The origin of the conductivity maximum vs. mixing ratio in pyridine/acetic acid and water/acetic acid.

Explanations are provided for the first time for the historically known locations of electrical conductivity maxima versus mixing ratio (mole fraction of acid, xA) in mixtures of (i) acetic acid with water and (ii) acetic acid with pyridine. To resolve the question for the second system, density-functional-based molecular dynamic simulations were performed, at 1:1, 1:2, 1:3, 1:5, and 1:15 mixing ratios, to gain vital information about speciation. In a zeroth-order picture, the degree of ionization (and hence conductivity) would be maximal at xA = 0.5, but these two examples see this maximum shifted to the left (water/acetic acid, xAmax = 0.06), due to improved ion stability when the effective dielectric constant is high (i.e., water-rich mixtures), or right (pyridine/acetic acid xAmax = 0.83), due to improved acetate stability via "self-solvation" with acetic acid molecules (i.e., acid-rich mixtures) when the dielectric constant is low. A two-parameter equation, with theoretical justification, is shown to reproduce the entire 0 < xA < 1 range of data for electrical conductivity for both systems. Future work will pursue the applicability of these equations to other amine/carboxylic acid mixtures; preliminary fits to a third system (trimethylamine/acetic acid) give curious parameter values.