A new vapor density technique for studying proton donor - acceptor interactions in the gas phase.

Abstract

Vapor density measurements are reported for 2,2,2-trifluoroethanol (TFE) and acetone and for the binary mixtures of TFE with water, methanol, ethanol, and 2-butanol and acetone with water, methanol, ethanol, and 2-butanol. Highly precise P-V-T data were collected with a new vapor density apparatus. An automated version of the apparatus is described. The repeatability of vapor pressure measurements made on replicate data sets is approximately (+OR-) 0.005 torr over a 200 torr range. The proton-donating ability of TFE in the binary mixture is enhanced by increasing molecular weight of ROH (where R=H, CH(, 3), CH(, 3)CH(, 2), or CH(, 3)CH(CH(, 3))). Equilibrium constants for the formation of the ROH: acetone complexes decrease with increasing R. However, the stepwise equilibrium constants for the addition of an acetone monomer to a heteroaggregate increases with increasing R. The contribution of polarizability and inductive effects are discussed in light of these findings. The self-association data are fitted to two-parameter models yielding an equilibrium constant for the formation of the dimer in both TFE and acetone and a stepwise equilibrium constant for the higher-order species in each vapor. Results for the mixed vapors are analyzed to obtain 1:1 heteroassociation constants and stepwise association constants for the addition of either a TFE monomer or acetone monomer to the heteroaggregate. Heats of association are reported for acetone and the acetone mixtures. Liquid-vapor curves for the binary mixtures including TFE are inferred from vapor density measurements obtained beyond the dew point.