Determination of the excess enthalpy of binary mixtures from the measurements of the heat of solution of the components: application to the perfluorohexane + hexane mixture

A calorimetric technique and a calculation procedure to obtain excess enthalpies HE of liquid mixtures is described and tested. It is based on a heat-flow calorimeter used as a titration calorimeter to measure the heat effects associated to small additions of one of the components, or a mixture of them, to the ampoule containing a known amount of the other component or of a mixture of them. The procedure takes into account the presence of a small vapour space and calculates the heat involved in the vapour–liquid equilibration after each addition. The experimental values are fitted with a parametric equation such as Redlich–Kister to determine the HE parameters by least-squares adjustment. The technique has been tested with respect to the systems ethanol+cyclohexane, dioxane+water and tetrachloromethane+benzene. The results show a good agreement with literature data for these mixtures, and confirm the capability of this technique to reliably replace the mix-flow technique when the components mix slowly, or show a mixture gap, or are available in small quantities. HE results are reported for the system perfluorohexane+hexane for which the mixing process is slow.