Water Sorption and Diffusivity in Ionic Liquids

Significance 

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.

Water Sorption and Diffusivity in Ionic Liquids - Advances in Engineering
Diagram of the IGAsorp gravimetric microbalance. Details can be found in Industrial & Engineering Chemistry Research 2019, 58, 1743-1753.

About the author

M. Alejandra Rocha is a full-time Process Engineer working in the Oil & Gas group at the engineering consulting firm Black & Veatch. She rejoined Black & Veatch in January 2019 after receiving her MSc degree with Honors in Chemical Engineering from the University of Kansas (KU) in 2018. During her time as a graduate student at KU, Alejandra’s research focused on the study of water sorption in ionic liquids utilizing microgravimetric balances.

 

About the author

Mark B. Shiflett is a Distinguished Foundation Professor in the Department of Chemical and Petroleum Engineering at the University of Kansas (KU). Professor Shiflett joined KU as a Foundation Professor in August 2016 after retiring from the DuPont Company. Professor Shiflett worked for DuPont for 28 years and was a Technical Fellow in the Central Research and Development organization which is located at the Experimental Station in Wilmington, Delaware. Professor Shiflett was also an adjunct professor at the University of Delaware in the Department of Chemical and Biomolecular Engineering. Professor Shiflett received his Ph.D. and M.S. degrees in chemical engineering from the University of Delaware in 2001 and 1998. He received his B.S. degree in chemical engineering from N.C. State University in 1989. Professor Shiflett is an inventor on 44 U.S. patents and has published over 80 articles on his research in both academia and DuPont. He was awarded the DuPont Bolton Carothers award in 2005, the ACS Hero of Chemistry award in 2006 and the University of Delaware presidential citation in 2007 for his development of hydrofluorocarbon refrigerant mixtures to replace chlorofluorocarbons which were linked to the depletion of the Earth’s ozone layer.

Professor Shiflett was elected in 2014 to be a Fellow in the American Institute of Chemical Engineers, in 2016 to be a Division Fellow in the American Chemical Society, and in 2018 to be a Fellow in the National Academy of Inventors for his significant professional accomplishments and contributions to the chemical engineering profession. Professor Shiflett received the American Institute of Chemical Engineers Institute award for Industrial Research in 2016 for the development of non-ozone-depleting refrigerants which have led to the healing of the Earth’s ozone layer, new applications using ionic liquids, an environmentally friendly TiO2 process and mentoring and educating chemical engineers.

Professor Shiflett is a licensed professional engineer in the State of Delaware and his research at KU focuses on developing environmentally friendly, energy efficient processes and products for the chemical industry.

Mark B. Shiflett, Ph.D., P.E.

Foundation Distinguished Professor
The University of Kansas
Chemical and Petroleum Engineering
Center for Environmentally Beneficial Catalysis
Life Sciences Research Laboratory
Building A, Office 110-F, 1501 Wakarusa Drive,  Lawrence, Kansas 66047
785-864-6719 office, 302-494-7505 mobile, [email protected] 
www.cebc.ku.edu

Reference

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 

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