the change in site of solubilization phenomenon of organic additives. These studies show that, with an appropriate R 4 NBr concentration present in combination with a surfactant, change of solubilization site even for nonpolar compounds (like hydrocarbons) is possible.