The stir-settle approach to semiautomated light scattering determination of liquid-liquid coexistence curves

Critical opalescence turbidity masks the precise location of liquid-liquid coexistence curve temperatures (T_cx) in light scattering data collected from stirred samples using semiautomated instrumentation. A modified semiautomated technique in which a stirred sample is fully equilibrated and then light scattering data is collected as a function of time with no stirring is described. Tests on three systems (aniline + hexane, methanol + cyclohexane, and isobutyric acid + water) showed that coalescence and sedimentation in the unstirred two-phase sample causes distinct temporal changes in the light scattering, most noticeably in light collected 2° off-incidence. With this experimental method, the coexistence curve temperatures of near-critical samples can be identified with ± 0.01 K uncertainty. Precise determination of T_cx can also be hampered by long mixing times. Equilibration lag measurements as a function of sample composition and ramping rate are also reported. These measurements show that the equilibration lag increases as the system composition moves away from the critical composition. In addition, the lag is asymmetric with respect to T_cx: equilibration of two phases into one phase requires more time (and results in a greater lag) than separation of one phase into two phases.