The liquid–liquid phase diagram of sulfur+biphenyl

Abstract

We have studied the liquid–liquid phase diagram of sulfur+biphenyl. Larkin, Katz, and Scott [J. Phys. Chem. 71, 352 (1967)] reported a liquid–liquid phase separation with an upper critical solution point (UCSP) at 381 K, complete miscibility between 381 K and a lower critical solution point (LCSP) at 494 K, and a polymerization line between the polymerization point of pure sulfur at 432 K and a point on the high temperature coexistence curve near the LCSP. We found that the phase transition temperatures for the higher temperature miscibility gap increased with time. We attribute this shift to chemical reaction between the sulfur and the biphenyl. Therefore, the transition temperatures which we obtain are necessarily upper limits to the true transition temperatures, but are in agreement with the results of Larkin et al. We find that the polymerization line meets the coexistence curve at 0.730±0.005 mole fraction sulfur and 498±2 K, whereas the LCSP, as indicated by the ratio of volumes of coexisting phases, is at 0.750±0.005 mole fraction sulfur and 496±3 K. Such a shift of the LCSP away from the end of the polymerization line is consistent with proximity to a nonsymmetrical tricritical point [Wheeler, J. Chem. Phys. 81, 3635 (1984), and references therein]. If we describe the shape of a branch of the high temperature coexistence curve by ‖x−xc ‖/xc =Bt β, where x is the mole fraction of sulfur, xc is the mole fraction at the critical point, and t is the reduced temperature ‖T−Tc ‖/Tc (with T the temperature and Tc the critical temperature), then the polymer-rich branch for sulfur+biphenyl is described well with β=0.25, the value appropriate for a nonsymmetrical tricritical point. The polymer-poor branch could not be described by any of the values of β thought to be possible (i.e., 0.25, 0.325, 0.50, 0.67, or 1.00). We searched carefully for a three-phase region near the LCSP and found none.