The Relationship between the Structure and Properties of Amino Acid Ionic Liquids

Ionic liquids based on different l-amino acids (glycine, l-valine, l-leucine, l-isoleucine, l-histidine, l-methionine, l-tyrosine, l-tryptophan, l-arginine, and l-threonine) and different cations (tetrabutylammonium (TBA), tributylmethylammonium (tBMA), didecyldimethylammonium (DDA), (2-hydroxyethyl)trimethylammonium (choline) (Chol), alkyl(C12-C14) dimethylbenzylammonium (benzalkonium) (BA), dodecyltrimethylammonium (DDTMA), hexadecyltrimethylammonium (HDTMA), octadecyltrimethylammonium (ODTMA) and 1-ethyl-3-methylimidazolium (EMIM)) have been synthesized and characterized by NMR and FTIR. Viscosity, specific rotation, surface activity, thermal stability (TG), and phase transformations (DSC) have been determined and compared with available data. Furthermore, benzalkonium, didecyldimethylammonium, dodecyltrimethylammonium, hexadecyltrimethylammonium, and octadecyltrimethylammonium amino acid ionic liquids have been shown to exhibit surface activity. The dissolution of cellulose in amino acid ionic liquids (AAILs) composed of various cations was also investigated. Cellulose was only dissolved in EMIM salts of amino acids. In particular, the influence of the cation type on selected physicochemical and spectroscopic properties were discussed. The article is a mini review on amino acid ionic liquids.


Introduction
Ionic liquids have been described as "designer solvents", and this means that their properties can be adjusted to the requirements of the process. The most important properties are negligibly low vapor pressure, high thermal stability, ability of dissolving various materials, and high ionic conductivity. Some properties, such as melting point, viscosity, density, and hydrophobicity, can be varied by simple changes of the structure of ions. Ionic liquids are used as solvents in many reactions and as electrolytes [1][2][3][4][5]. Several years of investigation on the synthesis and application of ionic liquids have shown that they can be the perfect solution for various technological problems. Their usefulness is determined by their structure, which can be modified in a wide range. Environmental aspects as well as "green chemistry" rules have meant that attention has focused on the use of renewable raw materials in the design and synthesis of ionic liquids. Thus, in recent years, ionic liquids have been prepared from amino acids, sugars, and terpenes [6][7][8][9][10][11][12]. An additional advantage of these raw materials is their optical activity, which can expand the potential applications of ionic liquids prepared based on them to bioprocesses, biotechnology, and asymmetric synthesis.
Amino acids are specific raw materials in the synthesis of ionic liquids. They can be the source of both cations and anions. They are chiral and have two chemically active centers suitable for modification [12][13][14]. Amino acid ionic liquids (AAILs) have a high hydrogen bonding ability useful for dissolution of biomaterials such as DNA, cellulose, and other carbohydrates [15]. The strong hydrogen bonding ability of AAILs as compared to conventional ionic liquids makes AAILs more demanding as chiral solvents and reagents for dissolution and stabilization of biomolecules (such as carbohydrates, cellulose, nucleic acids, etc.) in medicinal, synthetic, and pharmaceutical chemistry [16,17]. In industrial and pharmaceutical chemistry, ionic liquids based on amino acids have many different applications such as an intermediate in the synthesis of peptides, chiral solvents, and absorbents for acid gases [18]. Moreover, they have proved to be effective catalysts for many reactions, i.e., asymmetric aldol condensation between aldehydes and ketones in the presence of water [19], Diels-Alder reaction, and asymmetric Michael addition [20][21][22].
Due to their distinctive properties such as tunable hydrophobicity, hydrogen bonding ability (proton-donating/accepting properties), unique acid-base characteristic amino acid ionic liquids (AAILs) have tremendous potential in the field of enzymatic reactions, protein chemistry, and as templates for synthetic study.
TBA salts of these amino acids have been previously used by us to synthesize amino acid ionic liquid-supported Schiff bases, which have been shown to be capable of forming intramolecular hydrogen bonds. It was found that the balance between the NH and OH form in these ionic liquids depends on the amino acid [29].

Results and Discussion
Organic salts of amino acids were prepared by proton exchange in the amino acid carboxyl group by the organic cation, according to the previously described procedure (Scheme 1) [1,30].  The synthesis of amino acid ionic liquids, without tetrabutylammonium ionic liquids, consisted of two stages. In the first step, the halide anion in organic halides was exchanged for a hydroxide anion on the ion exchange resin.
In the second step, amino acid (1.2 equivalents) was dissolved in the aqueous solution of organic hydroxide (one equivalent). After the procedure described in Section 3.2 , the product was dried in a vacuum oven at 60 °C to a water content below 800 ppm (at least 24 h). All products were obtained with high yields (85-99%) and high purity. All AAILs were identified by NMR spectroscopy. The complete assignments of 1 H NMR and 13 C NMR were presented only for new tBMA, DDTMA, HDTMA, and ODTMA salts of amino acids because TBA, DDA, BA, Chol, and EMIM salts have been previously described [1,[23][24][25][30][31][32][33][34]. However, the spectroscopic and physicochemical data have never been compared to each other.

Spectroscopic Properties
The 1 H and 13 C chemical shifts for α-positions in amino acids and 13 C chemical shifts for the carboxyl group in DMSO-d6 are collected in Table 1. The synthesis of amino acid ionic liquids, without tetrabutylammonium ionic liquids, consisted of two stages. In the first step, the halide anion in organic halides was exchanged for a hydroxide anion on the ion exchange resin.
In the second step, amino acid (1.2 equivalents) was dissolved in the aqueous solution of organic hydroxide (one equivalent). After the procedure described in Section 3.2, the product was dried in a vacuum oven at 60 • C to a water content below 800 ppm (at least 24 h). All products were obtained with high yields (85-99%) and high purity. All AAILs were identified by NMR spectroscopy. The complete assignments of 1 H NMR and 13 C NMR were presented only for new tBMA, DDTMA, HDTMA, and ODTMA salts of amino acids because TBA, DDA, BA, Chol, and EMIM salts have been previously described [1,[23][24][25][30][31][32][33][34]. However, the spectroscopic and physicochemical data have never been compared to each other.

Spectroscopic Properties
The 1 H and 13 C chemical shifts for α-positions in amino acids and 13 C chemical shifts for the carboxyl group in DMSO-d 6 are collected in Table 1. na -not analyzed/not available; a) Data from [35]; b) Data from [34]; c) Data from [36].
The chemical shift values for H-α of amino acid anions were similar for all salts. Owing to the nature of the cation, the highest chemical shift values (δ2.69-3.57 ppm) were found for benzalkonium ionic liquids. In general, considering the kind of the anion, the lowest chemical shift values were exhibited by aliphatic amino acids (l-Val or l-Leu) and the highest by aromatic and/or heterocyclic amino acids (l-His, l-Trp, l-Tyr).
The largest differences in chemical shift value in 13 C spectra were observed for carbon C=O (C-2 ). The values were in the range of 172.2 up to 185.8 ppm. These values confirmed the ionic structure of compounds [29,[36][37][38]. The difference between the values observed for benzalkonium and other ionic liquids as about 2.4-9.3 ppm. The differences in chemical shifts for carbon C=O (C-2 ) are due to the presence of the aromatic ring in the benzalkonium cation. It seems that the aromatic ring of the benzalkonium cation is in the vicinity of this anion fragment and is positioned such that the carbon C=O and the proton H-α are under its influence. No apparent effect on carbon C-α offset suggests that it is outside its influence. A similar, but significantly less pronounced effect can be seen in EMIM derivatives. These effects are also caused by the length of the alkyl substituent, because ODTMA, HDTMA, and DDTMA show similar effects. The influence of the hydroxyl group on the choline derivative is definitely lower but there must be some interaction between the OH group and the COO − group.
No relationship between H-α and C-α chemical shifts was observed ( Figure 1). C-α is relatively insensitive to the type of cation and amino acid anion. Interestingly, all deviations refer to derivatives with an asymmetric cation. The largest deviations were found for tBMA, DDTMA, and HDTMA, respectively. The biggest influence on the chemical shifts carbon C-α was observed for tBMA cations. Chemical shifts in tBMA derivatives are significantly higher than in other derivatives. Perhaps this is related to some unsymmetrical interaction between longer cation fragments and carbon. FTIR spectra of respective amino acid ionic liquids are compared and collected in Table 2 (most distinctive IR bands). The data from the Table 2, i.e., the influence of the position of the characteristic bands depending on the type of cation, are presented to show the impact of the interaction between the cation and anion in the ionic liquid.   FTIR spectra of respective amino acid ionic liquids are compared and collected in Table 2 (most distinctive IR bands). The data from the Table 2, i.e., the influence of the position of the characteristic bands depending on the type of cation, are presented to show the impact of the interaction between the cation and anion in the ionic liquid. Table 2. Selected infrared (IR) bands of amino acid ionic liquids (cm −1 ).  The broad band ca. 3000-3600 cm −1 was assigned to vN-H and vO-H. The strong band at ca. 2960 cm −1 was assigned to vC-H The strong bands at ca. 1570 and 1390 cm −1 were assigned to v(COO − ) sym. and v(COO − ) as , respectively [29,35,39]. The differences of the IR bands for the derivatives of various amino acids and the same cations were negligible.

Physicochemical Properties
Most of the prepared organic salts of amino acids were colourless or slightly yellow liquids at room temperature ( Table 3) Table 3. The viscosity of the amino acid ionic liquids at 25 • C ranged from 330 to 16,856 mPa·s. TBA salts have the lowest viscosity among the studied compounds. The viscosity decreased significantly with increasing temperature (Figures 2-4). As indicated in Figures 2-4, the viscosity of the ILs is sensitive to temperature, e.g., the viscosity sharply changes when the ILs are in lower temperatures, the viscosity of the ILs with high viscosity values is especially sensitive to temperature. Such sensitivity of IL viscosity to temperature has been indicated in other studies [40][41][42][43][44]. The viscosities are largely dependent on the nature of the cation. The effect of cation type on AAIL viscosity for the threonine anion is reported in Figure 5. The viscosity values shows the salts with imidazolium cations are less viscous than the quaternary ammonium-based ILs. These trends are known and typical of other ILs with different anions [45]. Asymmetric N-substituted imidazolium ionic liquids owe their low viscosity to the synergistic effects of charge delocalization and planarity leads. The viscosity of tBMA and DDA are remarkably larger than that for other considered ILs, as it may be expected from the asymmetry of the alkyl substituents. There is also a significant difference in viscosity of the liquid with various anions. H-bonding is also a factor affecting the viscosities of ILs. Compounds with fewer hydrogen bonds have a lower viscosity [46]. In our case, fewer hydrogen bonds in ammonium AAILs do not decrease their viscosity relative to the imidazolium AAILs, which is associated with conjugated cation structure. The positive charge of the imidazolium cation is well distributed, which remarkably weakens the Coulomb interactions among ions. As a result, the viscosity of imidazolium AAILs are lower than that of quaternary ammonium AAILs. The asymmetry of the cation does have a significant impact on the viscosity. It is generally known that ionic liquids with asymmetric cations have a lower viscosity than those with symmetric cations. However, after a detailed analysis of literary data, it was shown that it depends on the type of substituent.  [47]. The molecular size and asymmetry of the anions also apparently influence the viscosity of AAILs. It is seen in Figures 2 and 3 that the viscosities of TBA and tBMA amino acids generally decrease in the order of Thr > Leu > Ile > Val. The highest viscosity was observed in liquids with Thr anions, which is associated with the presence of an additional polar -OH group in the structure. From Figure 1, a rapid decrease in the viscosities is found in the AAILs as the temperature increases. The influence of temperature on the viscosity is very significant at lower temperatures. At a higher temperature of 65 • C, the viscosity of [ [47]. The molecular size and asymmetry of the anions also apparently influence the viscosity of AAILs. It is seen in Figures 2 and 3 that the viscosities of TBA and tBMA amino acids generally decrease in the order of Thr > Leu > Ile > Val. The highest viscosity was observed in liquids with Thr anions, which is associated with the presence of an additional polar -OH group in the structure. From Figure 1, a rapid decrease in the viscosities is found in the AAILs as the temperature increases. The influence of temperature on the viscosity is very significant at lower temperatures.     [47]. The molecular size and asymmetry of the anions also apparently influence the viscosity of AAILs. It is seen in Figures 2 and 3 that the viscosities of TBA and tBMA amino acids generally decrease in the order of Thr > Leu > Ile > Val. The highest viscosity was observed in liquids with Thr anions, which is associated with the presence of an additional polar -OH group in the structure. From Figure 1, a rapid decrease in the viscosities is found in the AAILs as the temperature increases.         Figure 6 shows the relation between Tg and viscosity (at 25 °C) for different AAILs. All AAIls have a linear relationship between viscosity and Tg. Thus, for these ionic liquids, the side-chain structure did not affect the general relationship between Tg and viscosity.    Figure 6 shows the relation between Tg and viscosity (at 25 °C) for different AAILs. All AAIls have a linear relationship between viscosity and Tg. Thus, for these ionic liquids, the side-chain structure did not affect the general relationship between Tg and viscosity.   Figure 6 shows the relation between T g and viscosity (at 25 • C) for different AAILs. All AAIls have a linear relationship between viscosity and T g . Thus, for these ionic liquids, the side-chain structure did not affect the general relationship between T g and viscosity. All obtained AAILs are chiral with specific rotation listed in Table 3. Specific rotation was similar for most of the salts (with different cation) of the same amino acid ([α]λ T , Figure 7). Some trends were observed between specific rotation and molar mass (Figure 8). Such relationships suggest that [α] changes with the size of molecules (especially the size of the cation).Given that amino acids have the ability to rotate polarized light, it is reasonable that an increase in the molar ratio of the cationic part decreases the absolute value of the rotation.  [tBMA] [EMIM] [DDA] [BA] [Chol] [DDTMA] [HDTMA] [ODTMA] All obtained AAILs are chiral with specific rotation listed in Table 3. Specific rotation was similar for most of the salts (with different cation) of the same amino acid ([α] λ T , Figure 7). Some trends were observed between specific rotation and molar mass (Figure 8). Such relationships suggest that [α] changes with the size of molecules (especially the size of the cation).Given that amino acids have the ability to rotate polarized light, it is reasonable that an increase in the molar ratio of the cationic part decreases the absolute value of the rotation.
All obtained AAILs are chiral with specific rotation listed in Table 3. Specific rotation was similar for most of the salts (with different cation) of the same amino acid ([α]λ T , Figure 7). Some trends were observed between specific rotation and molar mass (Figure 8). Such relationships suggest that [α] changes with the size of molecules (especially the size of the cation).Given that amino acids have the ability to rotate polarized light, it is reasonable that an increase in the molar ratio of the cationic part decreases the absolute value of the rotation.  [tBMA] [EMIM] [DDA] [BA] [Chol] [DDTMA] [HDTMA] [ODTMA] Differential scanning calorimetry (DSC) showed the glass transition for the studied ionic liquids (Tg, Table 3). It has been observed that the glass transition depends on the cation structure. Ionic liquids with a symmetrical tetrabutylammonium cation showed higher glass transition temperature in comparison to those of tributylmethylammonium and 1-ethyl-3-methylimidazolium salts. Furthermore, the highest temperature of glass transition was found for threonine and histidine derivatives.
Some trends in the change of glass transition temperatures Tg with H-α chemical shift were observed, which are plotted for tBMA and DDA salts in Figure 9. These relationships may suggest that Tg depends on the structure of the amino acid anion. For amino acid ionic liquids with didecyldimethylammonium cation, along with a decrease in the glass transition temperature, there is increased H-α chemical shift (Figure 9, circle marked), which is contrary to other salts studied (such as tributylmethylammonium, Figure 9, triangles).  Differential scanning calorimetry (DSC) showed the glass transition for the studied ionic liquids (T g , Table 3). It has been observed that the glass transition depends on the cation structure. Ionic liquids with a symmetrical tetrabutylammonium cation showed higher glass transition temperature in comparison to those of tributylmethylammonium and 1-ethyl-3-methylimidazolium salts. Furthermore, the highest temperature of glass transition was found for threonine and histidine derivatives.
Some trends in the change of glass transition temperatures T g with H-α chemical shift were observed, which are plotted for tBMA and DDA salts in Figure 9. These relationships may suggest that T g depends on the structure of the amino acid anion. For amino acid ionic liquids with didecyldimethylammonium cation, along with a decrease in the glass transition temperature, there is increased H-α chemical shift (Figure 9, circle marked), which is contrary to other salts studied (such as tributylmethylammonium, Figure 9, triangles). Differential scanning calorimetry (DSC) showed the glass transition for the studied ionic liquids (Tg, Table 3). It has been observed that the glass transition depends on the cation structure. Ionic liquids with a symmetrical tetrabutylammonium cation showed higher glass transition temperature in comparison to those of tributylmethylammonium and 1-ethyl-3-methylimidazolium salts. Furthermore, the highest temperature of glass transition was found for threonine and histidine derivatives.
Some trends in the change of glass transition temperatures Tg with H-α chemical shift were observed, which are plotted for tBMA and DDA salts in Figure 9. These relationships may suggest that Tg depends on the structure of the amino acid anion. For amino acid ionic liquids with didecyldimethylammonium cation, along with a decrease in the glass transition temperature, there is increased H-α chemical shift (Figure 9, circle marked), which is contrary to other salts studied (such as tributylmethylammonium, Figure 9, triangles).  The glass transition temperature dependence on the type of amino acid for the various tested cations is presented in Figure 10. Generally, it can be seen that the glass transition temperature is dependent on the structure of the cation of the ionic liquid. Hydrogen bonding, van der Waals interactions, and the size of the amino acid anion caused increases in thermal stability [1,15]. It is also noted that the glass transition temperature generally increases with increasing molecular weight of the amino acid.
14 The glass transition temperature dependence on the type of amino acid for the various tested cations is presented in Figure 10. Generally, it can be seen that the glass transition temperature is dependent on the structure of the cation of the ionic liquid. Hydrogen bonding, van der Waals interactions, and the size of the amino acid anion caused increases in thermal stability [1,15]. It is also noted that the glass transition temperature generally increases with increasing molecular weight of the amino acid. Decomposition temperatures corresponding to 5% weight loss (Td 5% ) were in the range of 115.1 to 315.0 °C. 1-Ethyl-3-methylimidazolium salts of amino acids started decomposition at about 200 °C (except [EMIM][L-Thr] with Td 5% at 169.11 °C) and were the most stable among all prepared salts. AAILs with ammonium cations were less stable. They decomposed at about 120 °C.
Small dependencies were observed between decomposition temperatures corresponding to 5% weight loss ( Figure 11) or 50% weight loss (Figure 12) of the molecular weight. This relation indicates that the decomposition temperature depends on the structure of the compound studied and changes in the size of the molecules. This implies that the lower the molecular weight, the higher the stability of the compound. Figure 11. Plot of 5% (Td 5% ) weight loss temperature vs. molar mass for the compound studied. AAILs with ammonium cations were less stable. They decomposed at about 120 • C. Small dependencies were observed between decomposition temperatures corresponding to 5% weight loss (Figure 11) or 50% weight loss (Figure 12) of the molecular weight. This relation indicates that the decomposition temperature depends on the structure of the compound studied and changes in the size of the molecules. This implies that the lower the molecular weight, the higher the stability of the compound.

14
The glass transition temperature dependence on the type of amino acid for the various tested cations is presented in Figure 10. Generally, it can be seen that the glass transition temperature is dependent on the structure of the cation of the ionic liquid. Hydrogen bonding, van der Waals interactions, and the size of the amino acid anion caused increases in thermal stability [1,15]. It is also noted that the glass transition temperature generally increases with increasing molecular weight of the amino acid. Decomposition temperatures corresponding to 5% weight loss (Td 5% ) were in the range of 115.1 to 315.0 °C. 1-Ethyl-3-methylimidazolium salts of amino acids started decomposition at about 200 °C (except [EMIM][L-Thr] with Td 5% at 169.11 °C) and were the most stable among all prepared salts. AAILs with ammonium cations were less stable. They decomposed at about 120 °C.
Small dependencies were observed between decomposition temperatures corresponding to 5% weight loss ( Figure 11) or 50% weight loss (Figure 12) of the molecular weight. This relation indicates that the decomposition temperature depends on the structure of the compound studied and changes in the size of the molecules. This implies that the lower the molecular weight, the higher the stability of the compound. Figure 11. Plot of 5% (Td 5% ) weight loss temperature vs. molar mass for the compound studied.  Furthermore, the relationship decomposition temperatures corresponding to 5% weight loss and 50% weight loss and the type of amino acid for the various tested cations are shown in Figures 13 and 14. This relation indicates that the decomposition temperature depends on the structure of the compound studied and changes in the size of the molecules. This implies that the lower the molecular weight, the higher the stability of the compound.  [tBMA] [EMIM] [DDA] [BA] [Chol] [DDTMA] [HDTMA] [ODTMA] Figure 12. Plot of (T d 50% ) weight loss temperature vs. molar mass for the compound studied.
Furthermore, the relationship decomposition temperatures corresponding to 5% weight loss and 50% weight loss and the type of amino acid for the various tested cations are shown in Figures 13  and 14. This relation indicates that the decomposition temperature depends on the structure of the compound studied and changes in the size of the molecules. This implies that the lower the molecular weight, the higher the stability of the compound. Figure 12. Plot of (Td 50% ) weight loss temperature vs. molar mass for the compound studied.

15
Furthermore, the relationship decomposition temperatures corresponding to 5% weight loss and 50% weight loss and the type of amino acid for the various tested cations are shown in Figures 13 and 14. This relation indicates that the decomposition temperature depends on the structure of the compound studied and changes in the size of the molecules. This implies that the lower the molecular weight, the higher the stability of the compound.  [tBMA] [EMIM] [DDA] [BA] [Chol] [DDTMA] [HDTMA] [ODTMA] Figure 13. Plot of 5% (T d 5% ) weight loss temperature vs. amino acids.
The miscibility with conventional organic solvents and water was investigated and is summarized in Table 4. The solvents were ranked by decreasing polarity index [48]. [tBMA] [EMIM] [DDA] [BA] [Chol] [DDTMA] [HDTMA] [ODTMA] The miscibility with conventional organic solvents and water was investigated and is summarized in Table 4. The solvents were ranked by decreasing polarity index [48].  All AAILs were immiscible with nonpolar solvents such as benzene, diethyl ether, and n-hexane and were miscible with highly polar solvents such as water, mostly dissolved in acetone and ethanol.  Table 5). The surface tension reached a minimum value between 32.3 and 33.8 mN·m −1 for benzalkonium amino acid salts, between 27.9 and 29.4 mN·m −1 for didecydimethylammonium salts, and between 40.1 and 42 mN·m −1 for dodecyltrimethylammonium, hexadecyltrimethylammonium, and octadecylammonium salts at critical micellar concentration (CMC). The CMC values were in the range from 0.25 mmol·L −1 to 0.34 mmol·L −1 for benzalkonium, from 0.50 mmol·L −1 to 0.61 mmol·L −1 for octadecyltrimethylammonium, from 0.60 mmol·L −1 to 0.87 mmol·L −1 for didecyldimethylammonium salts, from 0.74 mmol·L-1 to 1.09 mmol·L-1 for hexadecyltrimethylammonium, and from 11.81 mmol·L −1 to 12.26 mmol·L −1 for dodecyltrimethylammonium depending on amino acid anion. The area occupied per molecule at interphase A min of [ODTMA][l-Met] (6.635·10 19 m 2 ) was higher than for other salts, indicating that the molecules of AAILs containing ODTMA were more loosely packed at the water-air interface. The opposite situation was observed for [BA][l-Met], where the area per molecule was the smallest. The area per molecule A min was higher for didecyldimethylammonium salts than for other salts of the same amino acid. The obtained ionic liquids were also tested as solvents of cellulose. We found that among the prepared AAILs, 1-ethyl-3-methylimidazolium salts of amino acids dissolved cellulose ( For comparison, the recently published results have shown much lower solubility of cellulose in choline salts of AA; the concentration of cellulose was lower than 5 mg g −1 ( Table 6). It is generally recognized that, in order to dissolve cellulose, its great number of inter-and intramolecular hydrogen bonds must be disrupted. Hydrogen bonding properties are important in solvents for the dissolution of cellulose. For this reason, EMIM-based ionic liquids have greater cellulose dissolution capacity. The obtained ionic liquids were also tested as solvents of cellulose. We found that among the prepared AAILs, 1-ethyl-3-methylimidazolium salts of amino acids dissolved cellulose ( have shown much lower solubility of cellulose in choline salts of AA; the concentration of cellulose was lower than 5 mg g −1 ( Table 6). It is generally recognized that, in order to dissolve cellulose, its great number of inter-and intramolecular hydrogen bonds must be disrupted. Hydrogen bonding properties are important in solvents for the dissolution of cellulose. For this reason, EMIM-based ionic liquids have greater cellulose dissolution capacity.

Structural Characterization of Obtained Amino Acid Ionic Liquids
The prepared AAILs were identified by 1 H NMR and 13 C NMR. The 1 H and 13 C NMR spectra were recorded in DMSO-d 6 or D 2 O on a BRUKER DPX-400 Avance III HD spectrometer (Billerica, MA, USA) operating at 400.13 MHz ( 1 H) and 100.62 MHz ( 13 C). TMS was used as an internal standard.
FTIR spectra (KBr) were recorder in the 4000-400 cm −1 range, employing a Thermo Fisher Scientific Nicolet 380 FT-IR Spectrometer (Waltham, MA, USA) as thin film on KBr tablets.

Water Content
The water content in AAILs was determined by coulometric Karl-Fischer titration, using a Metrohm 831 KF Coulometer (Herisau, Switzerland). The physicochemical properties such as viscosity, thermal stability (TG), and phase transformations (DSC) were also determined.

Viscosity
The viscosities of AAILs were measured at different temperatures (from 25 • C to 65 • C) using ARES (Advanced Rheometric Expansion System) Rheometric Scientific Rheometer (Piscataway, NJ, USA). During the viscosity measurement, the samples were blown with compressed dried synthetic air. This procedure protects against humidity absorption into ionic liquids.

Phase Transformations (DSC)
Phase transformation temperatures (glass transition temperature and melting point) were measured using a DSC analyzer (model Q-100, TA Instruments (New Castle, DE, USA)). Measurements were performed within the temperature range of −75 • C to 200 • C, in nitrogen atmosphere. The heating rate was 10 • C min −1 . The sample was loaded on an aluminum pan sealed with a pinhole cap. Phase transition temperatures were determined according to ISO 11357-1:2009(E) using the midpoint temperature. Indium and mercury were used as standards to calibrate the temperature. Heat calibration used indium.

Measurement of Surface Tension
Some of the prepared ionic liquids (with didecyldimethylammonium, benzalkonium, dodecyltrimethylammonium, hexadecyltrimethylammonium, and octadecyltrimethylammonium cation) exhibit surface activity. The critical micelle concentration and surface tension were measured by digital tensiometer (Krüss, K10ST, Hamburg, Germany) using the ring method at 25 • C. The values of the critical micelle concentration (CMC) and the surface tension at the CMC (γ CMC ) were determined from the intersection of two straight lines drawn in low and high concentration regions in surface tension curves using a linear regression analysis method. Surface excess concentrations (Γ max ) were calculated from the slope of linear portion of the γ-log c plots using the Gibbs isotherm: where: Γ max -surface excess concentration at the saturated interface, mol·m −2 , R-gas constant, 8.3144598(48) J·mol −1 ·K −1 , T-absolute temperature, K, c-concentration of the salt, mol·dm −3 .
From Γ max , the minimum surface occupied by a molecule at the interface A min can be calculated from the equation: where: N A -the Avogadro number.

Cellulose Solubility
To determine cellulose solubility, 0.1-0.5 mg sample was added to a glass vial containing 0.5 g IL at 60 • C under N 2 with stirring and was visually checked to determine whether it was soluble. If the solution was clear, the next portion of cellulose was added. The solubility was calculated when the solution remained heterogeneous for 24 h.
The cellulose concentration can be calculated by the formula: where: C cel -cellulose concentration [%], m cel -weight of cellulose dissolved in the ionic liquid [g], m cj -weight of ionic liquids [g].

Conclusions
Amino acid ionic liquids were synthesized and investigated. Their NMR and FTIR spectroscopic data were measured and compared. All investigated ionic liquids have ionic structure, which is confirmed by both the NMR and FT IR spectra. Physico-chemical properties such as specific rotation, thermal stability, glass transition, and surface activity were measured and discussed. Most of the analyzed salts of amino acids are liquids at room temperature. They are thermally stable in the temperature range 150-200 • C. Only 1-ethyl-3-methylimidazolium salts dissolve cellulose.