Regardless of its actual synthetic procedure, a protic ionic liquid (PIL) can be thought as stemming from a simple acid/base reaction: AH + B --> A- + BH+ where, in order to form a true PIL, the ensuing product has to be a fully ionized liquid.
From basic chemistry, it is well-known that the above reaction is an equilibrium process that can be more or less shifted to the right depending on the propensity of the acid/base pair to share the proton. At the moment, a straightforward relation between the position of said equilibrium in PILs and the nature of the constituent molecules and ions seems to have eluded the research community.
A crucial question therefore arises and part of its solution consists in assessing via first principle calculations the propensity for the involved ions to exchange the proton when in the bulk phase. This apparently trivial question has no simple answer: molecular indicators such as proton affinities or pKa are often not sufficient to predict the extent of the proton transfer equilibrium and complex many body effects due to the self-solvation properties of the liquid come into play.
The focus of this project is attempting to elucidate the features of proton transfer equilibria in several prototype (biocompatible) PILs, using accurate ab initio and molecular dynamics methods. Given the complexity of these materials and the fact that chemical bond breaking and formation is taking place during their equilibrium dynamics, the use ab-initio molecular dynamics would be mandatory. However, performance considerations, and our previous experience suggest that a semiempirical approach based on the DFTB (density functional tight binding) method is often sufficient to achieve a reliable description of the materials with the advantage of making this project feasible in a reasonable time span.
The inherent biocompatibility of the compounds that are the subject of this project makes them interesting candidates for a wide set of potential applications most of which are still in their research stage. Such biocompatible substances are broadly referred to as "4th generation ionic liquids" and might represent in the near future a viable opportunity to replace previous non-biocompatible materials. They often share the same peculiar and advantageous properties of the previous generations with the added value of biodegradability and low synthetic costs. In addition to the tunable chemical and physical properties of traditional ILs, they seem to show very specific biological properties that make them suitable for biopharmaceutical applications. [1]
Despite an increasing attention from the scientific community towards these compounds, their unexpected biochemical versatility emerges from an inherent complexity at the nanoscopic level which makes particularly difficult the understanding of how the specific chosen molecular architectures determine their bulk properties. This, in turn, justifies the need of fundamental studies of their structural and dynamical organization that might provide explanations of their ionicity, hydrogen bond driven aggregation, solvation properties and interaction with biomolecules.
Many of these properties are not readily available to experimental investigation due to the intrinsic difficulties in linking experimental bulk average properties such as conductivity, frictional properties, and ion mobilities to precise local nanoscopic geometric features. In this context, computational simulations as those planned in this project might provide the missing link between the bulk properties and the nature of the composing molecular entities. In other words, our project intends to make a step forward in elucidating the (rather non-linear) structure-properties relation that lie at the core of their possible practical use and that could be exploited to achieve a better design of these substances in specific applications.
[1] a) R. Md Moshikur, Md. R. Chowdhury, M. Moniruzzaman and M. Goto, Green Chem., 2020, DOI: 10.1039/D0GC02387F. b) J. M. Gomes, S. S. Silva and R. L. Reis Chem. Soc. Rev., 2019, 48, 4317-4335