Thermodynamic consistency near the liquid-liquid critical point


The thermodynamic consistency of the isobaric heat capacity per unit volume at constant composition C(p,x) and the density rho near the liquid-liquid critical point is studied in detail. To this end, C(p,x)(T), rho(T), and the slope of the critical line (dT/dp)(c) for five binary mixtures composed by 1-nitropropane and an alkane were analyzed. Both C(p,x)(T) and rho(T) data were measured along various quasicritical isopleths with a view to evaluate the effect of the uncertainty in the critical composition value on the corresponding critical amplitudes. By adopting the traditionally employed strategies for data treatment, consistency within 0.01 K MPa(-1) (or 8%) is attained, thereby largely improving the majority of previous results. From temperature range shrinking fits and fits in which higher-order terms in the theoretical expressions for C(p,x)(T) and rho(T) are included, we conclude that discrepancies come mainly from inherent difficulties in determining the critical anomaly of rho accurately: specifically, to get full consistency, higher-order terms in rho(T) are needed; however, the various contributions at play cannot be separated unambiguously. As a consequence, the use of C(p,x)(T) and (dT/dp)(c) for predicting the behavior of rho(T) at near criticality appears to be the best choice at the actual experimental resolution levels. Furthermore, the reasonably good thermodynamic consistency being encountered confirms that previous arguments appealing to the inadequacy of the theoretical expression relating C(p,x) and rho for describing data in the experimentally accessible region must be fairly rejected.



subject category

Chemistry; Physics


Losada-Perez, P; Perez-Sanchez, G; Cerdeirina, CA; Troncoso, J; Romani, L

our authors


This work is dedicated to the memory of Carlos Prieto Martinez, a truly charming man. He and his team at "Construcciones Emilio Surez Galicia S. A.," especially Pedro Vila Taboada, kindly collaborated with our research group. One of us (C.A.C.) is grateful to M. A. Anisimov, J. V. Sengers, and J. Thoen for valuable comments as regards the importance of correction-to-scaling terms. This research was supported by the "Ministerio de Educacion y Ciencia" under the "Programa General de Promocion del Conocimiento" (No. BFM2003-09295). Assistance to P. L.-P. and G. P.-S. by the Ministerio de Educacion y Ciencia under the "Programa Nacional de Formacion del Profesorado Universitario" (Nos. AP-2004-2947 and AP-2005-1959, respectively) is greatly appreciated.

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