On the Arrangement of Ions in Imidazolium-Based Room Temperature Ionic Liquids at the Gas−Liquid Interface, Using Sum Frequency Generation, Surface Potential, and Surface Tension Measurements

Abstract

The characterization of three ionic liquids [BMIM][BF 4 ], [BMIM][DCA], and [BMIM][MS] having a common cation and with anions of varying sizes and shapes was performed with three complementary surface techniques: sum frequency generation-polarization mapping, surface tension measurement, and surface potential measurement. Custom vacuum cells were designed for each technique to be able to perform measurements in a highly controlled environment minimizing the presence of water and other contaminants, which may compromise measured values. SFG results show evidence of having anions and cations present on the surface with the butyl chain of the cation positioned toward the gas phase and the imidazolium ring mostly parallel to the surface plane. Results from the surface potential measurements reveal the relative positions of the ions where the anions are located at a slightly lower plane compared to the cations. Observed values from the surface tension measurements denote surface intermolecular interactions indicative of both van der Waals and Coulombic interactions suggesting the presence of alkyl chains as well as ions on the surface. A model on the gas-liquid interface of ionic liquids is described based on the concurring results from these three surface characterization techniques, as well as current literature.

Surface Adsorption and Micelle Formation of Polyoxyethylene-type Nonionic Surfactants in Mixtures of Water and Hydrophilic Imidazolium-type Ionic Liquid

“…The cmc was also affected by the lengths of both the alkyl and polyoxyethylene chains, but a greater impact was observed for the alkyl chain length. These results indicate that micellization occurs as a result of solvophobic interaction between surfactant molecules in the water/bmimBF 4 25 . Previously, we studied the effects of water on interfacial properties of nonionic surfactants in a hydrophobic IL, 1-butyl-3-methylimidazolium hexafluorophosphate bmimPF 6 26 .…”

“…To the best of our knowledge, however, the effects of water on the interfacial properties of surfactants in bmimBF 4 have not been reported. In this paper, we report the solution properties of POE alkyl ether-type nonionic surfactants in bmimBF 4 in the presence of water, as a function of the water concentration in bmimBF 4 and the chain lengths of the surfactants.…”

“…However, there are only a few reports focusing on micelle formation of surfactants in ILs in the presence of water 24,25 . Wakeham et al have reported that micelle formation of nonionic surfactants in protic ILs depends strongly on solvent composiAbstract: The interfacial properties of polyoxyethylene alkyl ether-type nonionic surfactants (C n E m ) were studied in a hydrophilic room-temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF 4 ), in the presence of water. These properties were assessed using static surface tension, pyrene fluorescence, and dynamic light scattering measurements.…”

“…The other cac indicated the formation of micelles, where water is solubilized within the micelles. As a representative hydrophilic IL, the interfacial properties of surfactants in 1-butyl-3-methylimidazolium tetrafluoroborate bmimBF 4 have been previously reported 12,15,16 .…”

“…In this paper, we report the solution properties of POE alkyl ether-type nonionic surfactants in bmimBF 4 in the presence of water, as a function of the water concentration in bmimBF 4 and the chain lengths of the surfactants.…”