Experimental measurements and theoretical modeling of high-pressure mass densities and interfacial tensions of carbon dioxide + n-heptane + toluene and its carbon dioxide binary systems

Abstract

Experimental determination and theoretical predictions of the isothermal (344.15 K) mass densities and interfacial tensions for the system carbon dioxide (CO2) with heptol (n-heptane + toluene) mixtures varying liquid volume fraction compositions of toluene (0, 25, 50, 75, 100% v/v) and over the pressure range 0.1–8 MPa are reported. Measurements are carried out on a high-pressure device that includes a vibrating tube densimeter and a pendant drop tensiometer. Theoretical modeling of mass densities phase equilibria and interfacial properties (i.e., interfacial tension and interfacial concentration profiles) are performed by employing the Square Gradient Theory using an extension of the Statistical Associating Fluid Theory equation of state that accounts for ring fluids. The experimental bulk phase equilibrium densities and interfacial tensions obtained are in very good agreement with the theoretical predictions. Although there are no previous experimental data of these mixtures at the conditions explored herein, the results follow the same trends observed from experimental data at other conditions. The combination of experimental and modeling approaches provides a route to simultaneously predict phase equilibrium and interfacial properties within acceptable statistical deviations.For the systems and conditions studied here, we observe that the phase equilibrium of the mixtures display zeotropic vapor-liquid equilibria with positive deviations from ideal behavior. The mass bulk densities behave ordinarily whereas the interfacial tensions decrease as the pressure or liquid mole fraction of CO2 increases and/or the ratio toluene/heptane decreases. The interfacial concentration along the interfacial region exhibits a remarkable high excess adsorption of CO2, which increases with pressure and it is larger in n-heptane than in toluene. Toluene does not exhibit any special adsorption activity whereas n-heptane displays surface activity only at low pressure in a very narrow range for the case of CO2 + (25% n-heptane + 75% toluene) mixture.