CRITICAL VISCOSITY OF THE IONIC MIXTURE TRIETHYL N-HEXYL AMMONIUM TRIETHYL N-HEXYL BORATE IN DIPHENYL ETHER

We report measurements of the viscosity near the consolute point of tr iethyl n-hexyl ammonium triethyl n-hexyl berate in diphenyl ether. Unt il recently, this ionic mixture was the leading candidate for a ''mean -field'' ionic fluid composed of small molecules. The measurements of the coexistence curve of Singh and Fitter and the measurements of turb idity of Zhang et al. had indicated mean-field static behavior. In con trast, the present measurements show a critical viscosity enhancement similar to that seen in Ising fluids. Such an enhancement is not expec ted in either a mean-field fluid or a fluid with sufficiently long-ran ged forces. The measurements were made in two very different viscomete rs. Both viscometers achieved low shear rates by use of a flow impedan ce larger than in a conventional capillary viscometer. The first visco meter's impedance was a glass frit consisting of about 10(5) pores of 5.5 mu m diam each. The second viscometer's impedance was a single I m long, 203 mu m diam capillary. In both viscometers, the sample was se aled entirely in glass, in order to inhibit decomposition of the sampl e. (C) 1998 American Institute of Physics.