Micelle formation of imidazolium ionic liquids in aqueous solution

Ionic liquids (ILs) form a new class of compounds with a much varied industrial and technological application. Some research has been conducted in ecotoxicity of these compounds, but very little is known about their structure in aqueous solutions. It is known that some ionic liquids can form aggregates in aqueous solution. To determine how such structures influence the use, distribution and toxicity of ILs, first the critical micelle concentrations of the ILs need to be determined. Therefore the critical micelle concentration (CMC) for a number of imidazolium ionic liquids was determined in aqueous solution. The expected dependency of the CMC on the length of the alkyl chain of the investigated 1-alkyl-3-methylimidazolium ILs, was shown. The degree of ionization and the energy of aggregation were calculated and related to the alkyl chain length of the imidazolium. A significant correlation between retention times in reversed phase gradient HPLC, and the pCMC was observed. It was also shown that the CMC is directly related to the lipophilicity of the cation. (c) 2007 Elsevier B.V. All rights reserved.