# Diversity of Potential (Bio)Technological Applications of Amino Acid-Based Ionic Liquids
### General Structures, Synthesis, and Key Properties  
Since the AAs have/possess both amino and carboxylic acid functional groups, they can be converted into either a cation or an anion. Some examples from the literature of ILs containing an AA anion and various cations are shown in Scheme 1A [13; 14]. They are usually obtained in two steps: first, the organic halide is converted to organic hydroxide by ion exchange, and then the AA is neutralized by the organic hydroxide [13]. The ILs based on the AA cation are obtained in a single step by acidifying the neutral AA with a strong acid [15]. To minimize the H-bonding possibilities of AA and to reduce the melting point of the resulting salt, AA is sometimes first converted to AA ester chloride, which is then mixed with a metal salt to give the target product by metathesis [15; 16; 17]. Some examples of these types of AA ILs are shown in Scheme 1B. **Scheme 1. ** Left panel (**A**): examples of ionic liquids containing 1,3-disubstituted imidazolium (1), 1,1-dialkyl pyrrolidinium (2), tetraalkylammonium (3), choline (4), quaternary phosphonium (5), etc., with cations and an amino acid anion. Right panel (**B**): examples of ionic liquids containing an amino acid cation and chloride (6), nitrate (7), tetafluoroborate (8), hexafluorophosphate (9), trifluoroacetate (10), bisulfate (11), thiocyanate (12), bistrifilimide (13), salicylic acid (14), and saccharinate (15). General Information on Selected Physicochemical Characteristics of the Main Studied Groups of AA ILs  
* _Viscosity:_ Viscosity is a key factor influencing the applications of ILs, especially when used as solvents in catalysis, extraction, and related processes. As a result, considerable effort has been devoted to the design and synthesis of ILs with reduced viscosity. ILs containing the same anion and either imidazolium or amino acid ester cations have comparable viscosities [14]. They are less viscous than ILs based on quaternary ammonium cations [14]. Overall, ILs with asymmetric cations tend to be less viscous than those with symmetric cations [14]. AA ILs with strong hydrogen bonding through the carboxyl group generally exhibit high viscosity. This high viscosity is attributed to hydrogen bonding between the AA anions and the hydrogen atom at the carbon atom at the second position the imidazolium cation ring. For example, the minimum viscosity reported for 1-ethyl-3-methylimidazolium (Emim)-based AA ILs was 486 mPa-s at 25 \({}^{\circ}\)C [13; 18]. On the other hand, tetrabutylphosphonium (P\({}_{444}\)) is a relatively large cation, which leads to weakened interactions among the ILs. As a result, it does not exhibit effective hydrogen bonding with anions and is therefore less viscous than Emim-based AA ILs [19]. In addition, [PERSON] et al. reported that P\({}_{444}\)-based ILs are less viscous than ammonium-based ILs when comparing ILs with the same anions [20]. However, their viscosity is relatively high; for example, [P\({}_{444}\)][AAs] had a viscosity greater than 344 mPa-s at 25 \({}^{\circ}\)C [18]. On the other hand, mixtures of [P\({}_{444}\)][Lys] and [P\({}_{444}\)][Asp] in a ratio of 1:1 to 4:1 are characterized by a lower viscosity compared to the individual salts, due to hydrogen bonding between the amine group(s) of Lys and the carboxylic group of Asp. However, as suggested by [PERSON] et al., it is possible that these bonds do not form anion layers [21]. Among the series of tetraethylammonium (N\({}_{2222}\))-based ILs, those with hydrophobic AA anions such as Pro, Met, Ile, and Ala are less viscous than those with Thr or Ser anions [22]. Among them, [N\({}_{2222}\)][L-Ala] is found to be the IL with the lowest known viscosity of 81 mPa-s at 25 \({}^{\circ}\)C, which is 6 times lower than [Emim][Gly] [18]. Regarding the cation, low-molecular-weight cations, as well as asymmetric tetraalkylammonium-based AA ILs with short and flexible alkyl chain substituents, are less viscous due to a decrease in van der Waals interactions [22].