Cation isomerism effect on micellization of pyridinium based surface-active ionic liquids

Abstract

In this study, four pyridinium-based surface-active ionic liquids (SAILs) with small changes in the cation structure were synthesized, namely N-dodecylpyridinium bromide, N-dodecyl-2-methylpyridinium bromide, N-dodecyl-3-methylpyridinium bromide, and N-dodecyl-4-methylpyridinium bromide. Their micellization behaviour in aqueous solutions was investigated by isothermal titration calorimetry (ITC) in the temperature range between 278.15 K and 318.15 K. The experimental ITC data were analysed by applying an improved mass-action model, which provides the values for the critical micelle concentration (cmc), the degree of counterion binding (β), the average aggregation number (n), and the standard heat capacity (ΔMcpθ), enthalpy (ΔMHθ), entropy (ΔMSθ) and Gibbs free energy (ΔMGθ). Only minor differences were observed for the values of ΔMHθ, ΔMSθ and ΔMGθ, while some significant differences were found for the values of the average aggregation number (n) and ΔMcpθ.From MD simulations it was found that the presence of the methyl group on ortho-, meta- or para- position significantly affects the water organization around the pyridinium ring. Moreover, the hydration of the para-isomer is most pronounced and the hydration shell on this cation may also prevent the monomers from approaching each other. On the other hand, it turned out that the ortho-substituted pyridinium based SAIL has the most negative value of ΔMcpθ, which can be ascribed to the partial dehydration of the methyl group on the ortho- position during micelle formation. In contrast, for other systems, the alkyl chain was found to be mainly dehydrated while the pyridinium ring remains completely in contact with water even upon micellization.