Molecular Thermodynamics and Solvation Behaviour of Protic Ionic Liquid Systems

Protic ionic liquids (PILs) are a class of solvents prepared from the mixing of equimolar quantities of a Brønsted acid and base resulting in both neutral and ionic species in equilibrium with one another. Their evolving application as solvents for innovative processes requires further understanding of their properties and how they originate at the molecular level. Three topics remain widely debated concerning PILs: 1) the effects of low concentrations of water as an impurity, 2) the structure–property relations in PILs and 3) the connection between PILs and their precursor components in terms of both molecular interactions and bulk properties. In this work, these three topics are studied using a variety of experimental techniques and fundamental theory for selected representative PIL systems. To clarify the effect of water at low concentrations, the statistical thermodynamic theory of solutions has been applied to quantify the interactions between species solely from thermodynamic data. Results showed both a strong composition dependence of the effect of water on the liquid structure in aprotic and protic ILs, but also that water did not significantly weaken ion–ion interactions at low concentrations. After clarifying the effects of water at low concentration on PIL behaviour, it has been shown that incorporating hydrogen bond donor functionality to the cation can increase the ionic nature of acetate PILs. This increase in ionic nature provides an excellent rationalization for the effect of cation structure on the thermodynamic and solvatochromic properties of three PILs. By studying the effect of varying composition of precursor acid and base, a deeper insight into the molecular origin of trends in bulk properties and solvation behaviour can be found. Furthermore, it has been shown that the solvation environment is highly composition dependent, offering insight into a new strategy in the application of PILs and their precursor materials as tuneable solvation media.