Gas–Liquid Interface of Mixture of Ionic liquids and Benzonitrile Studied by Sum Frequency Generation Spectroscopy and Surface Tension



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Ionic liquid is one of the potential solvents which has wide applications in several fields such as electrochemistry, catalysis, gas separation, liquid extraction, etc. Mixing regular solvents to ionic liquid is important to expand the scope of ionic liquid. But the addition of molecular solvents at different concentrations can change the interfacial property of the mixture. Understanding the interfacial properties of such mixtures is significant in different aspects of their application. This dissertation investigates the gas-liquid interface of the mixture of [BMIM][PF6], [BMIM][DCA], and [BMIM][Tf2N] with benzonitrile using studied by sum frequency generation spectroscopy and surface tension measurements. The influence of benzonitrile on the surface structure of ionic liquid is investigated as a function of 0 to 1 mole fraction of benzonitrile. The SFG signal intensity from CH3 symmetric stretching and Fermi resonance modes of the terminal methyl group of the butyl chain of the cation of all three ionic liquids was noted to be decreased with the addition of benzonitrile while the peak intensity of CH stretching mode of vibration of benzonitrile constantly increased in case of [BMIM][PF6]. However, it follows a different trend in the case of [BMIM][DCA] and [BMIM][Tf2N] where the CH peak from benzonitrile barely increases on increasing the concentration of it even though the miscibility is quite similar in all three cases. The addition of benzonitrile does not result in any extra peaks or shifting of the peak frequency to the spectra of pure [BMIM][PF6], [BMIM][DCA], and [BMIM][Tf2N]. The presence of benzonitrile at the gas-liquid interface is further supported by the surface tension measurements. Orientational tilt angles of the terminal methyl group of the cation of ionic liquids are calculated from simulation and found to be lowered with the addition of benzonitrile. The effect of temperature on the surface structure of the binary mixture of [BMIM][PF6] and benzonitrile is also reported for both SFG and surface tension study. The temperature dependence of the interfacial tension is used to evaluate thermodynamic functions such as surface entropy and surface enthalpy, both found to be lowered with the increasing concentration of benzonitrile in the mixture of [BMIM][PF6] and benzonitrile.



Gas-Liquid Interface, Ionic Liquid, Benzonitrile, Sum Frequency Generation Spectroscopy, Surface Tension