Extended equations of the critical isotherm and the effect of gravity

О. Д. Альохін, Б. Ж. Абдікарімов, О. М. Бурмістров, Ye. G. Rudnikov

2020

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DOI 10.18524/0367-1631.2019.57.191903
Data confidence 70 medium
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Abstract

The model of the system in a critical state as Van der Waals gas of fluctuations of the order parameter at the critical temperature of substance, both homogeneous systems and inhomogeneous systems in the Earth's gravity field, has been approved. This model is in agreement with modern physical ideas on the critical state of substance and takes into account: 1) internal volume of fluctuations of the order parameter; 2) the attractive forces between fluctuations at distances greater than the correlation length; 3) the presence of complexes of quasiassociations of fluctuations of the order parameter when the system moving away from the critical point; 4) the role of fluctuations of the entropy in the form of the algebra of fluctuating variables. Based on the fluctuation theory of phase transitions and on the Van der Waals model of gas of fluctuations of the order parameter for the system, the extended equations of state have been proposed and analyzed at the critical temperature of both low-temperature (ethane and carbon dioxide) and high-temperature molecular liquids (benzene and n-heptane). Wherein, the experimental data of P-V-T measurements for homogeneous systems and the data of the phenomenon of the effect of gravity under the Earth's gravity field for inhomogeneous systems have been used for specified liquids. A comparative analysis of the data of P-V-T measurements and the data of phenomenon of the effect of gravity allowed to conclude that the amplitude of the asymptotic term of the equation of the critical isotherm for homogeneous system significantly exceeds the corresponding amplitude of the equation of the effect of gravity at the critical temperature. This result is explained in the paper by the appearance of an internal inhomogeneous critical field in the system under the action of the Earth's gravity field in the direction of the gravity field. It has also been shown that homogeneous and inhomogeneous molecular liquids under the Earth's field of gravity are characterized by the diametrically opposite field-altitude asymmetry of the extended equation of state at the critical temperature of the substance. This result is related in the paper with altitudinal asymmetry of the internal critical field in inhomogeneous system.