The nature of the ions present in mixtures of trialkylamines (B) with carboxylic acids (HA) is discussed. New ab initio molecular dynamics (AIMD) simulations of mixtures of triethylamine (TEA) with acids here reveal the same cation type, the large complex [B(HA)nHB]+, as seen earlier with pyridine (PYR). Since this indicates that the ionization equilibrium reaction 2 B(HA)n ⇄ B(HA)nHB+ + A(HA)n-1 − is perhaps more general than for pyridine systems, a new analysis derives ∆G for this equilibrium reaction from experimental conductivities, viscosities, and densities, as a function of xB (mole fraction of B in the acid/base mixture). The results show that large Walden constant values, which might be indicative of Grotthuss H+ conductivity mechanisms, are not needed except for formic-acid-rich mixtures.

Protic ionic liquids (PILs), made from anhydrous mixtures of Bronsted acids HA and bases B (HA + B → BH+ + A−), occasionally suffer from limited ionicity. In cases of “poor” PILs (<10% ionicity, e.g., using carboxylic acids), past simulations have hinted that ion-pair association, more than incomplete proton transfer, is at fault. To improve upon the Fuoss equation for predicting the degree of ion pairing, new electrostatic equations (including induced dipoles) are presented, for ion-pair and other associations that occur in anhydrous amine/carboxylic acid mixtures. The equations present the association Gibbs energies ΔGA (and thus the association constants KA) as functions of three fundamental properties: the acid/base mixing ratio (n = xA/xB), the HA-to-B proton-transfer strength (ΔpKa,ε=78), and the dielectric constant (relative permittivity) of the mixture (ε). Parameter values were obtained from fits to constant-dielectric quantum chemistry data (obtained and presented here). These ΔGA functions were then used to predict ΔGioniz values for the net ion-generating (autoionization) equilibrium in carboxylic acid/amine mixtures: [Formula: see text], where n = xA/xB and d = degree of disproportionation. The agreement with experiment was excellent, demonstrating that these equations could have useful predictive power.

Mixtures of amines with carboxylic acids give not only limited ionicity, but also fascinating profiles of conductivity vs mixing ratio. These acids, when mixed with pyridine or tertiary amines, produce conductivity maxima at 5:1 (not 1:1) mole:mole mixing ratios. Other historical cases of amine/acid mixtures have produced double maxima vs mixing ratio. A good theory to quantitatively explain these is, after over 100 years, still lacking. Here we report on our recent efforts to extend our 2018 pyridine/acetic acid theory to apply to trialkylamine/acid cases. Unlike the pyridine case, where the maxima in volume contraction, viscosity, ion concentration, and conductivity are all at the 5:1 ratio, the trialkylamine cases show more structural complexity; for instance, triethylamine with propionic acid produces these four maxima at four unique ratios (2:1, 3:1, 4:1, and 5:1 respectively). Our extension incorporates some excellent advances put forward by Huyskens in 1980 for these trialkylamine cases.