Free energies and entropies of transfer of hydrogen halides from water to aqueous solutions of tetrahydrofuran, dioxane, and 1,2-dimethoxyethane and the structuredness of the solvents

Abstract

Standard free energies (ΔGt0) and entropies (ΔSt0) of transfer of hydrogen iodide from water to some aqueous solutions of tetrahydrofuran (THF), dioxane (D), and 1,2-dimethoxyethane (DME) have been determined by measuring the emf's of the cell: Pt, H2 (g, 1 atm)|KOH (m1), KI (m2), solvent|AgI, Ag at seven equidistant temperatures ranging from 5 to 35 °C. In each of these ethereal solvent systems ΔGt0 values of HI, as well as of HCl and HBr obtained from the literature, and particularly of the individual ions, suggest that while H+ is increasingly stabilized, halide ions are increasingly destabilized due to the influence of cosolvent-induced larger "basicity" and smaller "acidity" of the mixed solvents compared to that of water, and both conformed to the expected order: D < THF < DME. Moreover, the relative order: Cl− > Br− > I− in all the solvent systems is ascribable to the combined effects of "acid–base" and "soft–soft" interactions and the superimposed quadrupolar interactions in the case of D and the charge transfer to solvent (CTTS) complexation effect, especially on I− in the case of THF. Analysis of the entropie contributions, TΔSt0, and particularly of the relative order of ΔY (≡TΔSt0(H+) + TΔSt,ch0(X−)) for X = Cl, Br, and I, in the light of the semi-quantitative theory proposed earlier by Kundu et al., reveals that at initial compositions, while THF promotes 3D structures of water, both D and DME break down the same; at higher compositions all the cosolvents disrupt the structure as usual due to packing imbalance. The nature and relative positions of ΔY–composition profiles also suggest that while increase of hydrophobic groups of the cosolvents increases the stabilization, increase in hydrophilicity or H-bonding sites decreases the stabilization of the 3D structure of water.