Hierarchical Aggregation in a Complex FluidThe Role of Isomeric Interconversion

There is an ever-increasing
body of evidence that metallic
complexes
involving amphiliphic ligands do not form normal solutions in organic
solvents. Instead, they form complex fluids with intricate structures.
For example, the metallic complexes may aggregate into clusters, and
these clusters themselves may aggregate into superclusters. To gain
a deeper insight into the mechanisms at play, we have used an improved
force field to conduct extensive molecular dynamics simulations of
a system composed of zirconium nitrate, water, nitric acid, tri-n-butyl phosphate, and n-octane. The important
new finding is that a dynamic equilibrium between the cis and trans
isomers of the metal complex is likely to play a key role in the aggregation
behavior. The isolated cis and trans isomers have similar energies,
but simulation indicates that the clusters consist predominantly of
cis isomers. With increasing metal concentration, we hypothesize that
more clustering occurs and the chemical equilibrium shifts toward
the cis isomer. It is possible that such isomeric effects play a role
in the liquid–liquid extraction of other species and the inclusion
of such effects in flow sheet modeling may lead to a better description
of the process.