Specific heat of binary liquid mixtures near the critical mixing point

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Abstract
The temperature dependence of specific heat c at constant pressure P of mixtures of critical composition xc of the systems methanol/ n‐heptan and isobutyric acid/water is measured in an isothermal Calvet calorimeter. Near the critical mixing temperature Tc(T > Tc) the temperature dependence of c of both systems can be represented by a logarithmic singularity to a good approximation, i.e. c ∼ ϵ−α; ϵ = (T – Tc)/Tc; 0 ⩽ α ⩽ 0.1 The values of excess specific heat near Tc calculated from the Fixman theory are in satisfactory agreement with the experimental data.Measurements of the temperature dependence of coefficient of thermal expansion αc of the system isobutyric acid/water show that αc diverges weakly. The critical exponent of αc has the same value as that of specific heat within the accuracy of the experiments (0 ⩽ α ⩽ 0.1). This is in agreement with postulates of Griffiths and Wheeler. The singular part of αc of the system isobutyric acid/water is negative (αc ∼‐ϵ−α). From this observation and theoretical considerations of Griffiths and Wheeler it can be predicted that the critical temperature of the system isobutyric acid/water should decrease with increasing pressure. This is in agreement with experimental data reported in the literature.