The Transition (vs ΔpKa) from Triple Ions to Free Cations in Poor Protic Ionic Liquids Made from Weak Acids

Abstract

In mixtures of amines with carboxylic acids, the limited ionicity at 1:1 stoichiometric mixtures is due to insufficient ionization or ion pairing in low-dielectric environments. Higher conductivities have historically been seen at roughly 4:1 acid/base mixing ratios, where simulations have revealed large (and thus more stable) ions: homoassociated anions (AH)(AH)(A−)(HA)(HA) and cationic triple ions (BH+)(AH)(A−)(HA)(HB+). Recent work in understanding the Gibbs energies for degree-of-ionization equilibria hints that there may be an onset, for systems with an acid-base pKa difference increasing towards 10, for the formation of free (unpaired) protonate-amine cations BH+. The transition with increasing ΔpKa is explored in this work, to aid in the prediction of ionicity and conductivity of protic ionic liquids.