PHYSICAL REVIEW E 102, 042610 (2020) High-pressure behavior of dielectric constant in a binary critical mixture Aleksandra Drozd-Rzoska * and Sylwester J. Rzoska † Institute of High Pressure Physics, Polish Academy of Sciences (X-PressMatter Lab), ul. Sokolowska 29/37, 01–142 Warsaw, Poland (Received 24 July 2020; accepted 18 September 2020; published 15 October 2020) An alternative design of the measurement capacitor for high-pressure studies of complex liquids or soft matter systems is presented. Subsequently, results for the precritical anomaly of dielectric constant in 1-nitropropane– octane critical mixture are reported. First, the pressure dependence of the critical consolute temperature T C up to P = 0.55 GPa was determined and portrayed using the derivative-based analysis. Second, temperature and pressure evolutions of dielectric constant on approaching the critical consolute point at (T C = 304.1K and P C = 403 MPa) were studied. They revealed that the pretransitional anomaly ε (P → P C ) is notably more pronounced than for ε (T → T C ). For both paths, the static domain extends even to as low frequency as f = 100 Hz, whereas for tests under atmospheric pressure, they require at least f = 100 kHz. The discussion of the impact of correction-to-scaling terms, including the unique case of the pressure paths, is also presented. DOI: 10.1103/PhysRevE.102.042610 I. INTRODUCTION High-pressure studies of soft matter [1] constitute a chal- lenging area of research, enabling the insight related to the density and free volume changes, whereas temperature in- vestigations are coupled to the activation energy shift [2,3]. For soft matter systems, compressing, even below 1 GPa, can create a significant change of properties [2,3], whereas, for the classical hard matter systems, tens or even hundreds of GPa are required [4–6]. For soft matter systems, critical liquids constitute a unique category, for which the sensitivity to exter- nal disturbations, including compressing, increases infinitely on approaching the critical point. In one-component liquids, there is a single gas-liquid critical point, defined by the crit- ical temperature (T C ) and pressure (P C ). Unique properties in the surrounding of the critical point are associated with multimolecular critical fluctuations, where size (correlation length ξ ) and lifetime (τ fl . ) increase infinitely for T → T C or P → P C [7–10]: ξ (T ) = ξ 0 |T − T C | −ν , τ fl . (T ) = τ 0 |T − T C | −zν , (1) ξ (P ) = ξ P o |P C − P| −ν , τ fl . (P ) = τ P o |P C − P| −zν , (2) where ν denotes the critical exponent of the correlation length and z is the dynamical exponent. For critical mixtures of lim- ited miscibility, discussed below, ν ≈ 0.625 and z = 3, what yields zν ≈ 1.875; ξ 0 and τ 0 are critical amplitudes. The similarity of Eqs. (1) and (2) is related to the iso- morphism postulate of critical phenomena [10], i.e., the parallel description of the precritical behavior for temper- ature and pressure paths of approaching the critical point. Such origins of the precritical behavior led to extraordinary * ola.drozdrzoska@gmail.com † sylwester.rzoska@unipress.waw.pl changes of susceptibility to external perturbations (compress- ibility) χ (P ) ∝|T − T C | −γ +··· (γ ≈ 1.23), the specific heat c p (T ) ∝|T − T C | −α +··· (α ≈ 0.115), or the order pa- rameter M (T ) ∝|T − T C | β +··· (β ≈ 0.325). The order parameter is related to the appearance/disappearance of an el- ement of symmetry: in the given case, related to the difference of density, refractive index, or dielectric constant between two phases [9,10]. The Physics of Critical Phenomena explained that the gas-liquid critical point and binary mixtures of limited mis- cibility with the critical consolute point belong to the same universality class (n OP = 1, d = 3), where n OP and d are for space and order parameter dimensionalities [7–11]. Simple magnetic systems with the Curie point and three-dimensional Ising model associated with the one-component order param- eter also belong to this universality class. All systems within a given universality class are described by the same (universal) values of critical exponents, and ratios of critical amplitudes for different physical properties, and linked via scaling equa- tions [9–11]. However, there is a notable difference between the gas-liquid critical point and the liquid-liquid critical con- solute point in binary mixtures of limited miscibility. The latter is associated with the continuous curve of critical points, emerging as the function of pressure [8,11]. It yields a unique possibility of adjusting the critical consolute temperature and properties of the tested mixture to experimental or application requirements. Amongst the variety of properties tested on approaching the critical consolute point, dielectric constant occupies a unique position [11–33]. This is the basic and classical dielec- tric property, established previously by Faraday [34]. Studies focused on the precritical anomaly of dielectric constant in bi- nary critical mixtures appeared surprisingly puzzling [11–33], and refs. therein, if comparing with similar research for other physical properties. The first clear indications for the de- cisive role of the measurement frequency and the biasing impact of the low-frequency Maxwell-Wegner effect asso- 2470-0045/2020/102(4)/042610(7) 042610-1 ©2020 American Physical Society