Volatility of deep eutectic solvent choline chloride:N-methylacetamide

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Thermodynamics, Transport, and Fluid Mechanics

Volatility of deep eutectic solvent choline chloride:Nmethylacetamide at ambient temperature and pressure Yu Chen, Dongkun Yu, Yanhong Lu, Guihua Li, Li Fu, and Tiancheng Mu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04723 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Industrial & Engineering Chemistry Research

Volatility

of

deep

eutectic

chloride:N-methylacetamide

solvent at

choline ambient

temperature and pressure Yu Chen*,‡,1, Dongkun Yu‡,2, Yanhong Lu1, Guihua Li1, Li Fu1, Tiancheng Mu*,2 1

School of Chemistry and Material Science, Langfang Normal University, Langfang 065000, P.R.

China 2

Department of Chemistry, Renmin University of China, Beijing 100872, P.R. China

ABSTRACT: Common organic volatile solvents would volatilize in air, thus causing air pollution and harming human health. Deep eutectic solvents (DESs) are one type of the green solvents with low volatility, deemed as the new solvents of the 21st century. Previous investigation showed that the DESs would undergo volatilization at high temperature and/or vacuum pressure. However, how DESs volatile at ambient temperature and pressure is unknown. Here the volatilization of DESs in air at ambient temperature and pressure was investigated by in situ FT-IR and 2D FT-IR. ChCl (choline chloride):N-methylacetamide was taken as an example of DESs and we found that it underwent severely volatilization even at ambient temperature and pressure. Moreover, the dynamics process of the volatilization was investigated by 2D FT-IR. It was found that N-methylacetamide volatilizes from ChCl:N-methylacetamide by destroying the hydrogen bond between free NH group and ChCl in ChCl:N-methylacetamide during the first 60 min. H-bonded NH II with strong H-bonding interaction with ChCl also undergoes a remark alternation at this turning point at ca. 60 min. During the period from 60 min and 190 min, there also exists free NH in the DES ChCl:N-methylacetamide. The shift of free NH group ceases after ca. 70 min. After 190 min, most of N-methylacetamide in ChCl:N-methylacetamide have volatilized, only trace of N-methylacetamide is left, therefore, the shift of IR absorption peaks is negligible. This works shows that we should pay more attention to the volatilization of DESs even at ambient temperature and pressure.

1. INTRODUCTION Deep eutectic solvents (DESs) are eutectic mixture containing Lewis or Bronsted acids and bases.1-3 They are ionic solvents with a variety of anionic and/or cationic species. The concept of DESs was first proposed by Abbott in 2003.4 The type of interaction between DESs is H-bonds between hydrogen-bond donors (HBDs) and hydrogen-bond acceptors (HBAs). The melting point of DESs is lower than either of the individual components and DESs are generally liquid at or around room temperature. For example, the melting point of choline: urea (1:2 mole ratio) is 12 oC, ca. 100 oC lower than that of urea (133 oC) and ca. 280 oC lower than that of choline (302 oC). DESs are versatile alternatives to ionic liquids,5 which are also green solvents with high stability,6 and have been applied in many fields such as biomass dissolution7 and gases capture8,9. Compared to ionic liquids, DESs are easier to synthesis and more biodegradable while keeping the high designability. Common organic solvents are easily volatile, resulting in air pollution. DESs are deemed as green solvent for the 21st century because they are less toxic, more bio-compatible and more biodegradable. Thus, DESs have been paid much attention and applied in many fields such as material preparation,10-12 1

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separation and dissolution,13-17 electrochemistry, reaction and catalysis18 etc.. Generally, DESs are deemed as non-volatile, which renders DESs the merits of no loss and non-pollution. However, are they really so stable? Compared to numerous reports on the stability of ionic liquids, 19-26, 27-33 The volatilization of DESs is seldom reported. Inspired by the research on the stability (including volatility and decomposition) of ionic liquids we would like to give a thorough investigation on the volatility of DESs. As far as we know, the only previous report related to volatility of DESs came from our group.27 We found that HBDs and HBAs undergo volatilization at higher temperature after the decomposition of DESs’ cations and anions via the weakening of H-bonding interaction at around 250 oC.27 However, the direct evidence of volatilization of DESs components was not given. Besides, the volatilization mechanism of DESs was not clear. Moreover, volatility of DESs at ambient temperature and and/or vacuum pressure is more important than at elevated temperature due to the fact that DESs are commonly stored, transported and used at ambient temperature and pressure rather than at high temperature and reduced pressure. Hence we would like to investigate the volatilization of DESs at ambient temperature and pressure. N-methylactamide-based DESs, for example, N-methylacetamide with three Li salts, LiTFSI, LiPF6, and LiNO3, have been used as the electrolyte of the lithium-ion battery and electric double layer capacitors.34-36 The volatilization of N-methylactamide-based DESs at room temperature might result in the expansion of electrolyte and battery, leading to the risk of air pollution and battery explosion. Furthermore, the volatilization of N-methylactamide-based DESs would have a profound effect on the chemical structure and physical properties of the electrolyte, thus hindering the normal working of lithium-ion battery and capacitors. Therefore, in this study, we select ChCl:N-methylactamide as a typical DES to investigate the volatilization of DESs at ambient temperature and pressure by FT-IR and 2D FT-IR spectra. ChCl:N-methylacetamide is one of the DESs composing of HBA (ChCl) and HBD (N-methylacetamide). The rationale for the choice of reagents molar ratio (1 mol ChCl to 6 mol N-methylacetamide) is due to the fact that lower mole fraction of N-methylacetamide would not be helpful for the formation of DES. Another reason for 1 mol ChCl:6 mol N-methylacetamide is to improve the detection of dynamic volatility process of N-methylacetamide from DES ChCl:N-methylacetamide. The atmospheric behavior of DES ChCl:N-methylactamide in air was studied via the change of infrared spectroscopy (IR) as a function of time in situ. The volatilization of DES ChCl:N-methylacetamide could be monitored by the intensity decrease and shift of IR absorption peaks, and notable volatilization of N-methylactamide could be observed from the Difference Spectra. Moreover, perturbation-correlation moving-window two-dimensional correlation techniques (PCMW2D-COS) are utilized to detect the evolutional mechanism of volatilization and atmospheric water absorption by ChCl:N-methylactamide. PCMW2D-COS IR spectra are composed of synchronous mode (s-PCMW2D-COS) and asynchronous mode (as-PCMW2D-COS) since PCMW2D-COS techniques are powerful for investigating the dynamic process.37-40

2. EXPERIMENTAL SECTION 2. 1. Materials Choline chloride (ChCl, 98%) was purchased from J&K Scientific Ltd. N-methylacetamide (99.9%) was purchased from Sinopharm Chemical Reagent Co., Ltd. The chemicals were used as received. The DES ChCl:N-methylacetamide (with the ratio of 1 mol ChCl to 6 mol N-methylacetamide) was synthesized by stirring the two components at 80 ℃ until a homogeneous liquid was formed, and no impurities were detected in the DES by NMR and FT-IR techniques. Excess of ChCl was added in ethanol at 60 oC to form oversaturated solution. Then the up-level clear solution was cooled in the ice-water mixture for the 2


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purpose of recrystallizing ChCl, followed by drying in vacuum at 60 oC. The above process was repeated for three times for obtaining purer ChCl. Finally, the purified ChCl was mixture with N-methylacetamide with a certain mole ratio to synthesize DES for the IR measurement. 2. 3. IR Spectra All the IR spectra related to DES as a function of time were measured in the presence of moisture air. Specifically, the films of samples were coated on the surface of KBr pellet and measured by FT-IR instrument (Bruker Tensor 27, Germany) from 0 min to 355 min with an interval of 5 min. Parameters of FT-IR were: 4 cm-1 resolution, 64 scans, and 700～4000 cm-1 range. 2.4. PCMW2D-COS IR Spectra PCMW2D-COS IR spectra were processed by 2D Shige software proposed by Prof. Morita and Prof. Ozaki.41 PCMW2D-COS IR spectra were composed of two types, i.e., synchronous (s-PCMW2D-COS) and asynchronous (as-PCMW2D-COS). We had previously used PCMW2D-COS IR techniques to detect the evolutional mechanism of water sorption37,38,40,42 and

iodine capture39 by ionic liquids.

3. RESULTS AND DISCUSSION 3. 1. The formation of DES ChCl:N-methylacetamide Pure potassium bromide (KBr) pellet was used as the substrate coated with DESs or reactants for FT-IR measurement.

However,

during

the

volatilization

process

of

N-methylacetamide

from

ChCl:N-methylacetamide mixture, the absorption of water from air by KBr might affect the detection of volatilization process. To exclude or minimize the effect of KBr hygroscopicity, we exposed KBr pellet overnight to the moisture atmosphere. From Figure 1a we can find that the IR spectra of KBr pellet as a function of time from 0 min to 355 min almost keep unchanged. It indicates that the sorption of water from air by KBr could be ignored during the volatilization process of N-methylacetamide from ChCl:N-methylacetamide. Therefore, the effect of the water in KBr on the volatilization of N-methylacetamide could be ignored, and the H-bonds mainly exist between N-methylacetamide and ChCl rather than related to KBr. Successful synthesis of DES ChCl:N-methylacetamide from ChCl and N-methylacetamide could be corroborated by FT-IR spectra (Figure 1b and Table 1). The IR position of H-bonded NH I for pure N-methylacetamide is around 3305.9 cm-1, but that for ChCl:N-methylacetamide is around 3296.2 cm-1. It indicates a strong red shift of IR absorption peak for N-methylacetamide after mixing with ChCl to form DES. It might be due to a stronger H-bond between NH of N-methylacetamide and Cl of ChCl than the H-bonds in pure N-methylacetamide. H-bonded NH II of pure N-methylacetamide around 3114.9 cm-1 also shows a red shift of 5.8 cm-1 in DES ChCl:N-methylacetamide (3109.3 cm-1). It again corroborated the stronger H-bond interaction between NH of N-methylacetamide and Cl of ChCl than that in pure N-methylacetamide. The stronger NH-Cl H-bond would weaken the N-H stretch vibration, leading to a lower value of IR absorption peak position of NH after the formation of DESs. Similarly, we could observe a slight red shift (about 1.9 cm-1) of amide II band (NH) from N-methylacetamide to ChCl:N-methylacetamide. However, the shift of IR absorption peak is undetectable for the CH3 stretch vibration (CH3,ss) in N-methylacetamide when forming DES (Figure 1b and Table 1). It is because CH3 group in N-methylacetamide is inert to the H-bonds disruption. The C=O of N-methylacetamide (amide I band) should show an obvious change in IR peak position due to an easier tendency of H-bonds formation. 3



However, from Table 1 we cannot see the shift of amide I band of C=O. It indicates that the surroundings of C=O in N-methylacetamide is similar to that in ChCl:N-methylacetamide. Namely, the H-bonding strength between C=O and ChCl in DES is comparable to that between between C=O and NH in N-methylacetamide. Two IR absorption peaks in ChCl:N-methylacetamide (marked by star) could be solely assigned to ChCl as seen from Figure 1b. The peak around 1487.1 cm-1 (CH3(-N) of ChCl) undergoes a sever red shift (ca. 7.8 cm-1) after forming DESs with N-methylacetamide. The possibility of forming direct H-bonds related to CH3 is low. This red shift of CH3(-N) of ChCl might be due to the indirect effect from the strong H-bonds between ChCl and N-methylacetamide. Finger area peak of ChCl around 954.7 cm-1 could be seen a slight blue shift of 2 cm-1, also indicating the strong H-bonds between ChCl and N-methylacetamide. Among all the above shift of IR peak position, H-bonded NH in N-methylacetamide is the highest (-9.7 cm-1), indicating that ChCl and N-methylacetamide forms DES mainly via the H-bonds between NH of N-methylacetamide and Cl of ChCl. 3.2. Conventional FT-IR and difference IR spectra Figure 2 shows the conventional FT-IR and difference IR spectra of ChCl:N-methylacetamide in the presence of atmosphere as a function of time from 0 min to 355 min with an interval of 5 min. From Figure 2 we can see that the intensity of IR spectra for ChCl:N-methylacetamide decreases notably in the whole process. The IR spectra have been assigned by Paulson and Noda.43,44 At the beginning, ChCl:N-methylacetamide possess the IR peak of free NH, H-bonded NH, symmetry stretching vibration CH3,ss, amide I C=O, amide II NH, asymmetry bending vibration CH3(-C/N)ab and CH3(-N of ChCl)ab, symmetry bending vibration CH3(-N)sb and CH3(-C)sb, amide III C-N, rocking vibration CH3(-N)r (Figure 2a). As the time elapses, the intensity of the IR absorption peaks of ChCl:N-methylacetamide decreases (Figure 2a). However, the intensity decrease of different types of IR absorption peak is different. Specifically, the absorption peak of amide II NH and amide III C-N of ChCl:N-methylacetamide almost disappears, while the intensity of free NH, H-bonded NH and CH3(-N of ChCl)ab decreases about two thirds. The other IR absorption peaks of ChCl:N-methylacetamide remain low at the end of 355 min. The variation of intensity for different absorption peaks of ChCl:N-methylacetamide as a function of time could be more clearly presented by Difference Spectra (Figure 2b). The spectroscopy of ChCl:N-methylacetamide at the begin of 0 min could set as the standardized spectrum. Difference Spectra of DES could be obtained by subtracting the standardized spectrum at 0 min of the DES from all other spectra of the process (from the beginning at 0 min to the end at 355 min) with the interval of 5 min. Thus, the first IR spectrum is a line, which could be seen in Figure 2b. From the Difference Spectra over the range from 1100 cm-1 to 3700 cm-1, we can see that the decrease of intensity for amide I C=O is the largest, while the decrease of intensity for CH3,ss is the lowest. The order of intensity decrease could be roughly listed as: amide I C=O > amide II NH > H-bonded NH > free NH > CH3(-C)sb > CH3(-N)sb > amide III C-N > CH3(-N)r > CH3(-N of ChCl)ab > CH3,ss. 3. 2. Volatilization of ChCl:N-methylacetamide Figures 3a shows the IR spectra of pure N-methylacetamide exposed to the air at the beginning (0 min) and end (355 min), separately. We can see that almost all the IR absorption peaks of N-methylacetamide disappear after exposed to the air for 355 min, which indicates that pure N-methylacetamide volatilizes to the air at ambient temperature and pressure. No detectable IR signal of water is detected at 355 min also 4


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suggest that water absorption by N-methylacetamide is negligible when exposed to the air. The possible reason is that high volatility of N-methylacetamide prevents water sorption from air. The volatilization of ChCl:N-methylacetamide could be seen in Figure 3b. The IR spectroscopy of ChCl:N-methylacetamide at the beginning (0 min) is presented in Figure 3b. Unlike pure N-methylacetamide, IR absorption peaks of ChCl:N-methylacetamide (from 3200 cm-1 to 3600 cm-1, and from 1400 cm-1 to 1700 cm-1) do not disappear after exposing to the air for 355 min. In the time period from ca. 300 min to 355 min, the IR spectra of ChCl:N-methylacetamide almost keep unchanged. It means that the volatilization of N-methylacetamide from ChCl:N-methylacetamide at ambient temperature and pressure could be deemed to be maximized. Namely, the IR spectra of ChCl:N-methylacetamide would still keep almost unchanged even the DES is exposed to air for more time. To explore the volatilization process of ChCl:N-methylacetamide, we first compare the decreased IR spectra in Difference Spectra style (Figure 2b) and the IR spectra of N-methylacetamide (Figure 3a). Result shows that they are similar, indicating the direct evidence of the volatilization of N-methylacetamide from ChCl:N-methylacetamide. Moreover, the IR spectra of ChCl:N-methylacetamide at 355 min is consistent with the IR spectra of pure ChCl (Figure 3c). It suggests that the only ChCl remains after the volatilization of N-methylacetamide at ambient temperature and pressure for 355 min, providing the indirect evidence of N-methylacetamide volatilization. Figure 3d gives the IR spectra of pure water. If ChCl:N-methylacetamide absorbs water from air, the intensity of characteristic IR spectra of absorbed water (ca. 3300 cm-1 and 1650 cm-1) in ChCl:N-methylacetamide would increase during the period from 300 min to 355 min. However, the IR spectra keep almost unchanged during this process as shown in Figure 2. It means that the water absorption from air by ChCl:N-methylacetamide would be negligible during this period. In a word, the volatilization of N-methylacetamide from ChCl:N-methylacetamide hinders the water absorption from air as discussed above. Moreover, the volatility of ChCl:N-methylacetamide is evident at ambient temperature and pressure. 3.3. Red shift and blue shift of DESs during volatilization During the process of volatilization for ChCl:N-methylacetamide, the shifts of IR spectra would own three categories of changes: red shift, blue shift and no shift. The shifts of IR spectra are usually caused by the H-bonds strength in DES. Red shift of a group means the increased H-bonding interaction related to this group. Namely, the stronger H-bonding interaction related to the investigated group would weaken this bond strength of groups thus leading to a red shift. Similarly, decreased H-bonding interaction would result in a blue shift due to the increasing the bond strength.45-47 The notable blue shift could be observed for H-bonded NH and CH3,ss during the volatilization process of N-methylacetamide from ChCl:N-methylacetamide at ambient temperature and pressure (Figure 4). H-bonded NH I and II increase the IR wavenumbers of ca. 24 cm-1 and ca. 15 cm-1 at the time point of ca. 70 min, while their IR absorption peaks disappear after ca. 70 min. It indicates that the total H-bonding strength of H-bonded NH groups in ChCl:N-methylacetamide system decreases. The possible reason is that the H-bonding strength between ChCl and N-methylacetamide is stronger than that of pure N-methylacetamide. When N-methylacetamide volatilizes, the stronger H-bonds between ChCl and N-methylacetamide is broken. This tendency is consistent with the conclusion draw by the above red shift of H-bonded NH groups of N-methylacetamide after mixing with ChCl to form DES in Figure 1b. The dividing point of ca. 70 min is also almost consistent with the dividing point of ca. 60 drawn from the PCMW2D-COS spectra as discussed below. 5



The possible explanation could be described as below. N-methylacetamide molecules contribute to the ChCl molecular networks with remaining the intact structure of ChCl when the concentration of N-methylacetamide is low.48 The low-concentrated N-methylacetamide could not disrupt the H-bonding networks among pure ChCl. The H-bonds related to NH groups in this range mainly exist among N-methylacetamide. In this range, due to the high H-bonds strength among N-methylacetamide, a high value of IR wavenumbers of H-bonded NH are detected as shown in 70 min after the volatilization of N-methylacetamide (Figure 4). Actually, it is also consistent with the fact that DESs are hard to be formed when we mix N-methylacetamide with ChCl in a low mole ratio because low content of N-methylacetamide cannot break the strong H-bonds existing in solid ChCl. When the content of N-methylacetamide increases (i.e., before the volatilization of N-methylacetamide), the stronger H-bonds between ChCl and N-methylacetamide would replace some of the weaker H-bonds among pure N-methylacetamide molecules, leading to a red shift of H-bonded NH. In this time period, the H-bonds networks among pure ChCl are almost totally disrupted by concentrated N-methylacetamide with stronger H-bonds between N-methylacetamide and ChCl, resulting in the success synthesis of liquid DES. IR absorption peak of CH3,ss also disappear after ca. 70 min, while the blue shift value is ca. 10 cm-1 at the time point of ca. 70 min. Another blue shift of IR spectra is CH3(-N)sb, but with a lower value of 4 cm-1 until the end of 355 min. As discussed above, CH3 is not directly affected by the H-bonds between N-methylacetamide and ChCl, thus the shift value of CH3 is lower than that of H-bonded NH (ca. 24 cm-1 and ca. 15 cm-1). Red shift of the IR absorption peaks of group amide I C=O and amide III C-N of DESs ChCl:N-methylacetamide as a function of time was observed. All of the other remaining groups show the shift of IR absorption spectra less than 2 cm-1, which indicates that these groups are less affected during the N-methylacetamide volatilization process. 3.4. PCMW2D-COS IR spectra The s-PCMW2D-COS IR spectra of ChCl:N-methylacetamide in Figures 5a and 5c show that before 190 min ChCl:N-methylacetamide releases N-methylacetamide, which could also be verified by IR spectroscopy because the release of N-methylacetamide decrease the concentration of N-methylacetamide in ChCl:N-methylacetamide. The reduced N-methylacetamide changes the H-bonding interaction between ChCl and N-methylacetamide, among pure N-methylacetamide molecules, and among pure ChCl. It would be reflected on the s-PCMW2D-COS spectra during the period before 190 min. However, only a weak ChCl:N-methylacetamide signal of s-PCMW2D-COS appears after 190 min, indicating that the volatilization of N-methylacetamide almost take places to the end. It is understandable because H-bonding interaction between ChCl and N-methylacetamide is very strong even there is only trace of N-methylacetamide. The strong H-bonds between ChCl and N-methylacetamide would fix the N-methylacetamide around ChCl molecules, leading to nearly no volatilization of N-methylacetamide in this period and a slight change of s-PCMW2D-COS. The turning point at ca. 60 min in the as-PCMW2D-COS IR spectra of ChCl:N-methylacetamide (Figures 5b and 5d) implies the dividing point (difference) of H-bonding interaction between ChCl and N-methylacetamide. The dividing point at ca. 60 min means the H-bonding interaction changed a lot at this time. During the first 60 min, the H-bonding interaction between ChCl and N-methylacetamide is weak. However, the strength of H-bonds between ChCl and N-methylacetamide increases after 60 min. The technique of as-PCMW2D-COS IR spectra is previously reported to determine the dividing point of water absorption process by ionic liquids.37,38 Discrimination of halogen bonds and induced force for the ionic liquids to capture iodine was also determined by the as-PCMW2D-COS IR spectra.39 Here, the 6


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dividing point at 60 min for H-bonding interaction between ChCl and N-methylacetamide is distinguished, as discussed below. In the wavenumbers range of 2800 to 3600 cm-1, we can focus on PCMW2D-COS IR spectra for the groups of free NH, H-bonded NH (two categories, I and II), and CH3,ss (two categories) as shown in Figure 6. The PCMW2D-COS IR spectra of free NH (ca. 3500 cm-1) mean that the decreasing rate for the intensity of free NH reduces with the time. Similarly, the H-bonded NH I with high wavenumbers (ca. 3296.2 cm-1) also shows a reduced rate of intensity decrease. However, the decrease of IR intensity for the free NH is faster than that of H-bonded NH I (Figure 6). It means that free NH group from the ChCl:N-methylacetamide is more slowly released than H-bonded NH I from ChCl:N-methylacetamide, which is consistent with that the interaction between free NH group and ChCl is weaker that that between H-bonded NH and ChCl.

H-bonded NH I (ca. 3296.2 cm-1) with higher IR peak value is less affected by

the H-bonding interaction with ChCl when compared to H-bonded NH II (ca. 3109.1 cm-1) that owns strong H-bonds with ChCl. Thus, H-bonded NH I appears no diving point while H-bonded NH II possesses one dividing point in the as-PCMW2D-COS IR spectra (Figure 6b); and the strength change of H-bonds between H-bonded NH II and ChCl results in this turning point at ca. 60 min. One turning point also occurs for the as-PCMW2D-COS IR spectra of two categories of CH3,ss at ca. 60 min (Figure 6b). In this case, it is not directly caused by the H-bonding interaction with ChCl due to the inert H-bonding ability of methyl. We attribute this turning point of CH3,ss to the indirect effect from the above H-bonding NH group. Before the change of the H-bonding interaction related to H-bonded NH, the vibration of CH3 groups would be adjusted to adapt to the alternation of H-bonded NH. The adjustment of CH3 groups would alter their vibration strength, which could be reflected by the IR intensity. This could be corroborated by a slight earlier turning point for CH3,ss than H-bonded NH II in Figure 6b. In the wavenumbers range from 1700 cm-1 to 1100 cm-1, no turning point is observed in the as-PCMW2D-COS IR spectra (Figure 7). In this range, the IR active groups include amide I C=O, amide II NH, CH3(-N of ChCl)ab, CH3(-C)sb, CH3(-N)sb, amide III C-N, CH3(-N)r. One possible reason for the non-detection of turning point lies in the range of IR fingerprint region for these groups. The fingerprint IR absorption of ChCl and N-methylacetamide is easier to overlap with each other. Another reason might be from the slight H-bonding interaction change for these groups. This is consistent with the above slight shift for these groups in Figure 4. Despite of the slight effect of H-bonding interaction during the volatilization process for these groups, intensity of amide I C=O also changes for the longest time (Figure 7). 3.5. Proposed mechanism From the discussion above we can propose the mechanism as shown in Scheme 1 for the volatilization of N-methylacetamide from ChCl:N-methylacetamideat at ambient temperature and pressure. We previously find that DESs could evaporate at high temperature.27 During the period of the first 60 min, N-methylacetamide volatilizes from ChCl:N-methylacetamide by releasing mainly free NH group from ChCl:N-methylacetamide. Besides, the H-bonded NH II with strong H-bonding interaction with ChCl undergoes a remark alternation at this turning point (ca. 60 min) detected by the PCMW2D-COS technique. The effect of H-bonded NH II on the CH3,ss could also be observed at the turning point. The turning point of PCMW2D-COS IR spectra is consistent with that of blue shift of IR absorption peak. Moreover, the intensity of IR spectra in the range of fingerprint region goes through a smooth change as reflected by the PCMW2D-COS technique during this time period. It means that these forms of vibration 7



for the fingerprint groups would hardly be changed as a function of time during the volatilization process. The shift of IR absorption peaks also undergoes little changes, consistent with the results of no dividing point in PCMW2D-COS spectra. The breakage of the H-bonds between ChCl and N-methylacetamide means that N-methylacetamide volatilizes from DES ChCl:N-methylacetamide mainly by overcoming the H-bonding interaction between ChCl and N-methylacetamide. The breakage of the H-bonds among N-methylacetamide means that N-methylacetamide volatilizes from DES ChCl:N-methylacetamide mainly by overcoming the H-bonding interaction among N-methylacetamide. Decrease in the intensity of free NH is positively related to the breakage of the H-bonds among N-methylacetamide due to the fact that free NH mainly exists among N-methylacetamide, while the breakage of H-bonds between ChCl and N-methylacetamide could be enormously influenced by the IR spectra of NH I. Thus, we can conclude that the breakage of H-bonds among N-methylacetamide occurs before the breakage of H-bonds between ChCl and N-methylacetamide. The earlier breakage of H-bonds among N-methylacetamide than breakage of H-bonds between ChCl and N-methylacetamide could also be corroborated by the shift of IR peak in Section 3.3. During the period from 60 min to 190 min, the PCMW2D-COS IR spectra signal of free NH group were detected, which shows that free NH exists in the DES ChCl:N-methylacetamide. However, the shift of free NH group ceased after ca. 70 min, reflecting that the free NH group is more stable during this period (from 60 min to 190 min), which is less affected by the H-bonding interaction with ChCl. The H-bonded NH II dissociates from the ChCl and tends to aggregates with itself, as indicated by the turning point in PCMW2D-COS IR. However, H-bonded NH I at large wavenumbers with large aggregates at the beginning would not display such turning point due to its high stability. The other fingerprint groups owns little change in this range due to the little difference for those two types of H-bonds: between ChCl and N-methylacetamide, and among pure N-methylacetamide molecules. The PCMW2D-COS IR spectra show that the volatilization of N-methylacetamide from ChCl:N-methylacetamide is negligible during the period from 190 min to the end (355 min). Therefroe, the shift of IR absorption peaks is also negligible in this period because only trace of N-methylacetamide retained in this period and ChCl would prevent remained N-methylacetamide being released. Volatilization of all N-methylacetamide from DES ChCl:N-methylacetamide might require a high temperature, low pressure and long time. In this period, the volatilization of N-methylacetamide could be ignored. Unlike the above two periods (the first ca. 60 min, and from 60 min to 190 min), N-methylacetamide could be assumed to loaded into the molecular networks of ChCl after exposed to air for 190 min. 3.6 Volatility of other DESs DESs varying in HBA (e.g., ChI:N-methylacetamide with the mole ratio of 1:6) and HBD (e.g., ChCl:oxalic aacid with the mole ratio of 1:6) would also own a high volatilization at ambient temperature and pressure, as shown in Figure 8.

4. CONCLUSION IR spectra provide the direct evidence of volatilization of DES ChCl:N-methylacetamide at ambient temperature and pressure. Among the shift of IR absorption peaks, H-bonded NH in N-methylacetamide is the highest (-9.7 cm-1), indicating that ChCl and N-methylacetamide form DES mainly via the H-bonds between NH of N-methylacetamide and Cl of ChCl. As the time elapses, the intensity of the above IR absorption peaks of ChCl:N-methylacetamide decreases. The order of intensity decrease could be roughly 8


Page 8 of 18

Page 9 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


listed as: amide I C=O > amide II NH > H-bonded NH > free NH > CH3(-C)sb > CH3(-N)sb > amide III C-N > CH3(-N)r > CH3(-N of ChCl)ab > CH3,ss. ChCl remains after the volatilization of N-methylacetamide from ChCl:N-methylacetamide in air, providing the indirect evidence of N-methylacetamide volatilization. The water absorption from air by ChCl:N-methylacetamide would be negligible in spite of the volatilization process of N-methylacetamide during this period. During the period of the first 60 min, N-methylacetamide volatilizes by releasing mainly free NH group from ChCl:N-methylacetamide. H-bonded NH II with strong H-bonding interaction with ChCl also changes at this turning point (at ca. 60 min). During the period from 60 min to 190 min, there also exists free NH in the DES ChCl:N-methylacetamide. The shift of free NH group ceases after ca. 70 min, reflecting that the free NH group is more stable during the period from 60 min to 190 min. After 190 min, the volatilization of N-methylacetamide from ChCl:N-methylacetamide is negligible. The shift of IR peaks is also negligible in this time period because only trace of N-methylacetamide retained in ChCl in this time period. We anticipate investigating the volatilization of other DESs comprehensively and seek the rules between chemical structure and degree of volatility in the near future.

AUTHOR INFORMATION Corresponding Author * Yu Chen, E-mail: [email protected]. * Tiancheng Mu, Email: [email protected].

Author Contributions ‡

Yu Chen and Dongkun Yu contributed equally.

Notes The authors declare no competing financial interests.



ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (21773307 and 51502125).



REFERENCES

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Page 12 of 18

Page 13 of 18

(b)

(a)

Absorbance

ChCl N-methylacetamide ChCl:N-methylacetamide

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


355 min

0 min

Cl

O Cl

N

OH

H N

N

OH

O

H3 C



CH3

H N

H3 C

3500

3000

2500

2000

Wavenumber / cm-1

1500

3600

1000



CH3

3200

2800

2400

2000

1600

Wavenumber / cm-1

1200

Figure 1. FT-IR spectra of potassium bromide pellet (after being exposed to air overnight before) in air as a function of time. All the above FT-IR spectra correspond to the time from 0 min to 355 min with an interval of 5 min (a). FT-IR spectra of ChCl:N-methylacetamide，its reactants (ChCl and N-methylacetamide (b).

Table 1. IR peak position and the corresponding shift for N-methylacetamide, ChCl, and DES ChCl:N-methylacetamide. N-methylacetamide

ChCl

ChCl:N-methylacetamid

ΔDES-Rea.

/ cm-1

/ cm-1

e / cm-1

/ cm-1

H-bonded NH I

3305.9

3296.2

-9.7

H-bonded NH II

3114.9

3109.1

-5.8

CH3,ss (left)

2947.1

2947.1

0

Amide I C=O

1651.0

1651.0

0

Amide II NH

1566.1

1564.2

-1.9

1487.1

1479.3

-7.8

954.7

956.7

2

CH3(-N)

13



(a)

CH3,ss

CH3

H-bonded NH

Ⅰ

amide Ⅰ NH2

free NH

CH3 (-C)sb

355 min

Ⅰ



CH3,ss

amide Ⅰ C-N CH3 (-N)r

0 min



Ⅰ

CH3 (-C/N)ab CH3 (-N)sb

0 min

CH3 (-N of ChCl)ab

0

amide Ⅰ C=O

H N

OH H3 C

Absorbance

O N

CH3 (-C/N)ab CH3 (-N)sb

Absorbance

Cl

(b)

free NH

Ⅰ H-bonded NH

355 min

CH3 (-N)r amide Ⅰ C-N CH3 (-C)sb

amide Ⅰ NH

amide Ⅰ C=O

3600

3300

3000

1800

1500

Wavenumber / cm-1

1200

3600

3300

3000

1800

1500

Wavenumber / cm-1

CH3 (-N of ChCl)ab

1200

Figure 2. Normal (a) and Difference (b) FT-IR spectra of DESs ChCl:N-methylacetamide as a function of time. The decrease in peak indicates the volatilization of N-methylacetamide from DES ChCl:N-methylacetamide. All the above FT-IR spectra correspond to the time from 0 min to 355 min with an interval of 5 min. The red ★ label could be solely assigned to the IR peak of CH3(-N) in ChCl.

(b)

(a)

ChCl:N-methylacetamide (0 min) ChCl:N-methylacetamide (355 min)

N-methylacetamide (0 min) N-methylacetamide (355 min)

O

Absorbance

Absorbance

0 min H N H3 C

CH3

0 min Cl

O N

H N

OH H3 C

CH3

355 min

355 min

3600

3200

(c)

2800

2400

2000

Wavenumber / cm-1

1600

3600

1200

3600

3200

N

2800

2000

Wavenumber / cm-1

2400

2000

1600

1200

2000

1600

1200

water (pure)

OH

2400

2800

Wavenumber / cm-1

Absorbance

Cl

3200

(d)

ChCl:N-methylacetamide (355 min) ChCl (pure)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 18

1600

1200

3600

H

3200

2800

O

H

2400

Wavenumber / cm-1

Figure 3. FT-IR spectra of N-methylacetamide (a) and DES ChCl:N-methylacetamide (b) at 0 min (the begin) and 355 min (the end). IR spectra of ChCl:N-methylacetamide at 355 min (up) and pure ChCl (down) (c). IR spectra of pure water (d).

14


Page 15 of 18

Shift of IR peak / cm-1

24

16

Cl

O N

H-bonded NHⅠ H-bonded NHⅠ CH3,ss Amide Ⅰ C=O Amide Ⅰ NH CH3-(N of ChCl) CH3-(N)sb CH3-(C)sb Amide Ⅰ C-N CH3-(N)r

H N

OH H3 C

CH3

8

0

-8 0

50

100

150

200

250

300

350

Time / min Figure 4. Shift of FT-IR spectra as a function of time, i.e., IR peak of ChCl:N-methylacetamide minus that of components.

(b)

190 min

3600

-2.37

3200 -1.59

2800 2400

-0.81

2000 1600

-0.04

1200

0.43

0

50

Wavenumbers / cm-1

(a) Wavenumbers / cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.25

3200 -0.16

2800 2400

-0.08

2000 0.01

1600

0.06

1200 0

100 150 200 250 300 350

Time / min

(c)

60 min

3600

50

100 150 200 250 300 350

Time / min

(d)

Figure 5. Planar (a and b) and stereoscopic (c and d) synchronous (a and c, s-PCMW2D-COS) and asynchronous (b and d, as-PCMW2D-COS) PCMW-2DCOS for ChCl:N-methylacetamide.

15


O N

H N

OH H3 C

CH3

-2.37

3400

-1.59

3200

-0.81

3000 -0.04

2800

0

50 100 150 200 250 300 350

0.43

Wavenumbers / cm-1

Cl

3600

(b) -0.25 free NH

3400

-0.16

Ⅰ

3200

H-bonded NH Ⅰ

3000 2800



3600

(a)

Page 16 of 18

 0

-0.08 0.01

CH3,ss

0.06

50 100 150 200 250 300 350

Time / min

Time / min

Figure 6. Planar synchronous (a, s-PCMW2D-COS) and asynchronous (b, as-PCMW2D-COS)

1700

(a)

Cl

O N

H N

OH H3 C

CH3

-2.37

1600 -1.59

1500 1400

-0.81

1300 1200 1100

-0.04

0

50 100 150 200 250 300 350

0.43

Wavenumbers / cm-1

Wavenumbers / cm-1

PCMW spectra for ChCl:N-methylacetamide in the range of 2800～3600 cm-1 .

1700

(b) Amide Ⅰ NH CHs(-N of ChCl)ab CHs(-N)ab CHs(-N)sb CHs(-C)sb

1500 1400 1300

-0.16 -0.08

Amide Ⅰ C-N

0.01

1200 1100

-0.25

Amide Ⅰ C=O

1600

CHs(-N)r

0

0.06

50 100 150 200 250 300 350

Time / min

Time / min

Figure 7. Planar synchronous (a, s-PCMW2D-COS) and asynchronous (b, as-PCMW2D-COS) PCMW spectra for ChCl:N-methylacetamide in the range of 1100～1700 cm-1 .

(a)

(b)

ChI:N-methylacetamide

ChCl:oxalic acid

0 min

0 min

Absorbance

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Wavenumbers / cm-1


180 min

3600

3300

3000

1500

1200

180 min

3600

3200

Wavenumber / cm-1

2800

2400

2000

Wavenumber / cm-1

Figure 8. Illustration of the effect of HBA (ChI:N-methylacetamide with the mole ratio of 1:6, a) and HBD (ChCl:oxalic aacid with the mole ratio of 1:6, b) on the volatility of DESs.

16


Page 17 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


before 60 min

H3 C

reactants of DESs

H3 C

O H3 C N

H3 C Cl

H3 C

H

N

N

O

H

CH3

H

H-bonds disruption

CH3

N Cl

O

O

H

N

O

N CH3 O

H3 C

H

H

O

O

CH3 N

N

O

H

CH3

CH3

N H3 C

H

H

CH3

(turning point at ca. 60 min)

after 190 min H3 C

H3 C

60-190 min

H3 C

O H3 C

O H3 C

N H

Cl

H

N

retained nanostructure

N

N

Scheme

1.

H3 C

CH3

Proposed

O

H

O

CH3

H N

N H3 C

N

O

Cl

O

H

CH3

H

H

H

O

O

CH3

N

process

for

volatilization

of

ChCl:N-methylacetamide.

17


CH3

N-methylacetamide

from


Page 18 of 18

For Table of Contents Only

Volatility of deep eutectic solvents H3 C

H3 C

O H3 C

H3 C

N H

O

H

N

O

CH3 O

H N

H3 C

Room temperature Atmospheric pressure

H3 C

O N H

Cl

H

N

O

O

N H3 C

H

H

Cl

O

N

CH3

N

CH3

H3 C

H

H O

N

O CH3

H N

CH3

H3 C

CH3

18


CH3

Volatility of deep eutectic solvent choline chloride:N-methylacetamide

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