The accuracy of liquid-liquid phase transition temperatures determined from semiautomated light scattering measurements

摘要

The synthetic-method determination of liquid-liquid coexistence curves using semiautomated light scattering instrumentation and stirred samples is based on identifying the coexistence curve transition temperatures (T _(cx)) from sudden changes in turbidity associated with droplet formation. Here we use a thorough set of such measurements to evaluate the accuracy of several different analysis methods reported in the literature for assigning T_(cx). More than 20 samples each of weakly opalescent isobutyric acid+water and strongly opalescent aniline+hexane were tested with our instrumentation. Transmitted light and scattering intensities at 2°, 24°, and 90° were collected simultaneously as a function of temperature for each stirred sample, and the data were compared with visual observations and light scattering theory. We find that assigning T_(cx) to the onset of decreased transmitted light or increased 2° scattering has a potential accuracy of 0.01 K or better for many samples. However, the turbidity due to critical opalescence obscures the identification of T_(cx) from the light scattering data of near-critical stirred samples, and no simple rule of interpretation can be applied regardless of collection geometry. At best, when 90° scattering is collected along with transmitted or 2° data, the accuracy of T_(cx) is limited to 0.05 K for near-critical samples. Visual determination of T_(cx) remains the more accurate approach in this case.