What Determines the Miscibility of Ionic Liquids with Water? Identification of the Underlying Factors to Enable a Straightforward Prediction Marco Kla ¨hn,* Claudia Stu ¨ ber, Abirami Seduraman, and Ping Wu Institute of High Performance Computing, 1 Fusionopolis Way, #16-16, Connexis, Singapore 138632, Republic of Singapore ReceiVed: January 4, 2010; ReVised Manuscript ReceiVed: January 22, 2010 Whether an ionic liquid (IL) is water-miscible or immiscible depends on the particular ions that constitute it. We propose an explanation, based on molecular simulations, how ions determine the miscibility of ILs and suggest a straightforward and computationally inexpensive method to predict the miscibility of arbitrary new ILs. The influence of ions on the solvation of water is analyzed by comparing molecular dynamics simulations of water in 9 different ILs with varying cation and anion constituents. The solvation of water in ILs is found to depend primarily on the electrostatic water-ion interaction strength, which, in turn, is determined mainly by two factors: primarily, by the size of the ions and secondarily by the amount of charge on the ion surface that is coordinated with water. It is demonstrated that large ions lead to weaker interactions with water, due to the involved delocalization of the ion charge. A large charge on the ion surface, which is determined by the chemical structure of the ion, strengthens water-ion interactions. We observe that whenever the interaction strength of water with ions exceeds a certain threshold, an IL becomes water-miscible. On the basis of these findings, a simple equation is derived that estimates the water-ion interaction strength. With this equation it is possible to predict most of the observed water-miscibilities of a sample of 83 ILs correctly. A linear increase of the water saturation concentration with the estimated water-ion interaction strength is observed in water- immiscible ILs, which can be utilized to predict the water concentration in new ILs. 1. Introduction Ionic liquids (ILs) are molten salts with melting points below 100 °C, and in many cases below room temperature. Typical cations are based on an aromatic heterocyclic compound in the center, e.g., imidazole or guanidinium, to which alkyl chains of varying length are attached. The large size of the cations, the resulting delocalization of their positive charge, and their asymmetric structure impede crystallization, which decreases the melting point. Anions are usually compact and inorganic, and are often fluorides or contain fluoroalkyls. ILs exhibit high viscosity, hampered self-diffusion, and negligible volatility due to strong interionic interactions. ILs are also characterized by a high ion conductivity and thermal and chemical stability. These characteristics, together with the possibility to adjust many properties of ILs by an independent variation of cations and anions, have brought massive attention to ILs. As a result, a broad variety of potential applications have been proposed. 1–4 The study of the behavior of ILs in the presence of water is of fundamental interest, since contact with ubiquitous water can be hardly avoided. It has been observed that some ILs are water- miscible, while other ILs are water-immiscible, even though their chemical structures might differ only slightly. Even in the case of immiscibility, ILs are generally hygroscopic and capable of solvating substantial amounts of water from air humidity or other accessible sources of water. The amount of water that water-immiscible ILs solvate at thermal equilibrium has been measured by several groups. 5–13 The presence of water in ILs modifies the characteristics of ILs in many ways: fundamental properties such as density, viscosity, and heat capacity are substantially affected. 14–17 Furthermore, the solvation properties of ILs are profoundly altered, where water can act either as a cosolvent to increase the solubility of polar compounds, e.g., alcohols, or as an antisolvent to prevent the solvation of various gases and nonpolar compounds. 13,14,18–21 Moreover, water affects the catalytic abilities of ILs by changing reaction barriers, reaction energies and thus also the selectivity of catalyzed reactions. 22 Essentially, given the enormous impact of water on IL properties, the variation of the water concentration in ILs provides an additional powerful parameter, with which ILs can be customized. Also, the solubility of ILs in water is of great importance. ILs are usually considered as a potentially green alternative for various conventional molecular liquids. Since the volatility of ILs is negligible, the risk of air-pollution is minimal. However, the solvation of ions in water, especially when water-miscible ILs are used, might pose the risk of water pollution and needs to be addressed. Indeed, it has been found that the solvation of ions from water-immiscible ILs in water is not negligible, even though their solubility was found to be 1-4 orders of magnitude smaller than the solubility of water in ILs. 7,9–13,15,23–27 Once these ions are solvated in water, living organisms might be affected due to the adsorption of the cationic lipophilic alkyl chains to the cell membrane. 28,29 The toxicity of various ions to aquatic organisms and human cells has been studied by several groups, where also ion modifications with reduced toxicity were suggested (see refs 10 and 30 and references therein). Knowl- edge of the mutual solubility of ILs and water is also essential for the use of ILs in the treatment of wastewater, which is * To whom correspondence should be addressed. Phone: (65) 6419 1468. Fax: (65) 6463 2536. E-mail: marco@ihpc.a-star.edu.sg and wuping@ ihpc.a-star.edu.sg. J. Phys. Chem. B 2010, 114, 2856–2868 2856 10.1021/jp1000557  2010 American Chemical Society Published on Web 02/10/2010