Aggregation Behavior of Ionic Liquids in Aqueous Solutions: Effect of Alkyl Chain Length, Cations, and Anions Tejwant Singh and Arvind Kumar* Central Salt and Marine Chemicals Research Institute, BhaVnagar 364002, India ReceiVed: April 5, 2007; In Final Form: May 17, 2007 Self aggregation of the ionic liquids, 1-butyl-3-methylimidazolium chloride [C 4 mim][Cl], 3-methyl-1- octylimidazolium chloride [C 8 mim][Cl], 1-butyl-3-methylimidazolium tetrafluoroborate [C 4 mim][BF 4 ], N-butyl- 3-methylpyridinium chloride [C 4 mpy][Cl], in aqueous solution has been investigated through 1 H nuclear magnetic resonance (NMR) and steady-state fluorescence spectroscopy. Aggregation properties were determined by application of mass action theory to the concentration dependence of 1 H NMR chemical shifts. Aggregation properties showed fairly good agreement with the previously reported results obtained from small angle neutron scattering, conductivity, and surface tension measurements. A detailed analysis of chemical shifts of water and various protons in ILs has been employed to probe the aggregate structure. Fluorescence spectroscopy provided important information about the critical aggregation concentration (cac) and the microenvironment of the aggregates. We could also observe a break point quite consistent with that of 1 H NMR and fluorescence spectroscopy at cac from the concentration dependence of refractive index measurements. Standard free energies of aggregation ΔG m o of various ILs derived using the refractive index/concentration profiles were found comparable to those of classical ionic surfactants. 1. Introduction Room-temperature ionic liquids (ILs), due to their unique physicochemical properties, are rapidly gaining interest as greener replacements for traditional volatile organic solvents used in chemical processes and have been the focus of many scientific investigations. 1-17 Some recent investigations have been made on a more molecular level, where the aggregation behavior of 1-methyl-3-alkylimidazolium salts in aqueous solution was investigated. 18-22 These investigations show that nano-inhomogeneity can be generated in aqueous solutions of ILs due to the amphiphilic nature of their cations. Self- aggregation similar to that of short-chain cationic surfactants has been observed in aqueous solutions of small-chain alkyl imidazolium-based ILs by Bowers et al. 21 through the measure- ments of small angle neutron scattering, conductivity, and surface tension in [C 4 mim][BF 4 ], [C 8 mim][Cl], and[C 8 mim][I]. They could establish the role of the chain length and anion on the aggregation behavior of these ILs in aqueous solution. However, nature of aggregates formed in the [C 8 mim][Cl] system could not be explained in a great detail because the SANS is dominated by the inter particle structure factor which does not contribute significantly to the scattering of aqueous solution of [C 8 mim][Cl] even for concentrations higher than critical aggregation. Therefore, further investigations are needed to define the role of alkyl chain length, anion and cation more specifically on the shape and size of the aggregates formed by ILs in the aqueous solutions. NMR, which is a very sensitive technique, has been used before to determine the critical aggregation concentration for classical surfactants. 23 It could be expected that NMR with its high resolution and large chemical shift range should be well suited for the study of the association of compounds having amphiphilic nature. Indeed it has been demonstrated in recent years that on aggregation there are considerable changes in both shielding and relaxation and it is possible to characterize the effects for a large number of protons in an alkyl chain or aromatic ring. 23-35 In this report 1 H NMR results of aqueous solutions of imidazolium- or pyridinium-based ionic liquids with different alkyl chain lengths and counterions are presented. Ionic liquids comprise solely of cations and anions with considerable intra- molecular hydrogen bonding between the protons of cationic ring and counterions as well as protons of alkyl chain attached to the aromatic ring (imidazolium or pyridinium) and counter- ions. 36-40 In the aqueous solutions, ILs form intermolecular hydrogen bonding with water, and there is a competition for the hydrogen bonding with the aromatic hydrogens of the cation for the solvent molecules and the anions. 41 From these studies it appears that specific interactions between the cation and anion or solvent media occur for the IL. However, the phenomena that affect their chemical shifts in dilute aqueous solutions, in particular, with respect to the post micellar or pre micellar regions is still ambiguous. Here, at aggregation concentrations we have observed a peculiar change in the chemical shift of the various protons, due to the specific arrangement of hydrophobic and hydrophilic domains similar to that of classical cationic surfactants. In this paper the results are deeply interpreted by looking at change in the magnitude of chemical shift of each proton of the ionic liquid, peak broadening or splitting before and after the critical aggregation concentration (cac). On the basis of NMR observations schematic models of aggregates are proposed for different ILs. In addition, steady-state fluorescence spectroscopy and re- fractrometry have been used to determine the cac’s in the aqueous solutions of ILs. In past, analysis of emission spectra from fluorescent probes has provided important details on the * Corresponding author. Telephone: +91-278-2567039. Fax: +91-278- 256-7562/-256-6970. E-mail: mailme_arvind@yahoo.com; arvind@csmcri.org. 7843 J. Phys. Chem. B 2007, 111, 7843-7851 10.1021/jp0726889 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/16/2007