Ionic liquids are highly polar solvents with negligible vapour pressure and low flammability that offer a potentially “green” alternative to volatile organic compounds (VOCs). However in order to confidently label this class of solvents as “green”, their affect on the environment must be thoroughly examined. As a result various toxicity, ecotoxicity and biodegradation studies have been carried out on ionic liquids. Although toxicity evaluations of ionic liquids have been widely reported in the literature, biodegradation data are comparatively scarce. In this tutorial review we present an overview of studies into the biodegradability of ionic liquids, including the various methods of biodegradation assessment, trends observed for structurally related ionic liquids, and applications of biodegradable ionic liquids in synthetic chemistry.

Although ionic liquids are generally referred to in a ‘green’ context, little is known about their degradation in the environment. Part I of this series focused on the biodegradability of the commonly used dialkylimidazolium ionic liquids and the effect of the imidazolium cation on biodegradability. We now report the influence of the anion on biodegradability. Preliminary investigations into the toxicology of ionic liquids are also described.

The design, preparation and evaluation of biodegradable ionic liquids containing ester or amide groups in the alkyl side chain are presented. Factors improving the biodegradation of surfactants were successfully applied to ionic liquids. These novel ionic liquids can be prepared from readily available starting materials in high yield. The introduction of a group susceptible to enzymatic hydrolysis greatly improves the biodegradation (OECD 301D ‘Closed Bottle Test’) compared with the commonly used dialkylimidazolium ionic liquids, bmimBF₄ and bmimPF₆ . For the 3-methyl-1-(alkyloxycarbonylmethyl)imidazolium bromide series, the greatest biodegradation was observed when alkyl = butyl, pentyl, hexyl and octyl. The corresponding amide analogs proved to be poorly biodegradable.

The importance of biodegradation data as part of the design of safer chemicals is presented using ionic liquids (ILs) as a model study.

Further research toward the discovery of biodegradable ionic liquids (ILs) is described herein. The biodegradability of the target ILs was evaluated using the ‘Closed Bottle’ and ‘CO₂ Headspace’ tests (OECD 301D and ISO 14593). This research has identified the first ILs which can be classified as ‘readily biodegradable’ under aerobic conditions.