Viscosities of concentrated electrolyte solutions

Hefter, G., May, P.M., Sipos, P. and Stanley, A. (2003) Viscosities of concentrated electrolyte solutions. Journal of Molecular Liquids, 103-104 (SPEC.). pp. 261-273.

Abstract

The variation of the viscosities of unassociated simple binary strong electrolyte solutions in water as a function of concentration and temperature have been modelled using a simple empirical expression of the type: log(η/ηo) = Bc + Dc2, where B = B1 + B2t and D = D1 + D2t. Such equations, containing only four adjustable parameters, fitted the viscosity data with average residuals < 0.9 % for a variety of electrolytes up to high concentrations (0 ≤ c/mol kg-1 ≤ 12) and over a wide temperature range (5 ≤ t/°C ≤ 90). Addition of a fifth term, B3t2, was necessary for the accurate description of electrolyte viscosities at higher temperatures (≤, 150 °C). Viscosities of ternary strong electrolyte solutions at constant ionic strength over the temperature range 25 ≤ t/°C ≤ 90 were reasonably well accounted for (with maximum residuals < 8 %) by assuming a simple ionic strength-based pro-rate additivity, analogous to Young's rule for thermodynamic properties. Departures from additivity appear to be largely related to the differences in the viscosities of the (binary) end-member solutions. Incorporation of a simple linear correction more than halved the residuals. The 'Young's rule' approach has been used to calculate the viscosities of the hypothetical pure NaAl(OH)4(aq) solutions and of NaOH/NaAl(OH)4 mixtures.