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, bmimBF 4 and bmimPF 6 . 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. 6 6G r e e n C h e m . , 2 0 0 4 , 6 , 1 6 6 -1 7 5G r e e n C h e m . , 2 0 0 4 , 6 , 1 6 6 -1 7 5 1 6 7 G r e e n C h e m . , 2 0 0 4 , 6 , 1 6 6 -1 7 5 1 6 8 G r e e n C h e m . , 2 0 0 4 , 6 , 1 6 6 -1 7 5 1 6 9G r e e n C h e m . , 2 0 0 4 , 6 , 1 6 6 -1 7 5 1 7 4

No abstract

Two series of long chain imidazolium- and pyridinium-based ionic liquids containing an ester functional group in the alkyl side chain, 3-methyl-1-alkyloxycarbonylmethylimidazolium bromides (C(n)EMeImBr) and 1-alkyloxycarbonylmethylpyridinium bromides (C(n)EPyrBr), were synthesized and their thermal stability, aggregation behavior in aqueous medium, and antimicrobial activity investigated. The introduction of an ester group decreased the thermal stability of the functionalized ILs compared to simple alkyl chain containing ILs (1-alkyl-3-methylimidazolium bromides and 1-alkylpyridinium bromides). Tensiometry, conductimetry, and spectrofluorimetry were applied to study the self-aggregation of the amphiphilic ILs in aqueous solution. The ILs investigated displayed surface activity and the characteristic chain length dependence of the micellization process of surfactants. As compared to simple alkyl chain containing ILs bearing the same hydrocarbon chain, ester-functionalized ILs possess higher adsorption efficiency (pC(20)) and significantly lower critical micelle concentration (cmc) and surface tension at the cmc (γ(cmc)), indicating that the incorporation of an ester group promotes adsorption at the air/water interface and micelle formation. The antimicrobial activity was evaluated against Gram-negative and Gram-positive bacteria and fungi. ILs containing more than eight carbon atoms in the alkyl chain showed antimicrobial activity. Their efficiency as antimicrobial agents increased with the hydrophobicity of the amphiphilic cation being the C(12) homologous the most active compounds. The incorporation of an ester group particularly increased the biological activity against fungi.

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 'CO 2 Headspace' tests (OECD 301D and ISO 14593). This research has identified the first ILs which can be classified as 'readily biodegradable' under aerobic conditions.