Associative behavior of ionic liquids in water

Associative behavior of ionic liquids in water

Ionic liquids (ILs) are liquid ionic compounds at room temperature that
have characteristics such as low vapor pressure, thermal stability, good
performance as a solvent for organic, inorganic and polymeric materials,
and can also have their physicochemical properties managed through
changes in their molecular structure. Many ILs present in their structure a
polar part and a nonpolar part that, when in aqueous solution, will have
hydrophobic and hydrophilic behavior, and the result of this interaction
may be an associative behavior between the cations of the system. The
presence and magnitude of this associative behavior is significantly related
to the size of the alkyl chain present in the molecule's structure.
The objective of this work is to evaluate, through computer simulations of
molecular dynamics, the associative behavior of ILs based on the
imidazole ring with different sizes of alkyl side chain solvated in water.
Computer simulations of molecular dynamics were carried out using the
GROMACS software package for ILs 1-butyl-3-methylimidazolium
chloride and 1-hexadecyl-3-methylimidazolium chloride, both solvated in
water. The results obtained by monitoring the minimum distance between
cations demonstrate an associative behavior for a pair of IL molecules that
have a larger 16-carbon alkyl chain, while a pair of IL molecules with a 4-
carbon alkyl chain does not present itself in the associated form in any
period of the simulation. A larger and more complex system containing 20
molecules of 1-hexadecyl-3-methylimidazolium solvated in water was
simulated and monitored. Radial distribution function (RDF) results were
obtained and show us a higher probability of finding the cations interacting
at smaller separation distances during the simulation period. It is assumed
that ionic liquids based on the imidazole ring with chloride as counter-ion
solvated in water demonstrate associative behavior when they have longer
alkyl chains and do not tend to associate when they have short alkyl chains