(345d) Prediction of Interfacial Tensions Between Demixed Solvents

The interfacial
tension between demixed fluids plays an important role in many applications
ranching from extraction over extractive distillation to interfacial chemical
reactions. All of these processes require mass transport over the formed
interface in a ternary system. Selective enrichment of one component in the
interface can have a large impact on the mass transport. Unfortunately, the
concentration profile in the interface can not be obtained by experimental
investigations. For this reason, a theoretical framework can be helpful for the
design and optimization of these processes.

Interfacial
properties can be handled using the density gradient theory[1],
where the Helmholtz-energy of the inhomogeneous system is developed in a Taylor
series. This theory was developed for the description of the interface established
between a liquid and a vapor phase. At constant temperature and pressure the
densities and the composition will be changed during this interface. If it is
assumed that the equilibrium between two liquid phases (LLE) does not depend on
pressure then only the concentration will be changed during the interface
between demixed liquids. Using this approach for binary mixtures the density
gradient theory can also applied for the description of the interface caused by
the LLE[2]. In
this situation the density gradients will be replaced by the concentration
gradients and the Helmholtz-energy by the Gibbs- energy.

In the suggested
contribution this theoretical framework of binary systems is extended to more
relevant ternary systems. The two selected ternary systems (system 1: water +
butan-1-ol + ethanol; system 2: water + acetone + toluene) show one demixing
gap starting from one binary sub-system. The first step in the theoretical
framework is the calculation of the corresponding LLE using the
Koningsveld-Kleintjens[3]
model. For these systems the final equation for the interfacial tension
contains only one adjustable parameter. This parameter is fitted to the
interfacial tension of the corresponding binary subsystem. The predicted
interfacial tensions of the ternary system are compared with experimental data
from the literature[4] and with own experimental
data obtained by spinning drop measurements. Both systems differ in the
enrichment behavior. Whereas ethanol will be enriched in the interface to a
small amount, the enrichment of acetone is much more pronounced.