`LOCAL COMPOSITION' EFFECTS IN WATER/ALCOHOL MIXTURES
CONTAINING ORGANIC SALTS. Joseph T. Slusher and Peter T. Cummings*. Division
of Physical and Chemical Properties, A111 Physics, Gaithersburg, MD 20899,
and *Department of Chemical Engineering, University of Tennessee,
419 Dougherty Engineering Bldg., Knoxville, TN 37996-2200, USA.
Estimation and correlation of the physical properties
of mixed-solvent electrolyte solutions is important because of their relevance
to, e.g., salt-effect distillation, ion-pair extraction operations,
ion-pairing chromatography, and electrochemical and biological systems.
In this work, we have studied the effect of tetraalkylammonium
bromides on the vapor-liquid equilibria of water/2-propanol mixtures, using
a simple Othmer-type recirculating still. In these systems, the enhancement
factor 
(where 
and 
are relative volatilities of the alcohol with and without salt, respectively)
changes sign with changing solvent composition. The molecular origins of
this behavior, which is unusual compared to inorganic salts (for which 
is
typically independent of solvent composition) was investigated using molecular
dynamics (MD) simulations of the liquid mixtures at 25o C. The
molecular distribution functions obtained via MD showed that both unlike
ion-pairing and preferential solvation of the ions are strong functions
of solvent composition in these solutions. Failure of existing engineering
models to predict (based on binary data) the salt-effect on VLE in
these systems is linked to the apparently complex dependence of `local compositions'
of the solvents around the ions found via MD.