Effect of amino acid addition on the micelle formation of the surface-active ionic liquid 1-tetradecyl-3-methylimidazolium bromide in aqueous solution

The effect of 3 amino acids on the self-assembly properties of the surface-active ionic liquid: 1-tetradecyl-3-methyl imidazolium bromide, [C(14)mim][Br], was studied in neutral and basic aqueous solution. The critical micelle concentrations were determined by different techniques, and the thermodynamic parameters of micellization (Delta G(mic)(0), Delta H-mic(0), and Delta S-mic(0)) were measured by isothermal titration calorimetry. The critical micelle concentration values decrease in the presence of all the amino acids used in this study. The most substantial effect was observed in the presence of L-tryptophan at pH 12, due to the electrostatic and pi-pi interactions with the imidazolium headgroup of the surfactant. Always entropy-driven micellization occurred. Both the enthalpy and entropy changes upon association to micelles increased when the pH was enhanced from 7 to 12. H-1 NMR measurements demonstrated that among the used amino acids, L tryptophan interacts most strongly with the headgroup of the surfactant cation. The knowledge on the surfactant-amino acid interactions significantly contributes to the deeper understanding of the effect of surfactants on the properties of peptides.