In this study, we have examined both the effect of alkyl chain length and anion composition on the 1-alkyl-3-methylimidazolium (C(n)mim, n = 4, 6, 8, 10, and 12) structure and orientation at the room-temperature ionic liquid (RTIL)/SiO(2) interface by sum-frequency vibrational spectroscopy (SFVS). Four different anions were investigated in this study: tetrafluoroborate (BF(4)), hexafluorophosphate (PF(6)), bis(trifluoromethylsulfonyl)imide (BMSI), and bis(pentafluoroethylsulfonyl)imide (BETI). It was found that the alkyl chain in BMSI and BETI RTILs showed a decrease in gauche defects with an increase in chain length, whereas the alkyl chains of the BF(4) and PF(6) RTILs have virtually no gauche defects regardless of chain length. The tilt of the alkyl chain lies predominantly perpendicular to the surface for all the RTILs examined. A strong correlation between the HCCH vs tilt angle and alkyl chain length was observed; as the alkyl chain is lengthened the HCCH vs lies more perpendicular to the SiO(2) surface. The results of this study suggest that the length of the alkyl chain dictates to a large degree the orientation of the imidazolium cation at the surface, regardless of anion composition. To a lesser extent, the HCCH vs tilt of the imidazolium ring of the cation also appears to be correlated to the surface charge density of the SiO(2). As the SiO(2) surface charge density becomes more negative the HCCH vs tilt angle lies more parallel to the surface.

This study presents the electrochemical properties of room-temperature ionic liquids ͑RTILs͒ at a variety of liquid-liquid interfaces. The RTILs used here consist of 1-alkyl-3-methylimidazolium ͑C n mim, n = 6, 8, 10, 12͒ cations with bis͑perfluoromethyl-sulfonyl͒imide ͑BMSI͒ and bis͑perfluoroethylsulfonyl͒imide ͑BETI͒ anions. Ion transfer across the polarizable 1,2-dichloroethane/ water and the neat RTIL/water interfaces were examined. The ion-transfer experiments suggested that the RTILs made with C n mim and BMSI or BETI are moderately hydrophobic with polarizable potential windows as large as 274 mV for C 12 mimBETI in 1,2-dichloroethane and 137 mV for the neat C 12 mimBETI/water interface. In addition, the effect of the junction potential at the aqueous Ag/AgCl reference electrode/RTIL interface was studied using the oxidation/reduction of ferrocene and decamethylferrocene. The junction potential at the reference electrode was found to decrease as a function of the cation chain length for both anions studied. The ⌬ o w G tr of transfer found for all the systems studied was ϳ3 kJ/mol per CH 2 group of the cation.

The surface and aqueous interfacial tensions for a series of water-immiscible room-temperature ionic liquids (RTILs) have been measured. The RTILs used in this study were based on 1-alkyl-3-methylimidazolium cations (Cnmim, n=6, 8, 10, and 12) and bis(perfluoromethylsulfonyl)imide (BMSI) and bis(perfluoroethylsulfonyl)imide (BETI) anions. It was found that the surface tensions of the RTILs increased with an increasing cation chain length similar to the behavior of n-alkanes. Interfacial tensions of the RTILs with aqueous solutions, however, were found to decrease with the cation chain length, which has been attributed to the increased surface activity of the longer chain cations. We have also demonstrated the first use of electrocapillary measurements to study the polarizable RTIL/aqueous interfaces. From the electrocapillary data, the potential of zero charge (PZC) for these RTIL/aqueous interfaces was determined, as well as the relative surface excess charge and capacitance. The PZC was found to be dependent upon the structure of the anions and cations with PZC values ranging from -357 mV for C6mimBETI and -161 mV for C10mimBMSI. The electrocapillary results also show that the cations of the RTIL are becoming increasingly surface-active as the alkyl chain on the cation is lengthened, thereby modulating the interfacial potential.

In this study 1-dodecyl-3-methylimidazolium (C 12 mim) bis(pentafluoroethylsulfonyl)imide (BETI) and 1-dodecylimidazolium (C 12 im) BETI hydrophobic room-temperature ionic liquids (RTILs) were synthesized and used as proton-conducting electrolytes in a nonhumidified feed gas electrochemical cell. The ionic conductivities of C 12 mimBETI and C 12 imBETI were similar and increased linearly with an increase in temperature from 20 to 130°C. However, when used in the electrochemical system the protic water-equilibrated C 12 imBETI had a larger maximum current and power density compared to the aprotic water-equilibrated C 12 mimBETI. The effect of water content on the reaction rates and thermodynamics of these hydrophobic RTILs was also examined. The efficiency of the C 12 mimBETI increased upon removal of water while that of the C 12 imBETI decreased in efficiency when water was removed. The water structure in these RTILs was examined using attenuated total internal reflection Fourier transform IR spectroscopy and depended on the chemical structure of the cation. These studies give further insight into the possible mechanism of proton transport in these RTIL systems.An increase in global energy demand coupled with the escalating costs of petroleum-based energy has prompted a growing interest in developing alternatives to fossil fuels. Proton exchange membrane hydrogen fuel cells (PEM-HFCs) are among the most attractive sources of alternative power due to their high output power density. Current drawbacks preventing the wide scale adoption of PEM-HFCs include the high cost of the Nafion membrane used in these devices and the low operating temperatures ( < 80°C). A low operating temperature is necessary to keep the Nafion membrane hydrated. At higher temperatures the water in the Nafion membrane evaporates and the membrane loses its ability to conduct protons. In addition, the low operating temperature requires ultrapure hydrogen gas to be supplied to the anode in order to prevent poisoning of the platinum catalyst. These limitations add to the overall cost of Nafion-based PEM-HFCs. To increase the operating temperature of a PEM-HFC, Nafion membranes containing imidazolium salts, 1,2 silica, 3 poly(tetrafluoroethylene), 4 zirconium phosphate, 5 and sulfated zirconia 5 have been explored in addition to alternative polymer membranes. 1,2,[6][7][8][9][10][11][12] The membranes that incorporate imidazole-or imidazoliumbased room-temperature ionic liquids (RTILs) are an attractive subset of these membranes, as they are conductive at temperatures of 100°C or higher using dry gases. 1,2,8,10,11 At these high temperatures the carbon monoxide poisoning of the platinum catalyst is reduced due to the thermal instability of the Pt-CO surface products, 13 allowing lower grade hydrogen fuel sources to be used. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptCost could further be reduced if a supplementary polymer membrane, or a liquid membrane cell, could be developed eliminating the need for the N...