David Feakins

Transition state treatment of the relative viscosity of electrolytic solutions. Applications to aqueous, non-aqueous and methanol + water systems

A transition-state treatment of the relative viscosity of electrolyte solutions is described. The following expression is found for the viscosity B-coefficient: B=(text-decoration:overlineV°1–text-decoration:overlineV°2)/1000 +text-decoration:overlineV°1[(Δµ°[graphic omitted]2–Δµ°[graphic omitted]1)/1000 RT]. text-decoration:overlineV°1 and text-decoration:overlineV°2 are the partial molal volumes of the solvent and solute respectively; Δµ°[graphic omitted]1 is the activation energy for viscous flow of the solvent, and Δµ°[graphic omitted]2 the “ionic activation energy” at infinite dilution.For aqueous solutions, at 25°C, the term (text-decoration:overlineV°1–text-decoration:overlineV°2)/1000 accounts completely, in the case of ammonium chloride, and partially in the case of potassium, rubidium and caesium chlorides, for the negative sign of the B-coefficient. When solutions in different solvents are compared, the well-known tendency for B to increase with text-decoration:overlineV°1 is at least partly explained by the form of the above expression; changes in µ°[graphic omitted]2 from water to methanol, for example, are less dramatic than changes in B. B-coefficients in the methanol + water system are consistent with a maximum in solvent structure around 20% methanol (w/w) at 25°C; whilst the µ°[graphic omitted]2-values support this interpretation, it is noted that solvent structure can influence B twice over, in µ°[graphic omitted]2 and (trivially) in µ°[graphic omitted]1.

Relative viscosities and quasi-thermodynamics of solutions of tert-butyl alcohol in the methanol–water system: a different view of the alkyl–water interaction

Viscosity B coefficients for tert-butyl alcohol (TBA) in water, methanol and mixtures of the two have been obtained at 25 and 35 °C. Hence solute contributions to the activation parameters for viscous flow of the solutions have been calculated, as well as the Gibbs energy, enthalpy and entropy of transfer from the ground-state solvent to the hypothetical viscous ‘transition-state solvent’. The enthalpies of mixing of the two transition-state solvents and their relative partial molar enthalpies L′1 and L′2 are also given.In water, B is large and positive and dB/dT large and negative; in methanol B is small and dB/dT near zero. The classical explanation of these observations, which is solely in terms of water–water (1,1) interactions in an ‘iceberg’ around the alkyl group, is inconsistent with the Compensation Principle because B is proportional to a Gibbs energy function, and this requires that solute–solvent (3, 1) interactions must determine B.It is proposed that in water the alkyl group occupies a ‘cage’ in which the strength of the bonding with respect to the pure solvent is of minor importance. The ‘caging’ of the alkyl group enhances its van der Waals interactions with its nearest neighbours. The expected dependence of such interactions on distance leads to energy–reaction coordinate diagrams in the form required to explain the inversion in sign of the solvation energies from ground-state solvent to transition-state solvent in highly aqueous media.These results are consistent with, though independent of, recent structural studies by neutron diffraction with isotopic substitution.

The viscosity and structure of solutions. Part 1.—A new theory of the Jones–Dole B-coefficient and the related activation parameters: application to aqueous solutions

For many years the lowering of the viscosity of water by certain electrolytes has been attributed to the action of their ions in breaking down its special three-dimensional structure. In this paper, the familiar viscosity B-coefficients for the alkali-metal chlorides in aqueous solutions and the related ionic molar contributions to the free energy, enthalpy and entropy of activation, namely Δµ°[graphic omitted]3, Δtext-decoration:overlineH°3 and ΔS°[graphic omitted]3, are re-examined. Particular attention is paid to the nature of the solvent in the transition state.The application of the compensation principle is first explored. If compensation between enthalpic and entropic contributions from solute-induced structural changes occurs in both ground- and transition-state solvents, Δµ°[graphic omitted]3, and hence B cannot by influenced by changes in the structure of the solvent of the type proposed, say, by Frank and Wen. Instead it is suggested that, in aqueous solution, an ion can coordinate more solvent molecules in the more weakly bonded transition-state solvent than in the ground-state solvent. This is particularly important for solutions containing the larger alkali-metal and halide ions, whose enhanced fluidity thus stems not from solvent–solvent bond-breaking in the ground state, but from ion–solvent bond-making in the transition state. Even if the compensation principle is not accepted, this explanation removes structurel changes in the solvent as a unique explanation of the observed trends in B-coefficients.

Enthalpies of transfer of N-methylformamide, formamide, and N,N-dimethylformamide from methanol to methanol + dimethylsulfoxide mixtures

The enthalpies of transfer of formamide, N-methylformamide and N,N-dimethylformamide from methanol to methanol+dimethylsulfoxide solvent systems have been measured. These data are analysed in terms of a recently developed model of solvation in mixed solvents. The results of the data analyses indicate that preferential solvation of the different functional groups of the amides differs, the carbonyl oxygen being preferentially solvated by methanol and the nitrogen protons are by dimethylsulfoxide.

Relationship between the entropy of transfer of a solute and the thermodynamic functions of mixed solvents

The entry of a solute into a solvent will affect the interactions between solvent molecules by making, breaking, strengthening or weakening solvent–solvent bonds. In a binary mixed solvent the relative partial molar entropies, Si, and excess entropies of mixing, ΔSE, are related to these interactions. Thus a relationship between the entropy of transfer, ΔS⊖t, of a solute and these thermodynamic parameters might be expected. A general relationship is developed and successfully applied to the ΔS⊖t for alkali-metal halides in methanol–water mixtures. The results of this analysis show that, for this system at least, ΔS⊖t is dominated by the effects of changes in solvent–solvent interactions, in contrast to the enthalpy of transfer, ΔH⊖t. Combination of the results of this analysis with those previously reported for ΔH⊖t allows a complete, quantitative explanation of the thermodynamics of solvation in the alkali-metal halides in methanol–water mixtures.

Enthalpies of transfer of tetrabutylammonium bromide as indicators of the structure of aqueous solvents: aqueous methanol, ethanol, propan-1-ol, 2-methylpropan-2-ol and 1,4-dioxane systems

Enthalpies of transfer for tetrabutylammonium bromide from water to aqueous methanol, ethanol and 1,4-dioxane are reported and analysed in terms of solvation theory. Combination of these analyses with those for aqueous propan-1-ol and 2-methylpropan-2-ol (TBA) mixtures shows that the extent to which tetrabutylammonium bromide disrupts the solvent structure increases systematically from aqueous methanol to aqueous TBA, with those for aqueous methanol and 1,4-dioxane being comparable. The concentration of the organic cosolvent at which the water structure breaks down decreases in a similar order. These changes in the solvation of tetrabutylammonium bromide are discussed in terms of the effects of the organic cosolvents on the aqueous structure.

Enthalpies of transfer of formamide, N-methylformamide and N,N-dimethylformamide from water to aqueous acetonitrile mixtures at 298 K

The enthalpies of transfer of formamide, N-methylformamide and N,N-dimethylformamide from water to aqueous acetonitrile mixtures have been measured. Analyses of these results shows that there is a marked discontinuity in the solvating properties of this system at an acetonitrile mole fraction of 0.3. This transition is characterized by changes in both the degree of preferential solvation of, and the extent of disruption of the solvent structure by, the solutes. Comparison of these results with those for other solvent systems indicates that in the water-rich domain the acetonitrile may cause greater rigidification of the water structure than do the simple alcohols, while the acetonitrile-rich media are less structured than the alcohols and more closely resemble purely non-aqueous mixtures.

Enthalpies of transfer of formamide, N-methylformamide, N,N-dimethylformamide and urea from water to some aqueous alcohol solvent systems at 298 K

The enthalpies of transfer, ΔtH⊖ of formamide, N-methylformamide and N,N-dimethylformamide from water to aqueous methanol, ethanol, propan-1-ol and 2-methylpropan-2-ol, and those of urea from water to aqueous propan-1-ol systems are reported. These data, and literature values for ΔtH⊖ of N-methylpyrrolidinone and urea, have been analysed in terms of solvation theory.Analysis of the ΔtH⊖ data points to there being a discontinuity in the solvating properties of the solvent systems at intermediate solvent compositions. In the water-rich domain the solutes are substantially randomly solvated, while they are preferentially hydrated in the alcohol-rich regions. It is also found that, in the water-rich solvents, the extent to which the solutes disrupt the structure of the solvent is a simple function of the size of the non-polar alkyl groups on the solute, increasing linearly with the size of the alkyl surface. The rate of this increase depends on the nature of the alcohol, and increases from aqueous methanol to aqueous propan-1-ol, and decreases again for the aqueous TBA system. In contrast, the extent of disruption of solvent structure shows more complex variations, dependent on the solvent system, in the alcohol-rich region.

Enthalpies of transfer of some non-electrolytes from acetonitrile to acetonitrile–methanol mixtures

The enthalpies of transfer of water, propan-1-ol, 2-methylpropan-2-ol, octan-1-ol, formamide, N-methyl-formamide, N,N-dimethylformamide, dimethyl sulphoxide and propylene carbonate from acetonitrile to methanol and acetonitrile–methanol mixtures are reported and analysed in terms of solvation theory. The analyses show that all of the solutes, except propylene carbonate, interact more strongly with methanol than acetonitrile, and are preferentially solvated by methanol in the mixed solvents. It is also found that the extent to which the solutes disrupt solvent–solvent bonding is greatest for the amides and dimethyl sulphoxide, which act only as hydrogen-bond acceptors, and is relatively insensitive to the size of the alkyl groups on the alcohols.

Enthalpies of transfer of 2-methyl-2-propanol from water to mixtures of water with methanol, ethanol and 1,4-dioxane

The enthalpies of transfer of 2-methyl-2-propanol (TBA) from water to mixtures of water with methanol, ethanol and 1,4-dioxane have been measured. The data are considered in terms of recently developed theory, and it is found that the enthalpies of transfer can be reproduced quantitatively over most of the composition range in each solvent system. The parameters recovered from the analyses indicate that the net effect of TBA on the solvent structure is a breaking of solvent-solvent bonds and that TBA is preferentially hydrated in the aqueous alcohol systems, but randomly solvated in the water+1,4-dioxane system. It is also found that the model parameters for TBA solvation in the alcohol systems are independent of the alcohol.