Abstract
A method to measure the critical temperature and critical pressure of substances, in particular, thermally unstable ones, is briefly described. The method is used to measure the temperature of attainable superheat of liquids with the help of a wire probe heated by electric current pulses. As the pressure increases, the temperature of attainable superheat tends to a critical temperature. The duration of the heating pulses is from 0.03 to 1 ms. A list of about 130 substances for which measurements of the critical properties were made is presented. The results of these measurements confirmed the scaling form of the relation between the critical constants of substances consisting of long chain molecules and the number of molecular units. Two methods to extrapolate the experimental data for the critical properties of the initial members of homologous series to heavier polymer homologs are proposed. One of the methods is based on the equation of state for the fluid of chain molecules. In this method, the extrapolating equations are power series in n, where n is the number of main units in a chain molecule. In the other method, the hypothesis of functional self-similarity and the presentation of scaling behavior of the critical constants of long chain molecules are used. Homologous series with the general formula R1 (CH2)nR2, where R1 and R2 are different end groups, are considered. We obtain equations to calculate, with good accuracy, the critical temperature and critical pressure of any member of any homologous series with the molecule structure R1(CH2)nR2 if they are known for one compound belonging to this series.
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Nikitin, E.D., Popov, A.P. Using the phenomenon of liquid superheat to measure critical properties of substances. J. Engin. Thermophys. 16, 200–204 (2007). https://doi.org/10.1134/S1810232807030137
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DOI: https://doi.org/10.1134/S1810232807030137