The application of a shear mode piezoelectric sensor to monitoring the high-pressure phase behaviour of asymmetric binary systems

We demonstrate for the first time that high-pressure phase diagrams can be measured in a simple, objective and fully automated manner for complex binary fluid mixtures. Supercritical CO2 is becoming of increasing industrial importance. Many of the important binary systems with CO2 are highly asymmetric, and exhibit a variety of the phase behaviour from liquid–liquid immiscibility to discontinuous critical loci. We investigate the possibility of mapping these different types of phase diagram using a single shear-mode piezoelectric sensor. We focus on two systems (1-octanol + CO2 and 2-octanol + CO2). We show that the existence of three phases (liquid–liquid–vapour) can be detected by the presence of two discontinuities when the impedance minimum of the sensor is measured as a function of fluid volume. This method is used to measure the three-phase coexistence curve quantitatively for the 1-octanol + CO2 system and the result is in good agreement with literature data. By analyzing the impedance minimum along the p–T phase boundary, we have found that the vapour–liquid critical locus of 2-octanol + CO2 is interrupted by the occurrence of a previously unobserved liquid–liquid immiscibility. For both systems, we have used the critical points, located either by the isochoric method or extrapolated from composition/T/p phase envelope to construct a p,T-projection with characteristic features which allow the particular mixture to be classified to its appropriate type (i.e., 1-octanol + CO2, Type III; 2-octanol + CO2, Type IV).