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The critical behaviour of the dielectric constant in the polar-polar mixture nitromethane + 3-pentanol. An unusual sign in its critical amplitude in the homogeneous region


J. Leys1, P. Losada-Pérez1, J. Troncoso2, C. Glorieux1 and J. Thoen1

1Laboratorium voor Akoestiek en Termische Fysica, Departement Natuurkunde en Sterrenkunde, Katholieke Universiteit Leuven, Leuven, Belgium
2Departamento de Física Aplicada, Universidad de Vigo (Campus de Ourense), Ourense, Spain

Keywords: critical behaviour, dielectric constant, binary liquid mixtures
property: dielectric constant
material: nitromethane, 3-pentanol

A systematic experimental study of the critical behavior of the dielectric constant ε of binary mixtures comprising different combinations, namely, polar + nonpolar, polar + polar and nonpolar + nonpolar compounds has been carried out motivated by the novel results, i.e a t2β singularity in the coexistence-curve diameter predicted by a recent scaling formulation [1]. In this work, we report on results for the polar + polar compounds, namely, the nitromethane + 3-pentanol binary mixture, for which ε has been measured in both the one and two-phase regions. Special attention will be given to the critical behavior in the one-phase region along the critical isopleth path (x = xc):

ε = εc + D1t1-α+D0t

where t = (T-Tc)/Tc, with Tc being the critical temperature and D1 and D0 the amplitudes of the 1 singularity and the regular linear term, respectively. On thermodynamic grounds, D1 can be expressed as:

D1 = –[(∂Tc/∂E´)p + εc (∂Tc/∂p)]A+/(1-α),

where – applies to upper consolute points (UCPs) and E´ = ε0E2/2, with ε0 being the permittivity of vacuum. A+ is the critical amplitude of the dimesionless isobaric heat capacity Cp,x2 in the one-phase region. (∂Tc/∂p) and (∂Tc/∂E´)p are the dimensionless slopes of the critical lines created by the pressure (at constant E) and the electric field (at constant pressure), respectively. To the best of our knowledge, only positive values for D1, i.e. ε(T) bending downwards near Tc, have been encountered experimentally. That implies a decrease of Tc when applying an external electric field, (∂Tc/∂E´)p < 0. For nitromethane + 3-pentanol, an “unusual” negative sign in the critical amplitude D1, i.e. ε(T) bending upwards near Tc, has been found for the first time in a UCP-type system. This implies that Tc increases when applying an external electric field (∂Tc/∂E´)p > 0. Additional mass density ρ measurements (and preliminary (∂Tc/∂p) ones) have been also carried out to confirm the validity of such experimental finding, which gains importance on recalling relevant studies about the role of electric fields in phase separating binary mixtures [2]. Evidence for the t2β singularity in the coexistence-curve diameter will also be provided.

References
  1. P. Losada-Pérez, G. Pérez-Sánchez, C. A. Cerdeiriña, and J. Thoen, Phys. Rev. E 81, 041121 (2010)

  2. Y. Tsori, Rev. Mod. Phys. 81, 1471 (2009)

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