Measurements of the Electrical Properties of a Binary Liquid Mixture

Abstract

Remarkable progress has been made in recent years in quantifying the behaviour of physical properties near a critical point. The dramatic increase in fluctuations near this point gives rise to anomalous features, which are found to be described by a set of critical exponents. Binary liquid mixtures exhibit a phase transition associated with the mixing of the two components. This transition has provided a convenient testing ground for the examination of critical point behaviour. However, the electrical properties in the region of the transition are either not well known, or the subject of conflicting reports. In this thesis, measurements of the dielectric constant of the cyclohexane/aniline mixture, as a function of temperature near TC, are reported. The conclusive result of these experiments is that there is no critical behaviour within the resolution of one part in 10 5. This is shown to be consistent with the predictions of the droplet model of Goulon et al (1979) for this mixture, but suggests that the decrease in TC in response to a large electric field, as measured elsewhere, may be of the wrong sign. A heating effect is suggested here as the likely cause. The results of a second experiment concerning the frequency dependence of the dielectric constant and conductivity of a near-critical mixture, in the range from 5Hz to l3MHz, are also presented. No critical behaviour of the dielectric constant as a function of temperature or frequency was found for the pure mixture, confirming the result of tie first experiment. However the low conductivity of this mixture meant that the resolution in this measurement was insufficient to provide any firm conclusion. Doping the mixture with a minute amount of ionic impurity produced a number of effects. [1] The low frequency conductivity increases with an exponent consistent with 1 – α, a value which has been predicted by a number of theories. [2] At higher frequencies an additional critical feature was found but could not be clearly identified. [3] The dielectric constant exhibits a peak around 10KHz which increases rapidly near TC. This is shown to be a combination of two effects; (i)that due to a leakage current effect and (ii) a feature which has a frequency dependent behaviour consistent with that of an electrode effect. Why this second feature should be critical in nature is not clear, although a possible reason is suggested. The experimental and theoretical background to these experiments is reviewed extensively. The kind of behaviour found here as a function of frequency may explain many of the inconsistent results found in previous work. The measurement techniques involved in these experiments are also fully described.