Excess molar enthalpies of methanol–cyclohexane and methanol–benzene from 454.2 to 523.0 K and up to 4 MPa

Abstract

Flow calorimetric measurements of the excess molar enthalpy, H^E_m, of [0.5CH₃OH + 0.5C₆H₁₂](g) and [0.5CH₃OH + 0.5C₆H₆](g) are reported. The measurements extend over the temperature range 454.2 to 523.0 K and pressures up to 4 MPa. Residual molar enthalpies of cyclohexane and benzene were calculated from the equation of state proposed by Kubic, and the residual molar enthalpy of methanol was calculated from a modification of Kubic's equation proposed by Massucci and Wormald. For methanol in its interaction with cyclohexane there is no hydrogen bonding, and the excess molar enthalpy can be fitted using the same one-fluid mixing rules and pseudo-critical constants for methanol which fitted H^E_m measurements on mixtures with n-pentane and n-hexane. However, for methanol–benzene the values of H^E_m calculated by this method are too large. Measurements of H^E_m for steam–cyclohexane and steam–benzene reported previously showed similar behaviour, and the difference between the experimental values of H^E_m for steam–benzene and those calculated from an equation of state was attributed to association between the water and benzene molecules. Using a similar approach we attribute the difference between the experimental and calculated values of H^E_m for methanol–benzene to weak association between the unlike molecules, and obtain a value of the equilibrium constant K₁₂(298.15 K)= 0.1773 MPa^–1 and ΔH₁₂=–14.7 kJ mol^–1 for the energy of the specific interaction between methanol and benzene.