Thermodynamics of autoionization of 2-methoxy ethanol + water mixtures and structuredness of the solvents

Abstract

Standard free energies (ΔG0) and entropies (ΔS0) of autoionization of aqueous mixtures of 10, 30, 50, and 70 wt.% of 2-methoxy ethanol (ME) have been evaluated from the autoionization constants (Ks) of the solvents as 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 15 to 45 °C. The observed increase in pKs and the related δ(ΔG0) (= sΔG0 − wΔG0) has been ascribed to be the effect of decreased dielectric constant, increased basicity and decreased acidity of the mixed solvents compared to that of water. Analysis of relative entropic contributions of autoionization, Tδ(ΔS0) (= T(sΔS0 − wΔS0)) and their chemical part, Tδ(ΔS0)ch in the aqueous mixtures of ME as well as the closely related cosolvents like ethylene glycol (EG) and 1,2-dimethoxy ethane (DME), appears to suggest that unlike EG but like DME, ME has an overall structure breaking propensity of three dimensional (3D) tetrahedral structures of water. But the transfer entropies of water ΔSt0(H2O) derived thereof, for these cosolvents suggest that while the structural changes induced by protic EG are seemingly obseured due to the involved multiple acid-base equilibria with water, and that by aprotic DME are disturbed by the formation of strong hydrogen-bonded DME–H2O complexes around 4–14 mol% DME, quasi-aprotic ME appears to induce some order due to the possible formation of H-bonded ME–water complexes around 10–15 mol% ME.