Discontinuities in Supercooled Ionic Melts

Abstract

DURING the course of a systematic investigation on the electrochemistry of boron trifluoride co-ordination compounds¹, discontinuities were observed in the temperature variation of viscosity on passing from the normal liquid to the supercooled state. In both temperature regions the viscosity could be expressed by an exponential relation of the form η = η₀exp(B/RT), but the activation energy B was approximately 10 per cent greater for the supercooled state. The temperature coefficient of the specific volume (v) shows no such discontinuity at the melting point, and this fact may be used for emphasizing the discontinuous behaviour of the viscosity by the application of Batchinski's equation² ρ = C/(v—b), where C and B are constants. For example, if the fluidity ϕ = 1/η is plotted as a function of specific volume for acetoxytrifluoroboric acid, H[CH₃.CO.OBF₃], there ia a well-defined discontinuity at a point corresponding to the melting point, 37.5° C. The change in slope is 34 per cent, compared with an increase of 10 per cent for the activation energy B for this compound.