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.
2.1. Spectroscopic Properties
The
1H and
13C chemical shifts for α-positions in amino acids and
13C chemical shifts for the carboxyl group in DMSO-d
6 are collected in
Table 1.
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
13C 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.
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.
2.2. Physicochemical Properties
Most of the prepared organic salts of amino acids were colourless or slightly yellow liquids at room temperature (
Table 3). Salts of histidine—[TBA][
l-His], [tBMA][
l-His], and tryptophan [TBA][Trp] were exceptions because they melt at 128.5 °C, 58.9 °C, and 129.6 °C, respectively. The viscosities of the amino acids were recorded at different temperatures and are summarized in
Figure 2,
Figure 3,
Figure 4 and
Figure 5. The viscosities at 65 °C are presented in
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 (
Figure 2,
Figure 3 and
Figure 4). As indicated in
Figure 2,
Figure 3 and
Figure 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. For example, the viscosity of [N
2222][
l-Ala] is 81 mPa∙s at 25 °C, while for [N
2224][
l-Ala] the viscosity is merely 29 mPa∙s, and for [N
2221][
l-Ala] it is 84 mPa∙s [
47]. The molecular size and asymmetry of the anions also apparently influence the viscosity of AAILs. It is seen in
Figure 2 and
Figure 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 [TBA][
l-Val] is 148 mPa∙s and that of [TBA][
l-Leu] is 149 mPa∙s, which are lower than that of [TBA][
l-Ile] (176 mPa∙s) and [TBA][
l-Thr] (316 mPa∙s). The strength of the momentum transfer of threonine ionioc liquid is more temperature-dependent than other tetrabutylammonium ILs. The viscosity variation of [TBA][Thr] and [tBMA][Thr] indicates that the interaction forces between the cation and anion are sensitive to temperature.
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]
λT,
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.
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).
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 (Td5%) 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 Td5% 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.
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
Figure 13 and
Figure 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.
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. Most of them were miscible with ethyl acetate and chloroform. Exceptions were DDTMA, HDTMA, and ODTMA salts, which were immiscible, and [tBMA][l-His], [TBA][l-Trp], [Chol][l-Trp], which were only partly miscible with these solvents, and [DDA][l-His] and [TBA][l-Thr] were partly miscible with chloroform, and [TBA][l-His] was partly miscible with ethyl acetate.
Amino acid ionic liquids composed of DDA, BA, DDTMA, HDTMA, or ODTMA cations showed surface activity in aqueous solution (
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
Amin 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
Amin 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 (
Table 6). It is known from the literature that commercially available 1-ethyl-3-methylimidazolium chloride [EMIM][Cl] dissolves cellulose, displaying 31.4 mg g
−1 solubility at 85 °C. However, [EMIM][Cl] is a solid with a melting point of 85 °C and requires heating above this temperature to dissolve the cellulose [
49]. In comparison with the 1-ethyl-3-methylimidazolium chloride, EMIM salts of AA require a lower temperature. Solubility of cellulose at 60 °C in [EMIM][AA] (
Table 6) is between 13.7 mg g
−1 for [EMIM][
l-Ile] to 43.2 mg g
−1 for [EMIM][
l-Thr]. 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.