Excess molar enthalpies of ethanol–hexane, ethanol–cyclohexane and ethanol–benzene from 453.5 to 522.9 K and up to 4.5 MPa

Abstract

Flow calorimetric measurements of the excess molar enthalpy, H^E_m, of [0.5C₂H₅OH + 0.5C₆H₁₄](g), [0.5C₂H₅OH + 0.5C₆H₁₂](g) and [0.5C₂H₅OH + 0.5C₆H₆](g) are reported. The measurements extend over the temperature range 453.5 to 522.9 K and are at pressures up to 4.5 MPa. Residual molar enthalpies of hexane, cyclohexane and benzene were calculated from the equation of state proposed by Kubic, and the residual molar enthalpy of ethanol was calculated from a modification of Kubic's equation proposed by Massucci and Wormald. For ethanol in its interaction with hexane or cyclohexane there is no hydrogen bonding, and the excess molar enthalpy can be fitted by applying simple one-fluid mixing rules using a pseudo-critical temperature of 345 K and a pseudocritical pressure of 4.65 MPa for ethanol. For ethanol–benzene the values of H^E_m calculated by this method are too large. Previous measurements of H^E_m for benzene with steam or methanol showed similar behaviour, and the difference between the experimental values of H^E_m and those calculated from an equation of state was attributed to association between the steam or methanol and the benzene molecules. Using a similar approach we attributed the difference between the experimental and calculated values of H^E_m for ethanol–benzene to weak association between the unlike molecules, and obtained a value of the equilibrium constant K₁₂(298.15 K)= 0.2197 MPa^–1 and a value of ΔH₁₂=–14.0 kJ mol^–1 for the enthalpy of the specific interaction between ethanol and benzene.