entropy Article Thermodynamic Derivation of Scaling at the Liquid–Vapor Critical Point Juan Carlos Obeso-Jureidini , Daniela Olascoaga and Victor Romero-Rochín * Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, Ciudad de México 01000, Mexico;
[email protected] (J.C.O.-J.);
[email protected] (D.O.) * Correspondence: romero@fisica.unam.mx; Tel.: +52-55-5622-5096 Abstract: With the use of thermodynamics and general equilibrium conditions only, we study the entropy of a fluid in the vicinity of the critical point of the liquid–vapor phase transition. By assuming a general form for the coexistence curve in the vicinity of the critical point, we show that the functional dependence of the entropy as a function of energy and particle densities necessarily obeys the scaling form hypothesized by Widom. Our analysis allows for a discussion of the properties of the corresponding scaling function, with the interesting prediction that the critical isotherm has the same functional dependence, between the energy and the number of particles densities, as the coexistence curve. In addition to the derivation of the expected equalities of the critical exponents, the conditions that lead to scaling also imply that, while the specific heat at constant volume can diverge at the critical point, the isothermal compressibility must do so. Keywords: liquid–vapor critical point; scaling hypothesis; critical exponents Citation: Obeso-Jureidini, J.C.; 1. Introduction Olascoaga, D.; Romero-Rochín, V. The full thermodynamic description of critical phenomena in the liquid–vapor phase Thermodynamic Derivation of transition of pure substances has remained as a theoretical challenge for a long time [1].