Hydrophobicity of counterions as a driving force in the self-assembly process: Dodecyltrimethylammonium chloride and parabens

The micelle formation of dodecyltrimethylammonium chloride (DTAC) in 0.01 M methyl- and ethyl-4-hydroxybenzoate (MeP and EtP, respectively) sodium salt aqueous solutions has been investigated at several temperatures in the range from 278.15 to 328.15 K by isothermal titration calorimetry, viscosity, electrical conductivity measurements, and NMR spectroscopy. It was found out, that hydrophobic counterion not only lowers the solubility of ionic amphiphile and thus its critical micelle concentration in aqueous solution, but interacts favorably with the surfactant molecules and enables the formation of higher organized structures also. Densely packed elongated micelles of higher aggregation number were found in the EtP sodium salt solution at low temperature, with their highest fraction in the solution at DTAC to EtP concentration ratio of approximately 1:1. The most negative heat capacity change determined for DTAC in presence of EtP can be related to the most pronounced hydrophobic effect due to the additional hydrophobic surface included into the dehydration process upon micellization and closed micelle formation with water molecules mostly excluded from their hydrophobic core. The slightly less hydrophobic MeP did not show such a behavior, although the formation of dynamic mixed micelle structures cannot be excluded for this system also. The role of counterion hydrophobicity as the main driving force for diverse rearrangements in the ionic amphiphile aqueous solutions is discussed.