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

Abstract

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.