# Diversity of Potential (Bio)Technological Applications of Amino Acid-Based Ionic Liquids
### General Structures, Synthesis, and Key Properties
Choline (Ch)-based AA ILs exhibit high viscosity, with reported values ranging from 102 to 107 mPa-s. In addition, for ILs based on AAs capable of participating in additional hydrogen bonding, such as Asp\({}^{-}\), Arg\({}^{-}\), Glu\({}^{-}\), and His\({}^{-}\), the viscosities reported by different research groups are not consistent. The discrepancies in these data in the literature have been attributed to insufficient drying during synthesis; therefore, the ILs may be hydrated to varying extents [23]. Typically, an increase in the size of the anion correlates with an increase in viscosity, which is attributed to stronger intermolecular forces such as anion stacking, tail aggregation, van der Waals interactions, hydrogen bonding, and \(\pi\)-stacking interactions. Consistent with this trend, [PERSON] et al. reported the lowest viscosity for [Ch][Gly] (121 mPa-s) and the highest for [Ch][Trp] (5640 mPa-s). Additionally, ILs containing negatively charged AA anions exhibited viscosities above 2000 mPa-s among a large series of [Ch][AA] [24]. It should be noted that the ILs that were prepared using AAs as cations or AA derivatives were either solids or liquids with extremely high viscosities, up to 4180 mPa-s at room temperature [18]. The incorporation of alkyl substituents at the nitrogen atom of the AA cation-based ILs results in a decrease in their viscosity. N,N,N-trialkyl-substituted AA ILs have been reported to exhibit lower viscosities (200 to 1000 mPa-s) at room temperature (r.t.) compared to other known AA ILs [25]. * _Solubility:_ The miscibility of room-temperature ILs with organic solvents is dependent on the side chain structure of the corresponding AA anion. For example, [Emim][Glu] and [Emimm][Asp], ILs with an AA anion with two carboxyl groups, are insoluble in chloroform [19]. AA ester saccharinates, nitrates, and halogenates are miscible with water, ethanol, and acetone [17]. * _Thermal properties and thermal stability:_ The thermal stability of AA anion-based ionic liquids decreases in the following order, depending on the cation: For the anion Ala, the decomposition temperatures (Td) for the cations above are as follows: 286 \({}^{\circ}\)C, 212 \({}^{\circ}\)C, 181 \({}^{\circ}\)C, 176 \({}^{\circ}\)C, 162 \({}^{\circ}\)C, and 150 \({}^{\circ}\)C [19]. The results are consistent with the data reported by [PERSON] et al. for [N222]-based AAs [22]. On the other hand, for a series of triethanolammonium [TEA] salts, the thermal stability varies with the type of AA anion and decreases in the following order: His > Arg > Glu2 > Pro > Trp > Ser > Gly > Leu > Ile > Thr > Met > Ala > Asp2 > Gln > Phe > Glu > Val > Lys > Asp > Asn [26]. The phosphonium-based AA ILs demonstrate lower melting points and viscosities than the ammonium-based AA ILs [19]. However, AA ILs containing phosphonium cation with the same alkyl substituents exhibit a higher melting temperature due to increased crystallinity. Most [Ch][AA] ionic liquids are stable up to 200 \({}^{\circ}\)C, with the exception of [Ch][Gly], which decomposes at 121 \({}^{\circ}\)C [23; 24]. In general, elongation of the AA anion side chain, as well as the introduction of hydroxyl or carboxyl groups, leads to an increase in the decomposition temperature, with consistent results across the reported studies. An exception is [Ch][His], for which the Td varies between 128 and 171 \({}^{\circ}\)C [23; 24]. The glass transition temperatures (Tg) of most tetraalkylammonium-based ILs are reported to be about 10 \({}^{\circ}\)C lower than those of phosphonium-based ILs with the same AA anion [18]. Interestingly, for a series of Gly-based [NTf2], [PERSON] et al. showed that the salts with N,N-dialkyl-substituted Gly or N,N-dialkyl-substituted Gly ethyl ester [GlyOEt] cations are more thermostable than [Gly][NTf2]. Esterification and alkyl substitution of the Gly cation lead to a decrease in the melting temperature of the salt, proportional to the number of alkyl substituents.