Generating Nonnative Structures in Binary Ionic Liquid Mixtures for Tunable Phase Equilibria Properties

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Ionic liquids are substances that are composed entirely of ions and can be designed to exist as liquids by combining a wide range of cations and anions. They have emerged as potentially environmentally benign chemicals due to their essentially nonvolatile nature and unique physicochemical properties. By judiciously selecting the cation, anion or functional groups on the ions, desired physical, chemical and biological properties can be imparted to ionic liquids making them truly designer solvents. Although such a combinatorial approach can potentially be exploited to generate as many as one million ionic liquids, a continuing challenge in the field of ionic liquids is that many ionic liquids display unfavorable transport properties limiting their use in industrial processes. Further, ionic liquids with transport properties in the range of interest for industrial applications, such as gas separations, usually contain fluorinated ions raising their production cost and environmental concern due to toxicity and poor biodegradability. Binary mixtures of ionic liquids with different compositions can potentially provide precise fine-tuning of the molecular interactions to generate desired properties. According to one estimate, there are one billion binary ionic liquid combinations; thus, it is highly desirable to predict a priori the properties of binary ionic liquid mixtures. However, at present, there is a lack of fundamental knowledge regarding the design rules that can be applied to arrive at such a conclusion. The proposed research is geared toward fulfilling this critical knowledge gap.The proposal is guided by the hypothesis that nonideality in the binary ionic liquid mixtures in terms of non-native molecular interactions occurs when two ionic liquids differing widely in their hydrogen bonding ability are combined; the nonideal behavior is reflected in the nonlinear dependence of solubility of gases on the composition of the binary mixture. The validity of the hypothesis will be tested by (i) determining the extent to which novel interactions are established for binary mixtures composed of ionic liquids differing widely in their hydrogen bonding ability and (ii) evaluating the effect of such interactions on the phase equilibria properties of binary ionic liquids mixtures with gases, such as carbon dioxide and methane. Molecular dynamics simulations will be carried out to elucidate molecular level interactions present in the mixtures composed of two ionic liquids while free-energy calculations and Monte Carlo methods will be employed for calculation of phase equilibria of substances in these mixtures. The proposed integration of research and education aims at creating an intellectually stimulating environment for training graduate and undergraduate students on understanding the role of molecular phenomena in designing efficient chemical systems. The proposed outreach plan is targeting students from diverse backgrounds and underrepresented populations in STEM fields.