We report for the first time a comprehensive study on the synthesis (supported by green chemistry metrics), aggregation properties, bacterial/fungal toxicities and preliminary data on biodegradation of a series of 24 L-phenylalanine derived surface-active ionic liquids (SAILs). The various cationic headgroups included pyridinium, imidazolium, and cholinium groups and enabled a comprehensive analysis of the effect of the alkyl ester chain (from C₂ to C₁₆) on the synthesis, toxicity, biodegradability, and surfactant properties of the novel SAILs. The evaluation of the SAILs revealed that a wide variety of properties were strictly dependent on the side chain length, including their bacterial and fungal toxicities (from low toxicity to high toxicity), and aggregation properties. Addition of the L-phenylalanine moiety which connects the lipophilic side chain to the cationic head group results in the phenyl group essentially contributing to the self-assembling properties. The interplay of dispersion interactions of the phenyl ring and the side chain hydrophobicity allows us to rank the novel SAILs (thus identifying the remarkable ones) as compared to other surfactants. The CMC values for the SAILs reported in this study are significantly (up to 10 times) lower than those reported for conventional surfactants with the same length of the side chain. Adsorption and micellization are among the factors affecting the toxicity of the studied SAILs. Preliminary biodegradation studies have shown that no clear trend was observed when comparing the closed bottle test results of the SAIL C₂ and C₁₀ derivatives. Medium chain length (C₆ to C₈) pyridinium SAILs have been recommended as the most prospective green alternatives for conventional cationic surfactants. These findings can contribute to designing new efficient amphiphiles with optimized antimicrobial activities and to employ them as potential environmentally benign mineralisable surfactants.

Ionic liquids (ILs) are perceived as a promising group of chemicals within the realm of green chemistry. They are increasingly of interest for open environmental applications. Rules for the design of biodegradable and preferably completely mineralizing ILs after their introduction into the environment are highly desirable. In this study, the impact of the length of the alkyl chain and of the cationic head group on the environmental biodegradability of L-phenylalanine ester (PheC_n) derived ILs using the pyridinium (PyPheC_n), imidazolium (ImPheC_n) and cholinium (CholPheC_n) head groups was systematically studied. This included high-resolution mass spectrometry monitoring of formed products of incomplete mineralization. Completely biodegradable ILs were only identified in the PyPheC_n series. PyPheC₂ and PyPheC₄ were fully mineralizable within 42 days with no detectable transformation products (TPs), whereas mineralization of longer chain length PyPheC_n ILs required more time. Our results show biodegradation by microorganisms was affected by an alkyl chain length of at least 10 carbon atoms. Biodegradation of the ILs started with biotic ester hydrolysis independent of the alkyl chain length or core scaffold. In contrast, the second step of microbial degradation was an amide bond cleavage which was only shown for ILs with a pyridinium or imidazolium core, even after a test prolongation up to 42 days. Hence, the nature of the cationic head group and the chain length impacted on the mineralization and biodegradability of the ILs. For ILs with an imidazolium core, amide bond cleavage resulted in the formation of recalcitrant 1-carboxymethylimidazolium (ImAc). Whereas for most ILs with a pyridinium core, amide bond cleavage resulted in 1-carboxymethylpyridinium (PyAc) which was mineralizable, although not readily biodegradable according to the test guideline 301D. In the case of cholinium ILs, no further degradation of cholinium phenylalanine amide TP was observed after the hydrolysis of the ester to the alkyl alcohol. The biodegradability of ILs decreased in the order PyPheC_n > ImPheC_n > CholPheC_n, but a key finding is that ImPhe is degraded to ImAc but not further (cf. favourable PyAc mineralisation). These results can be used to design ILs with the property to be effectively biodegraded and mineralized in the environment.