USE OF F-19 CHEMICAL-SHIFTS TO MEASURE DEVIATIONS FROM RANDOM MIXING IN BINARY-SOLUTIONS NEAR THE CONSOLUTE TEMPERATURE - SYSTEM HEXANE-PERFLUOROHEXANE

The 19F chemical shifts of the trifluoromethyl groups have been measured as a function of composition in hexane/perfluorohexane mixtures at 25, 35, 45, and 55 °C. The lowest temperature is just above the reported upper consolute temperature of 22.65 °C. Differences between the observed shifts and values anticipated for hypothetical ideal solutions are ascribed to the known volume increase on mixing and to the persistence of short-range order, with 1-1 and 2-2 contacts preferred over 1-2 contacts. Interpreted in this way, the data at each temperature provide a means of evaluating the quantity w/z appearing in Guggenheim's quasi-chemical treatment and equal to half the energy cost of converting a 1-1 pair and a 2-2 pair into two 1-2 pairs. The effects of imperfectly random mixing found at 55 °C are roughly 60% as large as those at 25 °C. © 1979 American Chemical Society.