Near-critical mixtures of limited miscibility are significant for chemical physics, soft matter physics, and a variety of challenging applications. Their basic properties can be tuned by compressing or a systematic change of one of the components. This report addresses these issues, based on experimental studies in nitro-compound (nitrobenzene, o -nitrotoluene, and 1-nitropropane) and n -alkane (from pentane to eicosane) critical mixtures. Studies reveal new patterns for the evolution of the critical consolute temperature ( T C ) and concentration ( x C , mole fraction) within the tested homologous series: T C ( n ) ∼ n 2 and x C ( n ) ∼ n 1/2 . They also show two paths of the high-pressure impact: (i) d T C ( P )/d P > 0 and overlapping of normalized T C ( P ) dependences and (ii) the crossover d T C ( P )/d P < 0 → d T C ( P )/d P > 0 with increasing n -alkane length. The consistent parameterization of all T C ( P ) dependencies is introduced. Supplementary nonlinear dielectric effect studies indicate a possible molecular origin of the phenomenon. The coexistence curve under high pressure is in the agreement with the isomorphism postulate for critical phenomena but with a surprisingly strong distortion from the Cailletet–Mathias law of the rectilinear diameter. The new and reliable method for estimating the critical concentration and temperature is proposed. It explores the analysis of relative volumes occupied by coexisting phases.
The transition from the isotropic (I) liquid to the nematic-type (N) uniaxial phase appearing as the consequence of the elongated geometry of elements seems to be a universal phenomenon for many types of suspensions, from solid nano-rods to biological particles based colloids. Rod-like thermotropic nematogenic liquid crystalline (LC) compounds and their mixtures with a molecular solvent (Sol) can be a significant reference for this category, enabling insights into universal features. The report presents studies in 4′-methoxybenzylidene-4-n-butylaniline (MBBA) and isooctane (Sol) mixtures, for which the monotectic-type phase diagram was found. There are two biphasic regions (i) for the low (TP1, isotropic liquid-nematic coexistence), and (ii) high (TP2, liquid-liquid coexistence) concentrations of isooctane. For both domains, biphasic coexistence curves’ have been discussed and parameterized. For TP2 it is related to the order parameter and diameter tests. Notable is the anomalous mean-field type behavior near the critical consolute temperature. Regarding the isotropic liquid phase, critical opalescence has been detected above both biphasic regions. For TP2 it starts ca. 20 K above the critical consolute temperature. The nature of pretransitional fluctuations in the isotropic liquid phase was tested via nonlinear dielectric effect (NDE) measurements. It is classic (mean-field) above TP1 and non-classic above the TP2 domain. The long-standing problem regarding the non-critical background effect was solved to reach this result.
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