Interaction of Water Vapor with the Surfaces of Imidazolium-Based Ionic Liquid Nanoparticles and Thin Films

Abstract

Understanding the interactions of humid air with ionic liquids (ILs) is critical for predicting how their physicochemical properties are affected by water. Using experimental and theoretical techniques, water vapor's interaction with aerosolized nanoparticles and thin films of [C 2 MIM][Cl] and [C 2 MIM][BF 4 ] was studied. Solutions were electrosprayed to produce dry particles. Particles' hygroscopic growth was quantified using tandem nanodifferential mobility analysis as a function of relative humidity (RH). This is the first report of the interaction of water with aerosolized IL nanoparticles. The particles' small size allows true IL−water vapor equilibrium achieved quickly. Growth curves for both ILs show steady water uptake with increasing RH. Water vapor uptake by IL thin films was also examined using ATR-FTIR spectroscopy. . They represent a large class of molecules that have unique and widely tunable physical and chemical properties. 1−3 For example, there is a significant interest in the use of ILs in green chemistry, 2,4,5 due to their negligible vapor pressure at room temperature, 6 and for many other applications including solvents in synthesis and catalysis, 1,6 chemical extraction, 1,2,5 and fuel cells. 7 Physical properties such as conductivity, viscosity, density, and surface tension of ILs are known to depend on the presence of water, both adsorbed and absorbed. 3,8 Water is ubiquitous in the environment, and hygroscopic ILs such as those with imidazolium-based cations can uptake significant amounts of water vapor from ambient air, which alters their physical and chemical properties. 1,3 Thus, quantification of water uptake by hygroscopic ILs and understanding how the presence of water affects the nature of the ion−ion and ion−water interactions are critical for use of ILs in future applications. 8−14 Numerous experimental and theoretical studies have investigated the interaction of imidazolium-based ILs with water. 3,8−35 It is understood that both the cation and the anion have an effect on how ILs interact with water, with the anion having a stronger effect. 3,17,19 Early experimental work by Seddon et al. investigated the miscibility of water with ILs containing 1-nalkyl-3-methylimidazolium ([C n MIM + ], n = number of carbons on the linear alkyl chain) cations and various anions at room temperature. 3 It was determined that, regardless of the alkyl chain length, all halide anions were fully soluble in water and all hexafluorophosphate ([PF 6 − ]) anions were insoluble. 3 However, alkyl chain length did have an effect on water miscibility of [C n MIM + ]-based ILs with tetrafluoroborate ([BF 4 − ]) anions; for chain lengths of n = 2 or 4, the ILs were fully miscible, but for n > 4, the ILs formed biphasic systems. 3 Cammarata et al. performed early attenuated total reflection (ATR) infrared (IR) studies on the molecular states of water

An ionic liquid-based actuator as a humidity sensor

“…The most significant change in impedance is the shift of real part towards higher values when the iEAP laminate is held in drier environment. This is in a good agreement with the previously reported physicochemical properties of IL-water mixtures: the addition of water increases significantly the conductivity of IL at relatively low water content [27][28][29][30].…”

“…However, there is a discrepancy: the physicochemical properties of ILs are typically determined in a dry environment, while the IL exposed to ambient air instantly absorbs water. Recently, the water-IL mixtures have attracted much interest [27][28][29][30]. As a rule, the water content in IL decreases its viscosity and therefore increases conductivity.…”

Thermophysical properties of aqueous solutions of tetraalkylphosphonium based ionic liquids at different temperatures and atmospheric pressure

“…The purity and other information of chemicals used in this work are given in table 1. SO 4 ] in water was found to be rather low and dissolution is a slow process in contrast to behavior expected from its hydrophilic nature [10].…”

“…The nature of ILs helps in designing ILs in terms of their physicochemical properties and hence it is possible to have properties suitable for particular application. Many ILs have been proved useful for their potential use in many ways [1]. The literature available on the physicochemical properties of ILs is found to be mainly focused on ammonium, imidazolium, pyridinium and pyrrolidinium-based ILs [1][2][3][4].…”

“…Many ILs have been proved useful for their potential use in many ways [1]. The literature available on the physicochemical properties of ILs is found to be mainly focused on ammonium, imidazolium, pyridinium and pyrrolidinium-based ILs [1][2][3][4]. On the other hand there is scarcity of the physicochemical properties of phosphonium based ILs either in pure form or in form of their solutions [3,[5][6][7].…”