The contour of excess molar enthalpy at a mole fraction of benzene x = 0.548 on the p-T plane in liquid state and near the critical points for the (benzene + cyclohexane) mixture
Introduction
The excess enthalpy HEm is important in order to understand the mixed state of fluids and intermolecular interaction. The HEm has been investigated for various binary mixtures under different conditions of temperature and pressure. The behaviour of the HEm of binary mixtures in the liquid state, near the critical points and gas state is quite different each other. The purpose of our study is to understand the total behaviour of HEm in those states. In previous studies, we approached this research idea using both theoretical and experimentally tools [1], [2], [3], [4]. In our theoretical investigations, we calculated the HEm in a model of a binary mixture interacting with the Lennard-Jones potential. This was accomplished by applying the Percus-Yevick integral equation for a wide range of temperature and pressure including the liquid state, near the critical points and the gas state [2]. The composition dependences of HEm were a parabolic curve in the liquid and gas states but were a distorted curve or an S-shaped curve near the critical points. Furthermore, near the critical points the temperature dependences of HEm had a local maximum and local minimum. The process of the change in the behaviour of the HEm accompanying state transitions and the origin of the specific behaviour of HEm near the critical points were discussed in detail.
In the experimental studies, we constructed a twin flow-type mixing calorimeter and measured the HEm of a (benzene + cyclohexane) mixture in the liquid state [3] and near the critical points [4]. The composition dependences of HEm in the liquid state were a positive parabolic shape. The maximum values of HEm over a range of compositions decreased as the temperature increased and were nearly independent of the pressure. The composition dependences of HEm near the critical point were a skewed curve or S-shaped curve, which is consistent with the theoretical result. The pressure dependences exhibited two local maxima near the critical points. In this study, we measured the HEm of the (benzene + cyclohexane) mixture near the vapour pressure curve of benzene and in the intermediate region between the liquid state and the state near the critical points. In order to understand the whole picture of HEm, we created the contour of HEm on the p-T plane over a wide range of temperature and pressure including near the critical points from the result of this study and the results of two our previous studies [3], [4]. Based on this contour, the three-dimensional behaviour of HEm was discussed with respect to the vapour pressure curves and the critical points.
Section snippets
Experimental
The calorimeter used in this work was a twin flow-type mixing calorimeter, which was described in the previous report [3]. The benzene and cyclohexane used in this work were special grade chemicals produced by Wako Pure Chemical Ind. Ltd., without further purifications. In Table 1, the mole fraction purities of benzene and cyclohexane are summarized [3].
The HEm of the (benzene + cyclohexane) mixture were measured at the mole fraction of benzene x = 0.548. The mole fraction was calculated as
The HEm measurements
Table 2 gives the HEm measurements of the (benzene + cyclohexane) mixture at x = 0.548. In this table, Uc(HEm) = 0.032 kJ·mol−1 at T = 568.2 K and p = (5.15 and 5.24) MPa is greater than Uc(HEm) = 0.015 kJ·mol−1 at the other temperatures and pressures. As shown in Fig. 1, T = 568.2 K and p = (5.15 and 5.24) MPa are in the vicinity of the extended line of the vapour pressure curve of benzene, and are extremely close to its critical point. In these two states, benzene does not undergo phase
Conclusion
We measured the HEm(x = 0.548) of the (benzene + cyclohexane) mixture near the vapour pressure curve of benzene and in the intermediate region between the liquid state and the state near the critical points. The pressure dependences at T = (478.2 to 573.2) K and the temperature dependences at p = (5.00 to 25.00) MPa for the HEm(x = 0.548) measured in this study and two previous studies were explained in detail. Using these results, we created the contours of HEm(x = 0.548) on the p-T plane over
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