Exploiting Cation Structure and Water Content in Modulating the Acidity of Ammonium Hydrogen Sulfate Protic Ionic Liquids

In this paper, we investigated the effect of cation structure and water content on proton dissociation in alkylammonium [HSO₄]⁻ protic ionic liquids (ILs) doped with 20 wt % water and correlated this with experimental Hammett acidities. For pure systems, increased cation substitution resulted in a reduction in the number of direct anion–anion neighbors leading to larger numbers of small aggregates, which is further enhanced with addition of water. We also observed spontaneous proton dissociation from [HSO₄]⁻ to water only for primary amine-based protic ILs, preceded by the formation of an anion trimer motif. Investigation using DFT calculations revealed spontaneous proton dissociation from [HSO₄]⁻ to water can occur for each of the protic ILs investigated; however, this is dependent on the size of the anion aggregates. These findings are important in the fields of catalysis and lignocellulosic biomass, where solvent acidity is a crucial parameter in biomass fractionation and lignin chemistry.