Molecular liquids versus ionic liquids: Thermodynamic insights into the interplay between inter-molecular and intra-molecular hydrogen bonding

Abstract

Inter- and intra-molecular hydrogen bonding (HB) was studied in series of molecular and ionic liquids, containing a five-membered aliphatic ring (hydroxy-alkyl-pyrrolidines) and a five-membered aromatic ring (hydroxy-ethyl-imidazole), as well as in a corresponding ionic liquids, also containing a five-membered aromatic ring (hydroxy-alkyl-imidazolium) with the hydroxyl attached to the alkyl chain of the cation. To quantify these types of hydrogen bonds, we measured the enthalpies of vaporisation using a number of experimental methods, including thermogravimetric analysis, quartz crystal microbalance, and gas chromatographic method. The method of homomorphs was applied to quantify the strength of inter-molecular hydrogen bonding. The group-additivity based “centerpiece” method was used to quantify the intra-molecular HB strength in both molecular and in ionic liquids. Two new methods based on the experimental vaporisation enthalpies and a well-balanced thermodynamic equations were developed and applied to estimate the intra-molecular hydrogen bond strength, and results of the three methods agreed well. The interesting interplay between intra- and intermolecular hydrogen bonding was observed and discussed for each ionic liquid.