The Proton Dissociation of Bio-Protic Ionic Liquids: [AAE]X Amino Acid Ionic Liquids

[AAE]X composed of amino acid ester cations is a sort of typically “bio-based” protic ionic liquids (PILs). They possess potential Brønsted acidity due to the active hydrogens on their cations. The Brønsted acidity of [AAE]X PILs in green solvents (water and ethanol) at room temperature was systematically studied. Various frameworks of amino acid ester cations and four anions were investigated in this work from the viewpoint of structure–property relationship. Four different ways were used to study the acidity. Acid dissociation constants (pKa) of [AAE]X determined by the OIM (overlapping indicator method) were from 7.10 to 7.73 in water and from 8.54 to 9.05 in ethanol. The pKa values determined by the PTM (potential titration method) were from 7.12 to 7.82 in water. Their Hammett acidity function (H0) values (0.05 mol·L−1) were about 4.6 in water. In addition, the pKa values obtained by the DFT (proton-transfer reactions) were from 7.11 to 7.83 in water and from 8.54 to 9.34 in ethanol, respectively. The data revealed that the cationic structures of [AAE]X had little effect and the anions had no effect on the acidity of [AAE]X. At the same time, the OIM, PTM, Hammett method and DFT method were reliable for determining the acidic strength of [AAE]X in this study.


Introduction
Protic ionic liquids (PILs) are an important subset of ionic liquids (ILs). PILs possess strong dissolvabilities, high thermal stabilities, designable structures and broad electrochemical windows [1,2]. They play important roles in fuel cells, electrochemistry, liquid-liquid extraction, gas capture, biological media and so on due to their acidity [3][4][5][6][7]. For the existence of active hydrogen in amino acid cations, proton dissociation occurs in different solvents to varying degrees [8]. Therefore, PILs have been considered as acidic catalysts for the replacement of hazardous acids in many catalytic reactions, including the esterification reaction, biomass conversion, transformation of CO 2 and Diels-Alder reaction. [9][10][11][12].
Bio-based ILs have been paid more attention in recent years due to their preferable green characters [13]. Some natural materials, including carboxylate salts, amino acids and sugars or sugar derivatives, have been employed as IL precursors in a green way [14]. Among these natural materials, amino acids and their derivatives are the most abundant natural sources containing quaternary nitrogens. Amino acid ionic liquids (AAILs) are fascinating for chemists in view of their close associations with chirality and biomolecules [15,16]. Some research has found that AAILs may be useful as potential solvents, catalysts, absorbents and selectors, etc. [17,18]. At the same time, AAILs can be used as acidic catalysts in the esterification of renewable valeric acid, styrene carbonate synthesis under CO 2 , the alkylation of indoles and so on [19][20][21]. Moreover, [AAE]X (AAE means the amino acid

Overlapping Indicator Method (OIM)
The overlapping indicator method (OIM) is a mature method to determine the pK a values [38,39]. The acid dissociation reaction of [AAE]X in water and ethanol can be simplified by the expression in Scheme 1.
Molecules 2020, 25, x 2 of 14 nitrogens. Amino acid ionic liquids (AAILs) are fascinating for chemists in view of their close associations with chirality and biomolecules [15,16]. Some research has found that AAILs may be useful as potential solvents, catalysts, absorbents and selectors, etc. [17,18]. At the same time, AAILs can be used as acidic catalysts in the esterification of renewable valeric acid, styrene carbonate synthesis under CO2, the alkylation of indoles and so on [19][20][21]. Moreover, [AAE]X (AAE means the amino acid ester cations, and X means the corresponding anions) AAILs have higher thermostabilities and lower melting points, as well as lower viscosities than those of their [AA]X (cations are amino acid) analogs and, hence, broader prospects in acid-involving processes [22]. In general, water and ethanol are considered as green media for acidic catalytic reactions, which is one of the "twelve principles" of green chemistry [23][24][25][26]. Acidic properties in solvents are very important to industrially relevant reactions [27,28]. The common PILs, such as imidazolium salts and quaternary ammonium salts, have been studied in many acidic catalytic reactions in solvents [29,30]. However, the precursors of [AAE]X are more abundant and bio-based compared to common PILs, which also possesses Brønsted acidity. Therefore, the proton dissociation of [AAE]X ILs in green solvents are interesting and important for their applications. However, the acidic characteristics of [AAE]X AAILs in solvents are still lacking. The acid dissociation constant (pKa) is one of the most significant physiochemical parameters. An accurate pKa value is important to select reaction conditions in catalytic chemistry [27,[31][32][33]. Determining the acidity of ILs in water and ethanol has become an intriguing topic, since water and ethanol are promising media for green chemistry [30,[34][35][36][37]. Herein, four different methods were employed to study the Brønsted acidity of [AAE]X PILs with different amino acid ester cations and anions in water and ethanol carried out.

Overlapping Indicator Method (OIM)
The overlapping indicator method (OIM) is a mature method to determine the pKa values [38,39]. The acid dissociation reaction of [AAE]X in water and ethanol can be simplified by the expression in Scheme 1.
The acidic dissociation constant (Ka) of HA + can be written as: where pKa (HIn) is the pKa value of the 4-nitrophenol indicator in water (7.15) and 2,4-dinitrophenol indicator in ethanol (8.21) [40,41]. It is easy to get the relationship between the UV/Vis absorption intensity and concentration of the indicator by the Lambert Beer law. Therefore, the concentrations of HIn, In − , HA + and A can be measured by the absorbed change of the indicator after adding the quantitative determinant. nitrogens. Amino acid ionic liquids (AAILs) are fascinating for chemists in view of their close associations with chirality and biomolecules [15,16]. Some research has found that AAILs may be useful as potential solvents, catalysts, absorbents and selectors, etc. [17,18]. At the same time, AAILs can be used as acidic catalysts in the esterification of renewable valeric acid, styrene carbonate synthesis under CO2, the alkylation of indoles and so on [19][20][21]. Moreover, [AAE]X (AAE means the amino acid ester cations, and X means the corresponding anions) AAILs have higher thermostabilities and lower melting points, as well as lower viscosities than those of their [AA]X (cations are amino acid) analogs and, hence, broader prospects in acid-involving processes [22]. In general, water and ethanol are considered as green media for acidic catalytic reactions, which is one of the "twelve principles" of green chemistry [23][24][25][26]. Acidic properties in solvents are very important to industrially relevant reactions [27,28]. The common PILs, such as imidazolium salts and quaternary ammonium salts, have been studied in many acidic catalytic reactions in solvents [29,30]. However, the precursors of [AAE]X are more abundant and bio-based compared to common PILs, which also possesses Brønsted acidity. Therefore, the proton dissociation of [AAE]X ILs in green solvents are interesting and important for their applications. However, the acidic characteristics of [AAE]X AAILs in solvents are still lacking. The acid dissociation constant (pKa) is one of the most significant physiochemical parameters. An accurate pKa value is important to select reaction conditions in catalytic chemistry [27,[31][32][33]. Determining the acidity of ILs in water and ethanol has become an intriguing topic, since water and ethanol are promising media for green chemistry [30,[34][35][36][37]. Herein, four different methods were employed to study the Brønsted acidity of [AAE]X PILs with different amino acid ester cations and anions in water and ethanol carried out.

Overlapping Indicator Method (OIM)
The overlapping indicator method (OIM) is a mature method to determine the pKa values [38,39]. The acid dissociation reaction of [AAE]X in water and ethanol can be simplified by the expression in Scheme 1.  , The acidic dissociation constant (Ka) of HA + can be written as: where pKa (HIn) is the pKa value of the 4-nitrophenol indicator in water (7.15) and 2,4-dinitrophenol indicator in ethanol (8.21) [40,41]. It is easy to get the relationship between the UV/Vis absorption intensity and concentration of the indicator by the Lambert Beer law. Therefore, the concentrations of HIn, In − , HA + and A can be measured by the absorbed change of the indicator after adding the quantitative determinant. The chemical equilibrium constant (K a ) can be obtained by the equation: The acidic dissociation constant (K a ) of HA + can be written as: where pK a (HIn) is the pK a value of the 4-nitrophenol indicator in water (7.15) and 2,4dinitrophenol indicator in ethanol (8.21) [40,41]. It is easy to get the relationship between the UV/Vis absorption intensity and concentration of the indicator by the Lambert Beer law. Therefore, the concentrations of HIn, In − , HA + and A can be measured by the absorbed change of the indicator after adding the quantitative determinant.
The pK a value is a quantitative parameter to insure the strength of the Brønsted acids. The lower pK a values means the stronger acidity of the PILs. The UV/Vis spectral absorbance of the indicator (sodium 4-nitrophenolate) after every titration in water is illustrated in Figure 1.
(The other UV-Vis spectra of the titration of [AAE]X is illustrated in Figures S1-S18). Figure 1a,c respectively represent the absorption spectra of the indicator after adding a quantitative indicator to the alkali liquor (sodium hydroxide). Figure 1b, The pKa value is a quantitative parameter to insure the strength of the Brønsted acids. The lower pKa values means the stronger acidity of the PILs. The UV/Vis spectral absorbance of the indicator (sodium 4-nitrophenolate) after every titration in water is illustrated in Figure 1. (The other UV-Vis spectra of the titration of [AAE]X is illustrated in Figures S1-S18.) Figure 1a,c respectively represent the absorption spectra of the indicator after adding a quantitative indicator to the alkali liquor (sodium hydroxide). Figure 1b, [42]. (b) Reference [43]. (c) Reference [44]. (d) Reference [45]. SD: standard deviation.  To obtain the influence of the anions on the Brønsted acidity, Cl − , NO 3 − , NTf 2 − and ClO 4 − were checked. The impact of the conformation of the cations on the Brønsted acidity of [AAE]X was also studied by the OIM. (Table 2)

Potential Titration Method (PTM)
To confirm the accuracy of the pK a values determined by the OIM, the potential titration method (PTM) was also used to measure the pK a as a comparative method [46]. All solutions are electrically neutral, i.e., the sum of all positive charges must equal the sum of all negative charges; thus, Since all salts are considered as being completely ionized, [K + ] equals the concentration of potassium hydroxide (after considering the dilution by the solution). Hence, The total concentration of acid taken is present in two forms, HA + and A. Consequently, By combining Equation (4), In these equations, [KOH] represents the concentration that the alkali would achieve by dilution if no other substance was present in the solution. The concentration of [AAE]X is 0.100 mol·L −1 , so we can ignore the activity coefficient. Then pK a can be obtained by combining Equations (3) and (10): The pK a values of [AAE]X by PTM are presented in Table 3.

pK a Values Measured by the OIM in Ethanol
To ulteriorly study the acidity of [AAE]X, the pK a values in ethanol were measured by the OIM with 2,4-dinitrophenol as the indicator. The principle of measuring the pK a values in ethanol is the same to that in water. The UV/Vis spectral absorbances of sodium 2,4-dinitrophenolate after titration every time in ethanol are illustrated in Figure 2. Additionally, the pK a values of [AAE]X PILs and the contrastive compounds in ethanol are shown in Table 4.

pKa Values Measured by the OIM in Ethanol
To ulteriorly study the acidity of [AAE]X, the pKa values in ethanol were measured by the OIM with 2,4-dinitrophenol as the indicator. The principle of measuring the pKa values in ethanol is the same to that in water. The UV/Vis spectral absorbances of sodium 2,4-dinitrophenolate after titration every time in ethanol are illustrated in Figure 2. Additionally, the pKa values of [AAE]X PILs and the contrastive compounds in ethanol are shown in Table 4.

Hammett Acidity
The Brønsted acidity associated with the Hammett acidity function (H 0 ) of [AAE]X was investigated in water to confirm the acidic strength of [AAE]X determined by the PTM and OIM [49]. Sodium 2,4-dinitrophenolate was used as an indicator for the determination of the Hammett acidity function by UV/Vis spectroscopy. For insuring the Brønsted acidity of [AAE]X, the protonated extent of the charged indicator bases (sodium 2,4-dinitrophenolate) in an aqueous solution (5 × 10 −5 mol·L −1 ), in terms of the measurable ratio [In − ]/[HIn], needs to be evaluated. In water, the Hammett acidity function can be expressed as the equation: where pK a (HIn) is the pK a value of the 2,4-dinitrophenol indicator in water (4.12) [50],  Table 5, Tables S1 and S2.

pK a Values Calculated by DFT
The theoretical and experimental pK a values of [AAE] + in water and ethanol are summarized in Table 6

Discussion
To     For the same [AAE]X, we found that the pK a by the PTM obtained by two different determination methods are near to zero. For example, the pK a value of [GlyC 1 ]NO 3 determined by the OIM are the same (7.67) to that from the PTM. The data suggest that both the PTM and OIM are reliable to obtain the pK a values of [AAE]X in water.
To ulteriorly study the acidity of [AAE]X, we measured the pK a values in ethanol by the OIM with 2,4-dinitrophenol as the indicator. The pK a values of [AAE]X in ethanol are between 8.54 and 9.05, which are obviously bigger than those in water (7.10 to 7.73). It may be generated by the weaker basicity of ethanol, which means that the weaker intermolecular interactions between the active hydrogens and solvent molecules lead to bigger pK a values. The relationship between the acidity and concentration is important for many applications, such as catalysis [9,37,51]. The [MIM]Cl (4.35) in water. Therefore, the acidic strength of [AAE]X may be almost the same as [MIM] + salt and stronger than the amino acid, EAN, [Et2N]NO3 and [Et3N]NO3. The results are matched well with the results of the pKa values in water determined by the PTM and OIM. The relationship between the acidity and concentration is important for many applications, such as catalysis [9,37,51]. The H0 of [ValC1]NO3 and [PheC1]NO3 in aqueous solutions at various concentrations were measured. In the UV/Vis spectra, a noticeable decrease of the maximum absorption peak was found, accompanied with adding [ValC1]NO3 and [PheC1]NO3 (Figure 4). The  Their nonlinear fittings are shown in Figure 5. The H 0 of [AAE]X gradually lowered with the concentration of [AAE]X rising. Based on these, the desired acidic strength can be obtained by choosing the appropriate concentrations. Their nonlinear fittings are shown in Figure 5. The H0 of [AAE]X gradually lowered with the concentration of [AAE]X rising. Based on these, the desired acidic strength can be obtained by choosing the appropriate concentrations.

Materials and Methods
General methods: All [AAE]Cl were purchased from Energy Chemical (Shanghai, China). Ethylamine (EAN) and ethanol (EtOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were obtained commercially as analytical-grade materials and used as received. Solvents were dried by standard procedures.
[AAE]X PILs were synthesized according to a literature procedure by the ion exchange reaction of [AAE]Cl precursors with corresponding salts. The synthesized [AAE]X PILs needed to be dried firstly and kept in vacuum before use.
The standard deviation (SD) was calculated by the equation: pKa determination by the PTM: A stock solution (0.010 mol·L −1 ) of [AAE]X PILs was prepared in ultrapure water. Then, the solution was titrated with aqueous KOH solution (0.100 mol·L −1 ). The

Materials and Methods
General methods: All [AAE]Cl were purchased from Energy Chemical (Shanghai, China). Ethylamine (EAN) and ethanol (EtOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were obtained commercially as analytical-grade materials and used as received. Solvents were dried by standard procedures.
[AAE]X PILs were synthesized according to a literature procedure by the ion exchange reaction of [AAE]Cl precursors with corresponding salts. The synthesized [AAE]X PILs needed to be dried firstly and kept in vacuum before use.
The standard deviation (SD) was calculated by the equation: pK a determination by the PTM: A stock solution (0.010 mol·L −1 ) of [AAE]X PILs was prepared in ultrapure water. Then, the solution was titrated with aqueous KOH solution (0.100 mol·L −1 ). The electric potential (E) (±1 mV) of the solution was obtained using an Ag-AgCl/glass combination electrode on an OHAUS Starter 2100 pH meter at 25.0 (±0.1) • C. Three standard buffer solutions with the pH values of 4.00, 6.86 and 9.18, respectively, were used to adjust the instrument before titration. pK a determination by OIM: The method determined the pK a of an "unknown" acid relative to that of an "indicator" acid (whose pK a was known) by monitoring the changes of UV/vis absorption of the indicator during titrations under standard conditions. (The indicators (In) here should show different UV/Vis absorbance between the HIn + and In species. Besides, in order to produce moderate changes in the titration, the pK a of HIn + should be close to the measured substance in each solvent.) There were two steps measuring the pK a values by the OIM. Firstly, the linear relation between the concentration of the indicator and absorbance could be achieved by adding the indicator to the alkali solution until the indicator was slightly excessive compared to the alkali. Secondly, an "unknown" acid was quantitatively added to the above solution to achieve the concentrations of HIn, Computational methods: The Brønsted acidity of [AAE]X coming from the -NH 3 group was determined by the above experimental data. In order to obtain a better understanding of the Brønsted acidities of [AAE]X PILs, the pK a values of [AAE]X were calculated by the density functional theory (DFT) using the Gaussian 09 suite program [52]. It was verified that the anions would have hardly any effect on the acidity of [AAE]X in water or ethanol by the experimental data. Therefore, the calculated acidity of [AAE] + could be considered as a simple and approximate method for determining the acidity of [AAE]X. The pK a values by theoretical calculation could be carried out by using the proton-transfer reaction (Scheme 3) [53,54]: Computational methods: The Brønsted acidity of [AAE]X coming from the -NH3 group was determined by the above experimental data. In order to obtain a better understanding of the Brønsted acidities of [AAE]X PILs, the pKa values of [AAE]X were calculated by the density functional theory (DFT) using the Gaussian 09 suite program [52]. It was verified that the anions would have hardly any effect on the acidity of [AAE]X in water or ethanol by the experimental data. Therefore, the calculated acidity of [AAE] + could be considered as a simple and approximate method for determining the acidity of [AAE]X. The pKa values by theoretical calculation could be carried out by using the proton-transfer reaction (Scheme 3) [53,54]: where the solution-free energy was calculated by: Then, it led to the following equilibrium in Equation (14): The calculation of the pKa was obtained from Equation (15): where the solution-free energy was calculated by: Then, it led to the following equilibrium in Equation (13): The calculation of the pK a was obtained from Equation (14): The final expression for the pK a can be written as  [53,58]. The liquidus Gibbs-free energies of AAEH + and AAE were obtained from the sum of the total electronic energies in water or ethanol and the thermal corrections to the gaseous Gibbs-free energies (G corr ).

Conclusions
The proton dissociation of [AAE]X PILs as a kind of Bio-PIL was systematically studied in green solvents, water and ethanol for the first time. The pK a values of [AAE]X PILs were from 6.99 to 7.52 in water and from 8. 54 Figure S1. (a) The increasing absorbance during the deprotonation of the acid indicator (4-nitrophenol) by the base.