Excess molar enthalpies for mixtures of carbon dioxide + a modifier (5 mol% methanol or 1-octanol) and hexane at 308.15 K and 12.40 MPa

The effect of adding a 5 mol% alcohol modifier to carbon dioxide being mixed with hexane at 308.15 K and 12.40 MPa is investigated using an isothermal high-pressure flow calorimeter. Excess molar enthalpies (HmE) are determined for the ternary mixtures [CO2(x1) + methanol(x2) + hexane(x3)] and [CO2(x1) + octanol(x2) + hexane(x3)] where x1/x2 = 95/5 at 308.15 K and 12.40 MPa. The ternary excess enthalpy data are then compared to excess enthalpy data for CO2 + hexane mixtures. Mixing becomes more endothermic for both alcohols; the HmE maximum value increases ca. 200 J mol−1. Excess enthalpy data for the methanol + hexane and octanol + hexane mixtures at 308.15 K and 12.40 MPa are also reported. For both alcohols, mixtures show endothermic mixing: HmE maximum values are ca. 500 J mol−1. HmE values are discussed in terms of the critical locus and phase equilibria of the systems involved. Binary excess enthalpy data for the five binary systems involved in this study were correlated using both a Padé function and the Peng–Robinson equation of state. These correlations are used to predict ternary data. Predictions obtained using the Peng–Robinson EOS in combination with the classical mixing rule are much more accurate than those obtained using the Padé function.