THE COLLINEARITY OF ISOCHORES AT SINGLE-PHASE AND 2-PHASE BOUNDARIES FOR FLUID MIXTURES
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abstract
Fluid isochores for mixtures of fixed overall composition generally change slope on passing accorss the isopleth (dew-bubble point curve, DBC) from the homogeneous to heterogeneous phase region on a pressure/temperature diagram. A thermodynamic proof is given which shows the isochores to be always collinear at the cricondentherm (or any temperature extremum), rather than at the mixture critical point. The proof agrees with a different, more mathematical proof given earlier by Griffiths. These thermodynamic proofs are supported by our new, high-precision density data for the CH//4/CO//2 and N//2/CO//2 equimolar binaries in addition to previously published measurements for **3He/**4He and for H//2/CH//4 at Duke University and Rice University, respectively. Collinearity of the isentropes at the cricondenbar is also demonstrated and supported by recent calculations using our CH//4/CO//2 data. Because fluid isochores for both pures and mixtures in the homogeneous region are approximately linear, most accurate equations of state (EOS) begin with this premise and then add correction terms for curvature. The present results contain thermodynamic constraints for such EOS.
