Effect of solubilization of aliphatic hydrocarbons on size and shape of rodlike C(16)TABr micelles studied by H-2 NMR relaxation

Multifield H-2 relaxation has been used to quantify the effect of solubilization of alkanes on the size and shape of the micelles in aqueous solutions of hexadecyltrimethylammonium bromide, C(16)TABr. Information about aggregate size and shape was deduced from the reorientation rate of the aggregates. The surfactant concentration was chosen so that the solutions without solubilizate contained moderately long rodlike micelles (axial ratio similar to 4). It was found that the change in aggregation behavior upon solubilization is substantially different for solubilizates of similar chemical composition but different structures. Solubilization of small amounts of cyclohexane, cyclooctane, cyclodecane, trans-decalin, n-hexane, 2,3-dimethylbutane, or adamantane causes growth of the micelles. Further addition of these solubilizates (except for adamantane, which has a lower solubility limit than the other solubilizates) causes a decrease in the aggregate size. On the other hand, solubilization of n-octane, n-decane, or n-dodecane causes a decrease in aggregate size even in small amounts. It was also found that when the rodlike micelles grow due to the presence of solubilizate molecules, they grow in the axial direction at a constant radius, while the decrease in size at higher solubilizate content occurs as a shortening of the rods accompanied by an increase in radius. The differences in effects are explained by different solubilization sites within the micelle for alkanes with different molecular volume and rigidity.