Building Blocks for Ionic Liquids: Vapor Pressures and Vaporization Enthalpies of Alkoxy Derivatives of Imidazole and Benzimidazole

The design of physical solvents for applications such as CO₂ capture has been an important research area as great emphasis is being placed on reducing greenhouse gas emissions. In gas treating with physical solvents, one of the most important solvent properties needed for efficient process operation is a low vapor pressure (≪100 Pa) at ambient temperature. We have identified alkoxy-functionalized imidazoles and benzimidazoles as candidates that can meet this criterion. Vapor pressures of alkoxy derivates of imidazole and benzimidazole have been determined as a function of temperature by the transpiration method. From these data, the molar enthalpies of vaporization (Δ_l^gH_m) were calculated. The measured data sets were successfully checked for internal consistency by comparison with vaporization enthalpies of the parent speciesdimethyl ethers of ethylene glycol, diethylene glycol, and triethylene glycol. We observe that for species with the same number of atoms in the side chain [e.g., 1-butylimidazole and 1-(2-methoxyethyl)-imidazole], replacing every third methylene group with an ether oxygen reduces vapor pressure by 50–75%.