Density and Pressure Dependence of the Critical Temperature near the Consolute Point of Isobutyric Acid (COOD)/Deuterium Oxide

Abstract

AbstractMeasurements of density of an isobutyric acid (COOD)/D2O mixture of critical composition (upper critical point; critical composition: xDA,c = 0.113; xDA: mole fraction of acid; critical temperature: Tc = 45.738°C) as function of temperature in the one fluid phase region are reported. The function p = P1 + P2t + P3t(1 ‐ α) is fitted to the data (p: density; P1, P2, P3: constants; t: reduced temperature difference (T–Tc)/Tc; α: critical exponent, α = 0.11). P3 has a positive value. Consequently, the coefficient dTc/dP (pressure dependence of critical temperature) has to be negative for thermodynamic reasons. This is confirmed by direct measurements. The experimentally determined values of P1, P3 and dTc/dP together with the value of the correlation length of local concentration fluctuations taken from the literature are consistent with the predictions of the amplitude relation of two‐scale factor universality. — For isobutyric acid/H2O, D2O mixtures of critical composition there exists a linear relation between the coefficient dTc/dP and the equivalent fraction of exchangeable protons Y (=nH/(nH + nD)). dTc/dP decreases from dTc/dP = −40.9 mK bar−1 for DA/D2O (Y=0) to dTc/dP = −55.0 mK bar−1 for HA/H2O (Y=1).