Solvation structure and transport of acidic protons in ionic liquids: Insights from computer simulations

I&EC 38

Mario G. Del Popolo, m.del-popolo@qub.ac.uk1, Jorge Kohanoff, j.kohanoff@qub.ac.uk1, and Ruth M. Lynden-Bell, rmlb@cam.ac.uk2. (1) Atomistic Simulation Centre - School of Mathematics and Physics, Queen's University Belfast, University Road, Belfast, BT7 1NN, United Kingdom, (2) University Chemical Laboratory, Cambridge University, Lensfield Road, Cambridge, CB2 1EW, United Kingdom
A description of the energetic, solvation structure and diffusion of acidic protons in ionic liquids is important for acid-based chemistry in ILs, and for the design of new proton-conducting materials. In this work we discuss the results of adding a molecule of HCl to dimethyl-imidazolium chloride. Ab-initio molecular dynamics simulations show that the acidic proton exists as a symmetric, linear [ClHCl-] species. Details of the solvation structure around this ion are given. The proton transport mechanism is investigated by applying a force along the asymmetric stretch coordinate until the ion breaks. This results in the exchange of one of the Cl atoms with a neighbouring chloride belonging to the solvent to form a new [ClHCl-] molecule. Such a process results in an effective hopping of the hydrogen atom. The reaction mechanism is discussed in terms of the structure of the reacting species and their solvation shells. An estimation of the associated free energy change is provided.