Asim K. Das

Ion—solvent interactions in acetonitrile + water mixtures

Abstract Standard Gibbs energies of transfer, Δ G ° t of alkali metal chlorides (M = Li, Na, K, Rb and Cs) and potassium bromide and iodide from water to the aqueous mixtures of 20, 40, 60 and 80 mass per cents of acetonitrile (ACN) have been determined at 25°C from emf measurements performed on the double cell comprising AgAg X and M (Hg) electrodes. The individual ionic contributions to Δ G ° t have also been evaluated using the reference electrolyte (RE = Ph 4 AsBPh 4 ) method, the required Δ G ° t s of the RE being obtained from the measured solubilities of salts viz . Ph 4 AsPic, KBPh 4 and KPic (Pic = picrate and Ph = phenyl). The observed increasing Δ G ° t values of the halide ions X − and their order Cl − ⪢ Br − ⪢ I − furnish the thermodynamic evvidence for the effect of the well known decreased H-bonding and the anion destabilizing propensities of dipolar aprotic cosolvent ACN. But the observed shallow minima in Δ G ° t -composition profiles for M + and H + are indicative of the result of the oppositg effects of water structure breaking propensities and the protophobic character of ACN and their relative order is the combined effects of acid—base, Born-type and soft—soft interactions. Moreover, while the distinctly pronounced stabilization of large tetraphenyl ionshas been ascribed as the combined effects of dispersion and cavity-forming interactions, the less pronounced solvation of Pic − has been attributed to the combined effects of increased dispersion interactions of benzene nucleus and of decreased H-bonding interactions of O-atoms of the substituents. These contentions have been further substantiated by comparing Δ G ° t ( i ) values of some selected ions in ethylene glycol—water mixtures.

Transfer free energies of some ions from water to dimethylsulfoxide-water and urea-water mixtures

The standard free energies of transfer (ΔGto) of some electrolytes from water to aqueous mixtures of dimethylsulfoxide (DMSO) and of urea have been split into the contribution from individual ions by use of the reference electrolyte Ph4AsBPh4 (RE), where Ph=phenyl. For each of the solvents, ΔGto(Ph4AsBPh4) was determined from the solubility products of the salts KBPh4, Ph4AsPi, and KPi, where Pi=picrate ion. The observed ΔGto(i) values for the individual ions are strikingly different from the corresponding values obtained by the simultaneous extrapolation (SE) procedure reported earlier.

Free energies of transfer of alkali halides from water to urea-water mixtures at 25°C from EMF measurements

The standard potentialssEo of M/M+ (M=Li, Na, K, Rb, and Cs) electrodes in aqueous urea solutions containing 12, 20, 30 and 37% by weight of urea have been determined at 25°C from emf measurements on the cell M(Hg)/MCl (m), solvent/AgCl−Ag, from the activities of metals in amalgams by use of a similar type of cell in water, and from the values ofsEo of the Ag/AgCl electrode determined earlier. The standard free energies of transfer of MCl, ΔGto(MCl), from water to the mixed solvents, computed by use of these values and those for the Ag−AgCl electrode, rise sharply from Li+ to Na+ but fall from Na+ to K+ and rather sharply from K+ to Cs+ with a maximum at Na+ in all the solvent compositions. This has been attributed to the superimposition of “soft-soft” interactions on the “electrostatic interactions” between the ions and the negative charge centers of the possible hydrogen-bonded solvent complexes in the mixed solvents. Comparison of ΔGto(i) values for individual ions, obtained by a “simultaneous extrapolation” procedure, with those in aqueous mixtures of methanol,t-butanol, and dimethyl sulfoxide leads to the conclusion that the solvation of these ions in all these solvents is chiefly dictated by the “acid-base” type of ion-solvent interactions.

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.

Standard potentials of M/M+(M = Li, Na, K, Rb or CS) electrodes in dimethyl sulphoxide + water mixtures at 25°C and related ion-solvent interactions in the solvents

The standard electrode potentials (sE°) of M/M+(M = Li, Na, K, Rb and Cs) electrodes in aqueous solutions of dimethyl sulphoxide (DMSO)(containing 10, 20, 40 and 60 wt. % DMSO) have been determined at 25°C from e.m.f. measurements of cells of the type: M(Hg)|MCl(m), solvent| AgCl—Ag, obtaining the activity of metals in amalgams with the help of a similar type of cell in water. The free energies of transfer of MCl, δG°t(MCl)(mole fraction scale) from water to each of these solvents computed from these values and those of sE°Ag—AgCl obtainable from the literature, rise sharply from Li+ to Na+, and fall from Rb+ to Cs+ with a maximum between Na+ and Rb+. This is considered as due to the superimposition of “soft—soft” interactions on the “electrostatic interactions” between the ions and the negative charge centres of hydrogen-bonded solvent complexes. The ΔG°t(i) values for individual ions, obtained by a method of “simultaneous extrapolation” are shown to be largely determined by the “acid-base” type ion-solvent interactions as observed in methanol + water mixtures.

Studies in isodielectric media Part—I. Standard potentials of the silver—silver chloride electrode in dimethyl formamide + ethylene glycol mixtures at 25°C

Abstract Standard potentials ( s E s ) of the silver—silver chloride electrode have been determined at 25°C in some approximately isodielectric media containing ethylene glycol (EG) and 10, 30 and 50% by weight of N - N -dimethyl formamide (DMF) from emf measurements performed on the cell: H 2 (g, 1 atm)/HCl (m). solvent/AgClAg. From these values the activity coefficients ( s s γ) of HCl at several concentration in each of the mixed solvents and the standard Gibbs energies of transfer (Δ G 0 t ) of HCl from EG to the mixed solvents have been computed. Comparison of the observed Δ G 0 t (HCl)-composition profile with those for three other related solvent systems studied earlier, reveals that the observed profile results from the opposing effects of the larger proton-accepting and the pronounced anion-desolvating propensities of DMF and its glycolic mixtures compared to pure EG and that the increased stabilization of proton is superseded by the increased desolvation of Cl − at higher proportions of DMF.

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.

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.