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Properties of ionic liquids--new solvents for green chemistry. (News Briefs).

Ionic liquids are a class of organic salts that are liquid at or near room temperature. Because they are nonvolatile and non-flammable, they have been proposed as recyclable "green solvents." The physical properties of ionic liquids may be tuned, for example, by altering the hydrophobicity, to affect reaction rates and selectivities. However, in spite of the many advantages that these fluids are predicted to offer, fundamental data on their physical and chemical properties are scarce. To provide U.S. industry with the knowledge base to exploit these solvents, NIST researchers have prepared several ionic liquids and have begun to study their effects on rate constants of fundamental chemical reactions.

Rate constants for several oxidation and reduction reactions were measured by the pulse radiolysis technique and compared with the rate constants for the same reactions in other solvents. The rate constants in several ionic liquids are much lower than those determined in aqueous solutions and close to those measured in low-polarity organic solvents such as ethanol. These low rate constants suggest a high degree of ion-association in the ionic liquids. On the other hand, the activation energy of the reaction is close to that measured in aqueous solutions but higher than that in alcohols. These two apparently conflicting results are explained by considering the microenvironments of the reactants, i.e., that each molecule and each site of a large molecule may be solvated by different ions of the ionic liquid.

Rate constants for several reduction reactions also are lower in ionic liquids than in water or alcohols. A striking difference (by four orders of magnitude) was found in one case, due to a combination of the effect of the ionic liquid on the rate of reaction and its effect on the reduction potentials of the two reactants, which in turn affect the rate constants. The effect of ionic liquids on reduction potentials is currently being assessed.

Results of this work can be found in the following references: J. Phys. Chem. A 105, 7607-7614 (2001) and J. Phys. Chem. A 106 (April 2002).

CONTACT: Pedi Neta, (301) 975-5635;
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Publication:Journal of Research of the National Institute of Standards and Technology
Date:Jan 1, 2002
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