Chapter 8
Dynamics of Fast Reactions in Ionic Liquids Alison M . Funston and James F. Wishart
Downloaded by CORNELL UNIV on May 30, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch008
Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973
Pulse radiolysis and laser flash photolysis are complementary tools for studying fast reactions in ionic liquids. Both techniques have been used to study solvation processes in quaternary ammonium ionic liquids, which extend into the nanosecond regime and influence the reactivity and energetics of radiolytically-generated excess electrons. The preparation and properties of ionic liquid families designed to further these studies is reported.
Introduction Ionic liquids form a new class of solvents which have potential uses in academic as well as industrial settings. One advantage of ionic liquids is that they may be tailored by selective combination of different cations and anions to meet the specific needs of an application or technique. A possible application of ionic liquids may be in the nuclear industry. For example, the incorporation of boron or chlorine, good thermal neutron poisons, into an ionic liquid could substantially reduce the risk of nuclear criticality in fuel cycle and radiological waste handling and decontamination operations (1). Experiments on several 102
© 2005 American Chemical Society
In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
Downloaded by CORNELL UNIV on May 30, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch008
103 imidazolium ionic liquids have found their radiochemical stability to be high (2). In order to determine the primary radiolytic species formed in the liquids and their subsequent reactivity, the technique of pulse radiolysis is ideal (3). In this technique a short pulse of ionizing radiation is focused into the solution of interest, producing radical species. The reactions of these species, either in the presence or absence of added solute, may be followed using a variety of techniques such as transient UV-visible and NIR spectroscopy, conductivity or electron spin resonance. The dynamics of very fast processes such as solvation and radical or excited state reactions may be resolved using the LEAF pulse radiolysis facility (4, J). The general processes occurring upon pulse radiolysis of neat ionic liquids have been described in detail previously (6), in addition the primary radiation chemistry (6-8) and the reactivity of the primary species formed in the ionic liquid methyltributylammoniumbis((trifluoromethyl)sulfo ( 0 - / 2 ) has been described in detail. More recently fluorescent solvatochromic probes such as CI53 have also been employed to study the solvation effects in ionic liquids. It was found in these and other studies of the dynamics of radical and excited state reactions in ionic liquids (73, 14) that diffusion and solvation processes in the ionic liquids occur on longer timescales when compared to the usual molecular solvents. This is due in some part to the more viscous nature of the ionic liquids, however it has also been found that the reactions occur at a rate faster than predicted by the diffusion of the reactants through the solvent using the modified Debye equation (15, 16), Equation 1, where the ratio of the radii of the reacting species is approximated as 1:1 £
