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Aromatic moments determine solubility in ionic liquids

Researchers propose new rule for predicting liquid-liquid solubility
Aromatic moments determine solubility in ionic liquids

Charge distribution in benzene and derivatives

Oeiras, 16.09.07

In a paper recently published in the Journal of Physical Chemistry, researchers from the Molecular Thermodynamics Laboratory propose a new rule of thumb to determine beforehand the mutual solubility of two liquid compounds.
In this work, the spatial distribution and intensity of electrical charges – something the chemists call dipole and quadrupole moments - in the benzene molecule, and derivative compounds, are quantitatively correlated to the solubility of each compound in an archetypal ionic liquid. The authors suggest that the underlying concept should now be tested in other liquid solutions as long as asymmetric distribution of charges plays a major role in intermolecular interactions.
Looking into the solubility of aromatic compounds in ionic liquids, researchers started by using the simplest aromatic molecule, benzene, and its twelve fluorinated derivatives - with distinct molecular dipole and quadropole moments - to see, by molecular modeling simulation, how they interact with the anion and the cation of the ionic liquid. In addition  to the hydrogen/fluor substitution effect, the thirteen compounds differ from each other in the spatial distribution and intensity of charges within the molecule and, as expected, the anions of the ionic liquid are preferentially found near the more positively charged regions of the aromatic compounds; the opposite is true for the cations. Electrical multipoles are thus acting as templates for the structuration of the solvent around the solute. When the solubility of the benzene derivative compounds in ionic liquids was experimentally determined, a direct quantitative correlation could be observed with the dipole and quadropole moments.

Ionic liquids are considered alternative, “green”, recyclable solvents because of their amazing solvent quality, low vapour pressure, non-flammability, and chemical stability. Understanding how these liquid salts interact with e.g. aromatic compounds – typically used in the chemical industry – is another step towards their generalized potential use.


Original paper

J. Phys. Chem. B, 2009, 113 (29), pp 9894–9900

On the Role of the Dipole and Quadrupole Moments of Aromatic Compounds in the Solvation by Ionic Liquids

Karina Shimizu, Margarida F. Costa Gomes, Agílio A. H. Pádua, Luís Paulo N. Rebelo and José N. Canongia Lopes



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