Kumardev Bose

Ion-solvent interactions in dimethylformamide + water mixtures

Abstract Standard Gibbs energies of transfer, ΔG°t, of MCl (M = Li, Na, K, Rb and Cs) and K X (X = Br and I) have been determined by use of the double cells comprising M(Hg) and AgXAg electrodes in some aqueous mixtures of DMF at 25°C. These values were dissected into individual ion contributions using TATB reference electrolyte assumption, as obtained by measuring the solubility products of the salts viz. KPic, KBPh4 and Ph4AsPic. The observed increasingly positive magnitudes of ΔG°t of the halide ions and the increased negative magnitudes of the cations including H+, reflect the well known anion-destabilization and the larger “basicity” and cationotropism of the aqueous mixtures of this co-solvent. And the observed distinctly flat regions of ΔG°t curves of cations over an intermediate composition are attributable to strong intercomponent interactions. Comparison of the results with those in DMSO—water are ACN—water mixtures reveals that the solvating characteristics of DMF—water are somewhat similar to that of DMSO—water but different from that of ACN—water mixtures. Moreover, while the observed relative behaviour of Pic− are found to be guided by the combined effects of dispersion interactions and anion destabilisation of these co-solvents, that of the large-sized tetraphenyl ion is, however, guided by the combined effects of dispersion interactions as well as the “cavity-forming” interactions as estimated in the light of scaled particle theory.

Free energies and entropies of transfer of hydrobromic and hydroiodic acids from water to t-butyl alcohol + water mixtures from electromotive force measurements at different temperatures (5—35°C)

Free energies, ΔG0t and entropies, ΔS0t of transfer of hydrobromic and hydroiodic acids from water to t-butyl alcohol + water mixtures containing 10, 20, 30 and 50 % t-butyl alchol by weight have been evaluated using the corresponding values of the standard potentials, E0, of silver–silver bromide and silver–silver iodide electrodes in these solvents obtained from electromotive force measurements at seven different temperatures ranging from 5 to 35°C performed on the cell Pt, H2(g, 1 atm)|HX(m), solvent|Ag-AgX, where X = Br or I. These and the corresponding values for HCl obtained from E0 data given in literature were used to evaluate ΔG0t for individual ions by the “extrapolation procedure”. In contrast with other aqueous organic solvents, humps or kinks in the ΔG0t against composition profiles occur at water-rich compositions reflecting the strong structural characteristics of this solvent system in that region. The variation of entropies of transfer with composition has been interpreted in terms of marked structure-promotion of water and consequent ion-dehydration at small concentrations of t-butyl alcohol and of the formation of alcohol–water complexes at somewhat higher contents of t-butyl alcohol. The irregular order Br– > I– > Cl– in water-rich regions for the entropy profiles is suggested to be due to the formation of charge-transfer complexes of the iodide ion with t-butyl alcohol.

Free energies and entropies of transfer of hydrogen halides from water to aqueous alcohols and the structure of aquo-organic solvents

Free energies ΔG0t and entropies ΔS0t of transfer of HCl from water to ethanol + water and propan-2-ol + water mixtures have been determined from the standard potentials E0 of the cell Pt, H2(g, 1 atm)|HCl (m), solvent|AgCl–Ag. ΔG0t values for hydrogen and halide ions have been obtained by a simultaneous extrapolation procedure applied to ΔG0t(HCl), ΔG0t(HBr) and ΔG0t(HI), values of the last two being taken from previous papers. The chemical parts of the ionic ΔG0t values in these and two other aquo-alcoholic systems have been interpreted in terms of ion-solvent interactions of the acid–base type modified by solvent structural effects.Characteristic humps observed in ΔS0t(HX)-composition profiles for aqueous ethanol, propan-2-ol and t-butanol mixtures have been shown, by means of a semi-quantitative theory, to result from a well-known feature common to these solvents, viz., promotion of water structure in highly aqueous compositions. This theory has then been used to obtain information regarding solvent structure in other aquo-organic solvent systems and to stress the usefulness of ΔS0t as a probe for solvent structuredness.

Proton-transfer equilibria in isodielectric acetonitrile–ethylene glycol mixtures at 298.15 K

The dissociation constants (Ka) of tris(hydroxymethyl)methylammonium (TrisH+) and p-nitroanilinium (pNAH+) ions and p-nitrophenol (pNP) have been determined at 298.15 K both in ethylene glycol (EG) and its isodielectric mixtures with acetonitrile (ACN), an e.m.f. method being used for TrisH+ and a spectrophotometric method for the other two acids. These Ka values have been used to derive the solvent effect on the free energies of dissociation of these acids. From independent solubility measurements on the neutral species: tris(hydroxymethyl)methylamine (Tris), p-nitroaniline (pNA), and pNP, the free energies of transfer of these species from EG to ACN–EG mixtures have been determined at 298.15 K. By a combination of the results of Ka and solubility measurements and with a knowledge of the free energies of transfer of the hydrogen ion evaluated earlier, the free energies of transfer of TrisH+, pNAH+, and pNP–(p-nitrophenoxide ion) in these solvents have been computed. Plots of free energies of transfer for the individual species against solvent composition reflect a wide range of solvation behaviour which has been interpreted in terms of the tendency of protic solvents like EG to solvate specifically hydrogen-bonding solutes and the tendency of dipolar aprotic solvents like ACN to solvate specifically solutes undergoing primarily dispersion interactions.

Standard potentials of the silver–silver bromide electrode in propan-2-ol + water mixtures. Free energies and entropies of transfer of hydrobromic acid

Standard potentials (E⊖) of the Ag–AgBr electrode in aqueous mixtures of 10, 20, 30 and 50 wt % propan-2-ol have been determined from e.m.f. measurements on the cell Pt, H2(g, 1 atm)| HBr(m), solvent | AgBr–Ag at seven temperatures from 5 to 35°C at 5°C intervals. These values were used to evaluate standard free energies (ΔG⊖t) and entropies (ΔS⊖t) of transfer of HBr from water to these solvents. The observed broad hump and pronounced maximum in the (ΔG⊖t)- and T(ΔS⊖t)-composition profiles respectively, being similar to those in t-butanol + water mixtures, are indicative of the similar effects of ion-solvent interactions as well as of the structural features of these solvents.

Free energies of transfer of alkali metal halides in isodielectric acetonitrile and ethylene glycol mixtures at 25C

Standard free energies of transfer (ΔGto) of alkali metal chlorides MCl (M=Li, Na, K, Rb, Cs) and of potassium bromide and iodide from ethylene glycol to its isodielectric mixtures containing 20, 40, and 60 wt. % acetonitrile have been determined from emf measurements at 25°C. The standard potentials of the M/M+, the Ag−AgBr, and the Ag−AgI electrodes have been calculated using auxiliary emf measurements. All electrolytes were found to be increasingly destabilized in mixed solvents of increasing acetonitrile content; the relative order of cation destabilization is Li+>Na+>K+>Rb+>Cs+ and that of anion destabilization is Cl−>Br−>I−, both following (a) the order of increasing softness of ions and increasing interaction with the soft base, acetonitrile,and (b) the order of decreasing hydrogen-bonding propensity of ions and decreasing interactions with the hard base, ethylene glycol.

Standard potentials of the silver-silver iodide electrode in aqueous mixtures of ethanol and propan-2-ol at different temperatures. Free energies and entropies of transfer of hydroiodic acid at 25°

Standard potentials of Eθ of the silver-silver iodide electrode in ethanol + water and propan-2-ol + water mixtures have been determined from the emf of the cell Pt, H2 (g, 1 atm)|HI(itm), solvent| Agl-Ag at seven temperatures ranging from 10 to 40° for ethanol and from 5 to 35° for propan-2-ol. Free energies ΔSgqitt of transfer HI obtained therefrom have been briefly discussed in the light of ion-solvent interactions and solvent structure.

Transfer free energies and entropies of hydrobromic acid in ethanol + water mixtures: structure of aquo-alcoholic solvents

Standard potentials (E⊖) of the Ag–AgBr electrode obtained from e.m.f. values of the cell Pt, H2(g, 1 atm)/HBr(m), solvent/AgBr–Ag at seven temperatures (10–40°C) have been used to compute standard free energies (ΔG⊖t) and entropies (ΔS⊖t) of transfer of HBr from water to ethanol + water mixtures containing 10, 20, 30 and 50 % by weight of ethanol. Variations of ΔG⊖t and TΔS⊖t with mole % alcohol have been compared in three alcohol + water systems with special reference to the effect of addition of alcohol on the three-dimensional structure of water. The entropy of transfer has been proposed as a sensitive probe for the elucidation of solvent structure.

Studies in isodielectric solvents. Part 1.—Standard potentials of the silver–silver chloride electrode in the acetonitrile + ethylene glycol solvent system at 25°C

Standard potentials (Eθ) of the silver–silver chloride electrode have been determined at 25°C in ethylene glycol and in its approximately isodielectric mixtures with 20, 40 and 60 wt. % acetonitrile, from e.m.f. measurements performed on the cell Pt, H2(g, 1 atm.)/HCl(m), solvent/Ag–AgCl. Standard free energies of transfer (ΔGθt), of HCl at 25°C from ethylene glycol to acetonitrile + ethylene glycol mixtures, obtained from these measurements have been interpreted in terms of the acid–base nature of the solvents. Comparison of the ΔGθt-composition profile with those of other isodielectric solvent systems has been used to confirm the idea of ion desolvation caused by dipolar aprotic solvents like acetonitrile.