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Environment

Lowering the Viscosities of Ionic Liquids

Substitution with silyl groups proves effective

by Michael Freemantle
November 10, 2005

A barrier to widespread adoption of room-temperature ionic liquids as solvents is that they are much more viscous than common organic solvents.

Incorporating silicon is a way to reduce the viscosity, a new study shows.

Hideaki Shirota and Edward W. Castner Jr. at Rutgers, the State University of New Jersey, prepared ionic liquids with trimethylsilylmethyl-substituted imidazolium cation (shown) and compared their room-temperature viscosities with those of ionic liquids with the analogous neopentyl group (J. Phys. Chem. B., 2005, 109, 21576). The viscosities of the Si-containing liquids are lower than those of the neopentyl analogs by a factor of 7.4 and 1.6, depending on the anion: tetrafluoroborate and bis(trifluoromethylsulfonyl)imide, respectively.

The researchers also explored the ultrafast intermolecular interactions and dynamics of the ionic liquids and obtained the electronic structures of the individual ions. They find that the alkylsilyl group is larger and more polarized than the alkyl group and that the net electrostatic interactions between cations and anions are weaker for the alkylsilyl cations than for the alkyl cations. As a result, viscosities are reduced.

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