Effect of counterions and headgroup hydrophobicity on properties of micelles formed by alkylpyridinium surfactants. 2. Microcalorimetry

We report the influence of counterions and headgroup hydrophobicity on the enthalpies and heat capacities of micelle formation by 1-alkyl-4-n-dodecylpyridinium surfactants in aqueous solution. For micelle-forming 1-alkyl-4-n-dodecylpyridinium iodide surfactants (alkyl = C-1-C-3), enthalpies of micelle formation show no trend with respect to 1-alkyl chain length. Changes in molecular architecture of the counterion, however, have a large influence on the thermodynamics of micelle formation. It is shown that next to solvation, London dispersion interactions, counterion (substituent)-water interactions, and the microenvironment of counterions adsorbed to micellar surfaces play an important role in determining the exothermicities of micelle formation. Isobaric heat capacities of micelle formation show that the formation of wormlike micelles for salicylate-containing cationic surfactants is due to a favorable microenvironment for salicylate counterions in the Stern layer of micelles.