Ionic liquids have in recent times found many applications, such as battery electrolytes, as green solvents and as cataslysts. Basically, ionic liquids are organic salts that exist in liquid form at near ambient temperatures. Interestingly ionic liquids possess negligible vapor pressure and have the ability to tailor both the cation and anion, which allows them to replace conventional organic solvents and be designed for specific applications. Unfortunately, the tunable physical and chemical properties of ionic liquids, such as viscosity, density, conductivity, and solvation ability, can be adversely affected by water contamination. Research has shown that the presence of water can be beneficial or detrimental depending on its ability to promote action of the respective ionic liquid.
“There are several studies investigating the interactions between water and ionic liquids, however most of these experiments involve deliberate water addition to the IL system. Realistically, the most common method of undesired water sorption occurs due to exposure to atmospheric conditions,” explains M. Alejandra Rocha.
Recently, Professor Mark B. Shiflett and M. Alejandra Rocha (recently graduated with an MSc) from the Department of Chemical and Petroleum Engineering at the University of Kansas investigated the in situ atmospheric water vapor absorption and desorption in three imidazolium-based ionic liquids, over a given range of temperatures and relative humidity conditions. The ionic liquids used were, (1-ethyl-3-methylimidazolium tetrafluoroborate [C2C1im]- [BF4], 1-butyl-3-methylimidazolium acetate [C4C1im][OAc], and 1-butyl-3-methylimidazolium chloride [C4C1im][Cl]). Their work is currently published in the research journal, Industrial & Engineering Chemistry Research 2019 , 58, 1743-1753.
Isothermal measurements were performed on the imidazolium-based ionic liquids using a Hiden IGASorp gravimetric microbalance, which measured total weight as a function of time. The ionic liquid [C2C1im][BF4] was selected in order to validate the gravimetric technique used, and [C4C1im][OAc] and [C4C1im][Cl] were chosen as these are known hydrophilic ionic liquids. The solubility data was correlated using the nonrandom two-liquid solution model, and the time dependent mass sorption data was analyzed to calculate diffusion coefficients and enthalpies of absorption.
The authors observed that the solubility of water in [C2C1im][BF4] agreed with published data and provided confidence that, the proposed method was reliable for measuring water sorption in ionic liquids. The researcher also noted that the solubility of water was the highest in [C4C1im][OAc] followed by [C4C1im][Cl] and lastly [C2C1im][BF4] at equivalent conditions.
In summary, the Rocha-Shiflett study demonstrated that the 1D diffusion model could provide satisfactory predictions and could therefore be used to determine the water−ionic liquid binary coefficients. A comparison of the diffusion coefficients for the three water-ionic liquid systems revealed the expected increase in diffusion with lower viscosity as temperature increased. Remarkably, however, it was observed that as water concentration increased, the diffusion of water in the [C2C1im][BF4]−water mixture decreased, while the diffusion of water increased in the [C4C1im][OAc]−water and [C4C1im][Cl]−water systems. The authors determined this was because the water−water hydrogen bonding energy began to exceed the [C2C1im][BF4]−water interactions and restricts water diffusion, while the water-water interactions weakened [OAc] and [Cl] interactions with water, thus increasing water mobility. Altogether, diffusing radius calculations using the Stokes−Einstein relationship support the hypothesis that a few water molecules through hydrogen bonding form clusters with the [OAc] and [Cl] anions, but much larger water/BF4− clusters/networks are occurring in the [C2C1im][BF4] system which increase in size with increase in water concentration.
Researchers interested in modeling the results using molecular simulations are encouraged to contact Professor Shiflett at [email protected]. For a list of other publications related to ionic liquids research please visit www.shiflettresearch.com.
M. Alejandra Rocha, Mark B. Shiflett. Water Sorption and Diffusivity in [C2C1im] [BF4], [C4C1im] [OAc], and [C4C1im] [Cl]. Industrial & Engineering Chemistry Research 2019, volume 58, page 1743−1753.Go To Industrial & Engineering Chemistry Research