Transition State Treatment of the Relative Viscosity of Electrolytic Solutions: Applications to Aqueous, Non-aqueous and Methanol + Water Systems

Abstract

A transition-state treatment of the relative viscosity of electrolyte solutions is described. The following expression is found for the viscosity B-coefficient: B=(V°1–V°2)/1000 +V°1[(Δµ°[graphic omitted]2–Δµ°[graphic omitted]1)/1000 RT]. V°1 and V°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 (V°1–V°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 V°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.