Pressure behaviour of dielectric permittivity on approaching the near-critical consolute point

– Results are presented of studies on dielectric permittivity (ε) for the isothermal pressure (P ) path of approaching the near-critical consolute point in 1-nitropropane–hexadecane solution. The pretransitional anomaly is well portrayed by the relation isomorphic to that applied in temperature (T ) studies under atmospheric pressure. However, the ε(P ) anomaly is much larger, with almost negligible influence of correction-to-scaling terms and the lowfrequency Maxwell-Wagner dispersion (even for f = 1 kHz), than it was observed in temperature studies at atmospheric pressure. This makes possible a reliable estimation of a critical exponent φ = 1 − α ≈ 0.88 which accounts for the critical anomaly. Discrepancies between the ε(P ) and the ε(T ) behaviour may be associated with different positions of isothermal pressure and isobaric temperature paths of approaching the critical consolute point. Introduction. – The behaviour of dielectric permittivity (ε) when approaching the critical consolute point in binary solutions has been the subject of studies already for nearly 6 decades [1-15]. The reason for such a long-standing interest was probably the lack of congruence both among experimental results and between theory and experiment (see [1-9] and references therein). This was undoubtedly associated with the weakness of the ε(T ) critical anomaly, comparable to that of the specific heat (cp ∝ (T − TC)), the diameter of the coexistence curve (d ∝ (T − TC)) or the density (ρ ∝ (T − TC)), where TC denotes the critical temperature and α ≈ 0.11 is the critical exponent of specific heat (see [16, 17] and references therein). It is noteworthy that for ε(T ) and ρ(T ) the critical effect in the homogeneous phase manifests itself in “bending down” from the almost linear behaviour far from the upper critical consolute temperature TC. In 1989 Thoen et al. [9] found that for lower frequencies (f) a strong, “parasitic” contribution due to the Maxwell-Wagner (MW) effect may antidote. It caused that for lower frequencies ε(T ) “bends up” from the almost linear behaviour far from TC. This implies that the critical anomaly predicted by theoretical models [5, 6], ε(T ) = εc + Ct+ C1t 1−α + C2t 1−α+∆1 + . . . , T > TC , (1) where εc is the dielectric permittivity at the critical point, C, C1, C2 are amplitudes: C1 is the critical amplitude and C2 is the amplitude of the first correction-to-scaling term [16] with