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

Abstract

[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 (pK_a) 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 pK_a 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 pK_a 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.

1. 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 CO₂ 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 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 (pK_a) is one of the most significant physiochemical parameters. An accurate pK_a 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.

2. Results

2.1. 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.

The reactions of the determinant (HA⁺) and indicator (In⁻) can be described by Scheme 2.

The chemical equilibrium constant (K_a) can be obtained by the equation:

$$K_{\mathrm{a}}=\frac{K_{\mathrm{a}}{(\mathrm{HA}}^{+})}{K_{\mathrm{a}}\left(\mathrm{HIn}\right)}=\frac{\left[\mathrm{A}\right]\left[\mathrm{HIn}\right]}{\left[{\mathrm{HA}}^{+}\right]\left[{\mathrm{In}}^{-}\right]},$$

(1)

The acidic dissociation constant (K_a) of HA⁺ can be written as:

$$\mathrm{p}{K}_{\mathrm{a}}({\mathrm{HA}}^{+})=\mathrm{p}{K}_{\mathrm{a}}\left(\mathrm{HIn}\right)-\lg \frac{\left[\mathrm{A}\right]\left[\mathrm{HIn}\right]}{\left[{\mathrm{HA}}^{+}\right]\left[{\mathrm{In}}^{-}\right]},$$

(2)

where pK_a (HIn) is the pK_a 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 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,d respectively represent the absorption spectra of the indicator after adding the quantitative [AAE]X of [GlyC₁]NO₃ and [PheC₁]NO₃. The absorption intensity at the maximum absorption wavelength of the 4-nitrophenolate anion continually decreased after quantificationally adding [AAE]X in the solvents. The pK_a values were obtained by the change of the absorption intensity at the maximum absorption wavelength. The pK_a values of [AAE]X and the contrastive compounds in water by OIM according to Equation (2) are listed in

Table 1. The acid dissociation constant (pK_a) values of [AAE]X and the contrastive compounds in water.

Compound	pKa	SD
[GlyC1]NO3	7.67	0.02
[GlyC2]NO3	7.73	0.02
[ValC1]NO3	7.54	0.03
[SerC1]NO3	7.23	0.04
[SerC2]NO3	7.10	0.02
[PheC1]NO3	7.20	0.01
[PheC2]NO3	7.26	0.04
[MIM]+	7.13 (a)	/
Gly	9.78 (b)	/
Phe	9.31 (b)	/
EAN	10.43 (c)	/
[Et2N]NO3	10.68 (c)	/
[Et3N]NO3	10.55 (c)	/
[Pyri]+	5.17 (d)	/

Conditions: 25.0 (±0.1 °C). (a) Reference [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⁻, NO₃⁻, NTf₂⁻ and ClO₄⁻ 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. The pK_a values of [AAE]X with different anions and cations.

Compound	pKa					Average pKa	SD
	1	2	3	4	5
[PheC1]Cl	7.25	7.26	7.26	7.25	7.24	7.25	0.01
[PheC1]NO3	7.21	7.20	7.21	7.21	7.20	7.20	0.01
[PheC1]NTf2	7.23	7.25	7.24	7.24	7.23	7.24	0.01
[PheC1]ClO4	7.21	7.20	7.16	7.20	7.21	7.20	0.02
[PheC1]Cl	7.25	7.26	7.26	7.25	7.24	7.25	0.01
[D-PheC1]Cl	7.17	7.16	7.18	7.16	7.18	7.17	0.01
[PheC1]NO3	7.21	7.20	7.21	7.21	7.20	7.20	0.01
[D-PheC1]NO3	7.16	7.15	7.18	7.20	7.18	7.17	0.02

)

2.2. 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,

[HA⁺] + [K⁺] + [H⁺] = [OH⁻] + [X⁻].

(3)

Since all salts are considered as being completely ionized, [K⁺] equals the concentration of potassium hydroxide (after considering the dilution by the solution). Hence,

[HA⁺] + [KOH] + [H⁺] = [OH⁻] + [X⁻].

(4)

The total concentration of acid taken is present in two forms, HA⁺ and A. Consequently,

c₀ = [X⁻] = [HA⁺] + [A].

(5)

By combining Equation (4),

[HA⁺] = c₀ + [OH⁻] − [KOH] − [H⁺].

(6)

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):

$$\mathrm{p}{K}_{\mathrm{a}}\left(\mathrm{HA}\right)=-\lg (\frac{a_{\left({\mathrm{H}}^{+}\right)}{a}_{\left(\mathrm{A}\right)}}{a_{{(\mathrm{HA}}^{+})}})=\mathrm{pH}-\frac{c_0-[{\mathrm{HA}}^{+}]}{[{\mathrm{HA}}^{+}]}.$$

(7)

The pK_a values of [AAE]X by PTM are presented in

Table 3. The pK_a values of a part of [AAE]X by the overlapping indicator method (OIM) and potential titration method (PTM).

Compound	pKa		SD
	OIM	PTM
[GlyC1]NO3	7.67	7.67	0.00
[GlyC2]NO3	7.73	7.82	0.09
[PheC1]NO3	7.20	7.19	0.01
[PheC2]NO3	7.26	7.21	0.05
[PheC1]Cl	7.25	7.19	0.06
[SerC1]NO3	7.23	7.24	0.01
[PheC1]NTf2	7.24	7.12	0.12

Conditions of PTM: [AAE]X (0.100 mol·L⁻¹) and KOH (0.500 mol·L⁻¹), 25.0 (±0.1) °C.

.

2.3. 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. The pK_a values of [AAE]X and the contrastive compounds in ethanol.

Compound	pKa	SD
[GlyC1]NO3	9.05	0.02
[ValC1]NO3	8.86	0.04
[SerC1]NO3	8.88	0.03
[PheC1]NO3	8.54	0.01
[PheC2]NO3	8.61	0.03
[PheC1]NTf2	8.61	0.03
[D-PheC1]NO3	8.59	0.01
[EtNH3]+	12.0 (a)	/
[Et2NH2]+	10.7 (a)	/
[Et3NH]+	10.22 (b)	/
[MIM]+	7.50 (b)	/
[Pyri]+	4.30 (a)	/

Conditions: 25.0 (±0.1 °C). (a) Reference [47]. (b) Reference [48].

.

2.4. Hammett Acidity

The Brønsted acidity associated with the Hammett acidity function (H₀) 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:

$$H_0=\mathrm{p}{K}_{\mathrm{a}}\left(\mathrm{HIn}\right)+\lg (\frac{[{\mathrm{In}}^{-}]}{\left[\mathrm{HIn}\right]}),$$

(8)

where pK_a (HIn) is the pK_a value of the 2,4-dinitrophenol indicator in water (4.12) [50], and [In⁻] and [HIn] are the molar concentrations of the unprotonated and protonated forms of the 2,4-dinitrophenolate indicator, separately.

The Hammett acidity functions (H₀) of some [AAE]X and the contrastive compounds in water are listed in

Table 5. The Hammett acidity functions (H₀) and pK_a values of some [AAE]X and the contrastive compounds in water.

Compound	H0	pKa (a)
[ValC1]NO3	4.37	7.54
[PheC1]NO3	4.33	7.20
Phe	6.44	9.31
[MIM]Cl	4.35	7.13 (c)
EAN	u.d. (b)	10.43
[Et2N]NO3	u.d. (b)	10.68
[Et3N]NO3	u.d. (b)	10.55

(a) Measured by the OIM. (b) Undetected. (c) Reference [42].

, Tables S1 and S2.

2.5. pK_a Values Calculated by DFT

The theoretical and experimental pK_a values of [AAE]⁺ in water and ethanol are summarized in

Table 6. The theoretical and experimental pK_a values of [AAE]⁺ in water and ethanol.

[AAE]+	pKa(Cal) (a)	pKa(Exp) (b)	pKa(Cal) (c)	pKa(Exp) (d)
[GlyC1]+	7.11	7.67	8.54	9.08
[GlyC2]+	7.15	7.73	8.60	/
[ValC1]+	7.83	7.54	9.34	8.85
[SerC1]+	7.40	7.23	8.96	8.88
[SerC2]+	7.69	7.10	9.27	/
[PheC1]+	7.24	7.20	8.78	8.50
[PheC2]+	7.46	7.26	9.05	8.61
[D-PheC1]+	7.24	7.17	8.78	8.58

(a) The calculated pK_a values in water. (b) The experimental pK_a values in water. (c) The calculated pK_a values in ethanol. (d) The experimental pK_a values in ethanol of [AAE]NO₃.

. The pK_a values of [AAE]⁺ are from 7.11 to 7.83 in water and 8.54 to 9.34 in ethanol, separately. Additionally, the pK_a values of [AAE]X determined by the OIM are from 7.10 to 7.73 in water and from 8.54 to 9.05 in ethanol, separately. The theoretical values of [AA]X obtained are consistent with their experimental values. Therefore, the DFT method is fast and convenient to calculate the acidity of [AAE]X.

3. Discussion

To systematically explore the acidity of [AAE]X, eight [AAE]⁺ and four anions (including nitrate (NO₃⁻), chloride (Cl⁻), perchlorate (ClO₄⁻) and trifluoromethanesulfonate (NTf₂⁻)) were studied from the viewpoint of the structure–property relationship. The cations and anions of [AAE]X used in this work are shown in Figure 3. All [AAE]X PILs were synthesized and characterized by the referenced method [15]. Being convenient for studying the structure–property relationship, the glycine methyl ester cation ([GlyC₁]⁺) was chosen as the fundamental [AAE]+ framework. Other cations, including the glycine ethyl ester cation ([GlyC₂]⁺), serine methyl ester cation ([SerC₁]⁺), serine ethyl ester cation ([SerC₂]⁺), phenylalanine methyl ester cation ([PheC₁]⁺), phenylalanine ethyl ester cation ([PheC₂]⁺), valine methyl ester cation ([ValC₁]⁺) and D-phenylalanine methyl ester cation ([D-PheC₁]⁺), could be viewed as the derivatives of [GlyC₁]⁺.

According to the pK_a values of [AAE]X and the contrastive compounds in water by the OIM, the acidic strength of [GlyC₁]NO₃ is the weakest in the studied [AAC₁]X (amino acid methyl ILs) and depends on the biggest pK_a values. The acidic strengths of [ValC₁]NO₃, [SerC₁]NO₃ and [PheC₁]NO₃ are stronger than that of [GlyC₁]NO₃, maybe due to the steric effect of the side chains in [ValC₁]NO₃ and [PheC₁]NO₃ and the hydrogen bonding in [SerC₁]NO₃. The acidic strength of [AAE]X has little difference in the same magnitude range, since the side chain of [AAE]X may exist in hydrogen bonding or steric hindrance with the –NH₃ group. The pK_a values of [GlyC₁]NO₃, [GlyC₂]NO₃, [SerC₁]NO₃, [SerC₂]NO₃, [PheC₁]NO₃ and [PheC₂]NO₃ are 7.67, 7.73, 7.23, 7.10, 7.20 and 7.26, respectively. The methyl ester and ethyl ester group hardly affects the acidity of [AAE]X due to the low maximum difference (0.23) between methyl ester and ethyl ester. The pK_a values of [AAE]X range from 7.10 to 7.73, which are smaller than those of glycine (Gly, 9.78), phenylalanine (Phe, 9.31), ethylammonium nitrate (EAN, 10.43), diethylammonium nitrate ([Et₂N]NO₃, 10.68) and triethylammonium nitrate ([Et₃N]NO₃, 10.55). The electron-withdrawing inductive effect of the ester group may result in the stronger acidity of [AAE]X than those of their precursors (amino acids), EAN, [Et₂N]NO₃ and [Et₃N]NO₃, whose acidity also depend on the protonated amino group.

The pK_a values of [AAE]X range from 7.10 to 7.73 in water, which are almost equal to that of imidazolium ([MIM]⁺, 7.13) salts. The acidic strength of [AAE]X is between pyridinium ([Pyri]⁺, 5.17) salts and EAN (10.43), [Et₂N]NO₃ (10.68) and [Et₃N]NO₃ (10.55).

The pK_a values of [PheC₁]Cl, [PheC₁]NO₃, [PheC₁]NTf₂ and [PheC₁]ClO₄ are 7.25, 7.20, 7.24 and 7.20 by the OIM, respectively. The pK_a values of [PheC₁]X with different anions are almost the same, to some extent. It seems like anions have insignificant effects on the acidity of [AAE]X PILs. In other words, the cation and anion of [AAE]X in water may be dissociated, because water is a typical high-polar solvent (ε = 80.100) [43]. Different from anions, the pK_a values of [L-PheC₁]NO₃, [D-PheC₁]NO₃, [L-PheC₁]Cl and [D-PheC₁]Cl are similar.

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 [GlyC₁]NO₃ 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 pK_a values of [GlyC₁]NO₃, [ValC₁]NO₃, [SerC₁]NO₃ and [PheC₁]NO₃ are 9.05, 8.86, 8.88 and 8.54, separately. There is also a tiny difference of the pK_a values that may be due to the side chain of [AAE]X. The pK_a values of [AAE]X are smaller than [EtNH₃]⁺ (12.0), [Et₂NH₂]⁺ (10.7) and [Et₃NH]⁺ (10.22) and bigger than [Pyri]⁺ (4.30). Therefore, the acidic strength of [AAE]X in ethanol is between [Pyri]⁺ and [EtNH₃]⁺, [Et₂NH₂]⁺ and [Et₃NH]⁺. The acidic strength of [AAE]X is slightly weaker than [MIM]⁺ (7.50) in ethanol, depending on the pK_a values.

To confirm the acidic strength of [AAE]X determined by the PTM and OIM, the Brønsted acidity associated with the Hammett acidity function (H₀) of [AAE]X was investigated in water. The maximum absorption peak of sodium 2,4-dinitrophenolate decreased as the acidity of the solution increased. The H₀ values of [ValC₁]NO₃, [PheC₁]NO₃ and phenylalanine (Phe) are 4.37, 4.33 and 6.44 in water, separately. This means the acidic strength of [AAE]X is almost same and is stronger than that of their precursors (amino acids). The acidity of EAN, [Et₂N]NO₃ and [Et₃N]NO₃ are so weak that the decrease of the indicator’s absorbance was not detected. The H₀ of [AAE]X is almost the same as [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 pK_a 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 H₀ reduced when the concentration increased. The relationship between the concentration of [ValC₁]NO₃ and [PheC₁]NO₃ and the H₀ can be obtained using the fitting equations. Their fitting equations are:

$$H_0=-1.982\lg \left(c^{0.5}\right)+3.014R^2=0.993\mathrm{and}$$

(9)

$$H_0=-1.471\lg \left(c^{0.5}\right)+3.319R^2=0.991.$$

(10)

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.

4. 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:

$$\mathrm{SD}=\sqrt{\frac{{\left({\mathrm{A}}_1-\overline{\mathrm{A}}\right)}^2+{\left({\mathrm{A}}_2-\overline{\mathrm{A}}\right)}^2+\cdots +{\left({\mathrm{A}}_{\mathrm{i}}-\overline{\mathrm{A}}\right)}^2}{\mathrm{i}-1}}; \overline{\mathrm{A}}=\frac{{\mathrm{A}}_1+{\mathrm{A}}_2+\cdots +{\mathrm{A}}_{\mathrm{i}}}{\mathrm{i}}.$$

(11)

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, In⁻, HA⁺ and A. The UV/vis absorption (A) (±0.0001) of the indicator during the titrations was obtained using a BFRL UV-1601 UV/VIS spectrophotometer 25.0 (±0.1) °C.

Hammett acidity function: The Hammett acidity function of the ILs was investigated on a BFRL UV-1601 UV/VIS spectrophotometer. Samples were measured in sealed 1-cm quartz cuvettes (Helma). The dyes of sodium 2,4-dinitrophenolate were used as the indicator and molecular probe for the determination of the H₀ with 5.0 × 10⁻⁵ mol·L⁻¹. Absorbance values of the indicator after adding acid in an aqueous solution were recorded between 330 and 500 nm at 25.0 (±0.1) °C. The concentration of [AAE]X PILs was 0.050 mol·L⁻¹.

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 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]:

where the solution-free energy was calculated by:

$$△{\mathrm{G}}_{\mathrm{sol}}^{*}=△{\mathrm{G}}_{\mathrm{g}}^{*}+△{\mathrm{G}}_{\mathrm{sol}}^{*}\left({\mathrm{HRef}}^{+}\right)+△{\mathrm{G}}_{\mathrm{sol}}^{*}\left(\mathrm{AAE}\right)-△{\mathrm{G}}_{\mathrm{sol}}^{*}\left({\left[\mathrm{AAE}\right]}^{+}\right)-△{\mathrm{G}}_{\mathrm{sol}}^{*}\left(\mathrm{Ref}\right).$$

(12)

Then, it led to the following equilibrium in Equation (13):

$$K_{\mathrm{a}}=\frac{{\left[\mathrm{AAE}\right][\mathrm{HRef}}^{+}]}{\left[{\left[\mathrm{AAE}\right]}^{+}\right]}={\mathrm{e}}^{\frac{-△{\mathrm{G}}_{\mathrm{sol}}^{*}}{\mathrm{RT}}}.$$

(13)

The calculation of the pK_a was obtained from Equation (14):

$$\mathrm{p}{K}_{\mathrm{a}}\left({\left[\mathrm{AAE}\right]}^{+}\right)=\frac{△{\mathrm{G}}_{\mathrm{sol}}^{*}}{2.303\mathrm{RT}}-\lg \left(\mathrm{Ref}\right).$$

(14)

The final expression for the pK_a can be written as

$$\mathrm{p}{K}_{\mathrm{a}}\left({\left[\mathrm{AAE}\right]}^{+}\right)=\frac{△{\mathrm{G}}_{\mathrm{sol}}^{*}}{2.303\mathrm{RT}}-\mathrm{K},$$

(15)

where K was a correction value dependent on the experimental values. The structures of [AAE]⁺, AAE, H₂O, H₃O⁺, Et₂OH and Et₂OH₂⁺ were optimized in the gas phase at the B3LYP/6-311++G(d,p) level [55,56,57]. On the basis of the optimized structures, the solvation-free energies of [AAE]⁺ and AAE in water or ethanol were calculated with the polarizable continuum model (PCM) at the B3LYP/6-311++G(d,p) level [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).

5. 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 to 9.05 in ethanol by the OIM, respectively. The acidity of [AAE]X determined by the PTM, Hammett method and DFT method coincided with those by the OIM. All the methods revealed that the acidic strength of [AAE]X was the almost same to [MIM]⁺ and between [Pyri]⁺ and [EtNH₃]⁺ in water. Additionally, the acidic strength of [AAE]X was slightly weaker than [MIM]⁺ and between [Pyri]⁺ and [EtNH₃]⁺ in ethanol. The certain Brønsted acidity of the bio-PILs [AAE]X will help them to be considered as feasible acidic catalysts with green and recoverable features. This insight into the proton dissociation will prompt PILs being applied widely.