Thermodynamics of autoionization of aqueous tetrahydrofuran and 1,2-dimethoxyethane and the structuredness of solvents

Abstract

Autoionization constants (Ks) of aqueous mixtures of tetrahydrofuran (THF) and 1,2-dimethoxyethane (DME) containing 10, 30, and 50 wt.% cosolvent in each case have been determined from emf measurements of the cell: Pt, H2 (g, 1 atm)|KOH (m1), KBr (m2), solvent|AgBr, Ag at seven equidistant temperatures ranging from 5 to 35 °C. The standard free energies (ΔG0), entropies (ΔS0), and enthalpies (ΔH0) of autoionization of the solvents were also evaluated from these data. Relative free energy data, δΔG0(≡sΔG0 − wΔG0), for these solvents as well as those for dioxane (D) – water mixtures taken from the literature, when coupled with the previously determined transfer free energies of H+, ΔGt0(H+), yielded ΔGt0(OH−)app (≡ΔGt0(OH−) − ΔGt0(H2O)) values in the mixed solvents. Relative magnitudes of ΔGt0(H+) and ΔGt0(OH−)app and their non-Born parts, ΔGt,ch0(H+) and ΔGt,ch0(OH−)app in particular, suggest that the "basicity" of these aqueous cosolvents decreases in the order DME > THF > D and their "acidity" in the reverse order, as expected from structural and electronic consideration of these cosolvent molecules. Analysis of the relative entropie contributions, Tδ (ΔS0) (≡T (sΔS0 − wΔS0), for the autoionization of these aqueous cosolvents and in particular ΔSt0(H2O) values derived there from, suggests that while THF promotes three dimenstional (3D) ice-like water structures at initial compositions and D induces breakdown of the 3D structures right from the beginning, DME breaks down water structures at initial compositions, but induces some order around 4–14 mol% DME by forming the possible H-bonded bidentate DME–water complexes. And beyond certain compositions, depending upon the relative size and shape, all the cosolvents break down water structure due to packing imbalance.