Ionic-liquid-based surfactants with unsaturated head group: synthesis and micellar properties of 1-(n-alkyl)-3-vinylimidazolium bromides

Structural versatility is an important reason for the interest in ionic liquids (ILs) and ionic-liquid-based surfactants, ILBSs. We report here on the synthesis, characterization, and micellar properties of a series of ILBSs that carry unsaturation in the head group, 1-C-n-3-vinlyimidazolium bromide, C(n)VnImBr, C-n = C-10, C-12, C-14, and C-16, respectively. We studied this series at 298.15 K using surface tension, ultraviolet-visible (UV-vis) spectroscopy, and steady state fluorescence of solubilized methyl orange, MO, and pyrene, respectively. We studied the electrical conductance of C(n)VnImBr at 298.15 to 313.15 K. From the results of surface tension and conductivity, we calculated the area per surfactant at solution/air interface; the critical micelle concentration (cmc); the degree of counter-ion binding; and the enthalpy, entropy, and free energy of micellization. These properties showed the expected dependence on the length of C-n, and indicated that micellization is an entropy-driven process. We used fluorescence data to calculate the cmc, microscopic polarity of the interfacial region, and the micelle aggregation number. The UV-vis spectra of MO were used to calculate the cmc and probe dye-ILBS interactions in the pre- and post-micellar regimes. The aggregation behavior of C(16)VnImBr was compared with its saturated counterpart 1-(n-hexadecyl)-3-ethylimidazolium bromide, with 1-C-n-3-methylimidazolium bromides, and with "conventional" cationic surfactants, alkyltrimethylammonium bromides. The vinyl group is less hydrophobic than the ethyl moiety.