Surface Tension of 50 Deep Eutectic Solvents: Effect of Hydrogen

Jun 24, 2019 - Deep eutectic solvents (DESs) are liquid eutectic salts of two or more ... (9) Distillation, absorption, and extraction also need the d...
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Cite This: Ind. Eng. Chem. Res. 2019, 58, 12741−12750

Surface Tension of 50 Deep Eutectic Solvents: Effect of HydrogenBonding Donors, Hydrogen-Bonding Acceptors, Other Solvents, and Temperature Yu Chen,*,§,† Wenjun Chen,§,‡ Li Fu,*,† Yingze Yang,† Yaqing Wang,‡ Xiaohong Hu,† Fangen Wang,† and Tiancheng Mu*,‡ †

Department of Chemistry and Material Science, Langfang Normal University, Langfang 065000, Hebei, China Department of Chemistry, Renmin University of China, Beijing 100872, China

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S Supporting Information *

ABSTRACT: Surface tension is a key physiochemical property for the application of deep eutectic solvents (DESs) in the field of interface and colloid. However, the surface tension of DESs has not been systematically studied. Here, a comprehensive investigation on the surface tension of 50 typical DESs was carried out. The effect of hydrogen-bonding donors (HBDs) and hydrogen-bonding acceptors (HBAs) on the surface tension was investigated. Furthermore, the surface tension of mixed systems of DESs with other solvents, water, water+salt (e.g., KCl), ethanol, acetone, isopropyl alcohol, and ethyl acetate (EtAc), was studied. It was found that both HBDs and HBAs had a significant influence on the surface tension of DESs. The presence of crystal water in the salt component of DESs would decrease the surface tension of DESs. Besides, the surface tension of DESs increased significantly when the water mole fraction was higher than 0.9, which was well consistent with the tendency of IR spectra. However, the surface tension of DESs decreased continuously with the increase of the mole ratio of other investigated solvents. of surface tension. Also, the flotation of a needle on the water surface is ascribed to the high surface tension of water. Due to the low surface tension of disinfectants, it could be easy to bind to the cell walls of bacteria to kill them. The efficiency of toothpaste is affected by its surface tension; a low value of surface tension helps the spread of toothpaste in the mouth. The surface tension plays a key role in wetting, bubbling, permeability, and lubrication.9 Distillation, absorption, and extraction also need the data of surface tension for better designing of chemical processes. Surface tension of some DESs has been reported,9−20 for example, ChCl (choline chloride):malonic acid (1:1),19,20 ChCl:phenylacetic acid (1:2),19 ChCl:alkanolamine (1:6, 1:8, and 1:10),11 ChCl:lactic acid (ChCl:LA, 1:2),12 allyltriphenyl phosphonium bromide:diethylene glycol (1:4, 1:10, and 1:16), allyltriphenyl phosphonium bromide:triethylene glycol (1:4, 1:10, and 1:16),15 and ChCl:D-glucose (1:1, 1.5:1, 2:1, and 2.5:1).14 The mole ratio of component in DESs has a profound effect on the surface tension of DESs. Specifically, a high ratio of HBA would increase the surface tension of DESs.13 Mjalli et al. applied the parachor/Othmer equation17 and Guggenheim empirical equation21 to predict the surface tension of DESs.

1. INTRODUCTION Deep eutectic solvents (DESs) are liquid eutectic salts of two or more components, including Lewis/Brønsted acid and base.1−3 The components of acid and base in DES act as hydrogen-bond donor (HBD) and hydrogen-bond acceptor (HBA), respectively. DESs are deemed as green solvents of the 21st century due to their wide liquid-range, high biocompatibility, and biodegradability.4 The toxicity of DESs is dependent upon starting materials composition and mole ratio of the resulting mixture. After reasonable design, DESs could be tuned to their own negligible toxicity by selecting biocompatible starting material and certain mole ratios. Moreover, DESs could be easily prepared from readily available raw materials with low cost and simple procedures. Till now, DESs have been used in many fields, such as biomass utilization, gas absorption and conversion, and electrochemistry.2,5−7 Surface tension (γ) is defined as the energy required to increase the unit area of a new surface or as the force to close unit length in the surface of a liquid.8 It is one of the methods to evaluate the cohesive forces or interactions between components at the surface.8 The surface tension could make liquid hold up a weight (e.g., water striders) and aggregate themselves together. Mosquito eggs float on the water, resulting in the easy breeding of mosquito offspring. However, the breeding would be inhibited after the spray of other substances such as kerosene on the water due to the decrease © 2019 American Chemical Society

Received: Revised: Accepted: Published: 12741

February 14, 2019 May 22, 2019 June 24, 2019 June 24, 2019 DOI: 10.1021/acs.iecr.9b00867 Ind. Eng. Chem. Res. 2019, 58, 12741−12750

Article

Industrial & Engineering Chemistry Research

surface tension of DESs. All the DESs were prepared with the reported methods.22 The DESs were prepared in the glovebox for the purpose of refraining from impurities from the air. DESs were used directly to measure surface tension without further purification. The water content in DESs without water components was less than 950 ppm by Karl Fischer titration. Note that the water content of some DESs could not be measured by a Karl Fischer titrator (e.g., FeCl3-containing DESs), thus their water content was not determined. 2.2. Measurement. Surface tension of DESs was measured by tensiometer QBZY-1 (Wilhelmy platinum plate methods, Shanghai Fangrui Instrument Co., Ltd.) with the range of 0− 600 mN·m−1 and the accuracy of ±0.1 mN·m−1. Specifically, QBZY-1 was stabilized for 30 min before the measurement, calibrated with pure water at 20 °C, and correlated with buoyancy. Then, about 3 mL of DESs was loaded into a container and sealed before the surface tension measurement. After that, the Wilhelmy platinum plate (2.2 cm length and 1 cm width) was cleaned by pure water, followed by being fired by an alcohol lamp until the platinum plate was totally red. The advantage of a Wilhelmy platinum plate (the Wilhelmy plate method) over a du Nouy platinum ring (the du Nouy ring method) was the larger measuring range of surface tension, less deformation during the measuring process, and higher precision. After the platinum plate was heated with an alcohol burner and turned red, it was cooled for about 15 s. Finally, the surface tension of DESs was measured automatically by QBZY1 in 4 s. Only several seconds are needed for measuring surface tension, indicating that the interference from the air23,24 could be ignored. Before the measurement, after measurement, and during the heating process, the DESs container was sealed to avoid the disturbance from the air. The reported values of surface tension were obtained by averaging three replicate measurements. The error bar was depicted with the corresponding standard deviations. All the figures were displayed with the vertical range as the surface tension ranged from 36 to 67 mN·m−1 for a good comparison except for PEGbased DESs. The temperature was controlled by a watercontained thermostat coupled with mechanical agitation (501A with the range of R.T. ∼ 95 °C with the precision of ±0.5 °C, Shanghai Pudong Rongfeng Scientific Instrument Co., Ltd.) by flowing the water through the flexible pipe and hollow cylindrical glass bottle (in which the DESs container was placed and sealed) for about 10 min for the purpose of stabilizing the temperature. 2.3. IR Spectra. The IR spectra were measured by FT-IR instrument Bruker Tensor 27, Germany (2 cm−1 resolution, 40 scans, and 600−4000 cm−1). The DESs samples were placed within two pieces of KBr pellets for IR measurement to refrain from possible contaminants from air. The effect of KBr pellets on the IR spectra was ignored due to the low temperature and short-time exposure. D2O rather than H2O was used to conduct IR for the purpose of reducing the overlap of IR peaks.

However, the prediction method might not be applied for all systems, and the precision of the predicted data is not very optimistic. Moreover, the experimental data are not systematic, and the investigations have been carried out in different groups and by different methods, which makes the comparison of data more complex.8 Therefore, a consistent and systematic investigation on the surface tension of DESs is still important. Here, the surface tension of 50 typical DESs (Scheme 1) was measured, and the effect of HBDs, HBAs, temperature, water, Scheme 1. Chemical Structures of Components for DESs Investigated

organic solvent, inorganic salts, mole ratio, and component on the surface tension was investigated. Among all 50 DESs, only the surface tension of ChCl:glycerol, ChCl:EG, and ChCl:LA had been studied before. Apart from reporting ChCl-based DESs, our manuscript measured the surface tension of many new types of DESs, such as ChBr-based, ChI-based, PEGbased, TEAC-based, LiTf2N-based, and metal chloride-based DESs. Particularly, the effect of crystal water, organic solvent, and inorganic salts solution (in the whole mole fraction range) on the surface tension of DESs was for the first time investigated, to the best of our knowledge.

2. MATERIALS AND METHODS 2.1. Materials. Choline chloride (ChCl, 98%), tetrabutylammonium chloride (TEAC, 98%), betaine (98%), and lithium bis(trifluoromethylsulfonyl)imide (LiTf2N, >99%) were purchased from J&K Scientific Ltd. The D2O (99.9 atom %D) was supplied from J&K Scientific Ltd. Benzamide (99%) and FeCl3·6H2O (99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. Choline bromide (ChBr, 98%), choline iodide (ChI, 98%), formic acid (98%), acetic acid (98%), propionic acid (99.5%), urea (99%), thiourea (99%), acetamide (98.5%), ethylene glycol (EG, 98%), glycerol (99%), oxalic acid (98%), malonic acid (98%), glutaric acid (99%), citric acid (99.5%), lactic acid (LA, 98%), phytic acid (70 wt % solution in water), anhydrous N-methylacetamide (NMA, >99.9%), ZnCl2 (98%), MgCl2·6H2O (98%), poly(ethylene glycol) 200 (PEG, 99.5%), ethyl acetate (EtAc, 99%), ethanol (99%), acetone (>99%), isopropyl alcohol (>99%), phenol (>99%), KCl (>99%), and anhydrous FeCl3 (>99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Distilled water was used as the effect of water on the

3. RESULTS AND DISCUSSION The surface tension of DESs at 20 °C is shown in Table 1. Below, we discuss the effect of HBDs, HBAs, temperature, water, other solvents, mole ratio, and component on the surface tension. 3.1. Effect of HBDs on the Surface Tension. The effect of HBDs on the surface tension of DESs at 20 °C is shown in Figure 1. Figures 1a, 1b, and 1c present the surface tension of ChCl-/ChBr-based, PEG-based, and ZnCl2-/FeCl3-/MgCl212742

DOI: 10.1021/acs.iecr.9b00867 Ind. Eng. Chem. Res. 2019, 58, 12741−12750

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Industrial & Engineering Chemistry Research Table 1. Surface Tension of DESs Investigated no. 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

name of DESs ChCl:LA:phenol ChCl:LA:phenol ChCl:LA:phenol ChCl:LA:phenol:H2O ChCl:LA ChCl:LA ChCl:EG ChCl:EG:ZnCl2 ChCl:EG:FeCl3·6H2O ChCl:ZnCl2 ChCl:glycerol ChCl:glycerol:FeCl3· 6H2O ChCl:glycerol:ZnCl2 ChCl:glycerol:MgCl2· 6H2O ChCl:phytic acid ChBr:LA ChBr:EG CHI:glycerol betaine:LA:phenol betaine:LA:phenol betaine:LA:phenol betaine:LA:phenol:H2O betaine:EG betaine:LA betaine:formic acid TEAC:propionic acid TEAC:acetic acid glucose:LA:H2O glucose:LA:H2O ZnCl2:LA ZnCl2:EG FeCl3·6H2O:LA FeCl3·6H2O:oxalic acid FeCl3·6H2O:EG FeCl3·6H2O:urea FeCl3·6H2O:glycerol MgCl2·6H2O:EG MgCl2·6H2O:glycerol LiTf2N:NMA LiTf2N:glycerol PEG:propionic acid PEG:acetamide PEG:LA PEG:benzamide PEG:citric acid PEG:malonic acid PEG:urea PEG:glutaric acid PEG:thiourea PEG:oxalic acid

mole ratio/ mol:mol

surface tension/ mN·m−1

1:2:3 1:2:2 1:2:1 1:2:1:1 1:4 1:2 1:2 1:2:0.06 1:2:0.06 1:2 1:2 1:2:0.06

42.5 43.5 43.9 44.3 44.4 47.4 49.4 50.1 50.9 51.5 57.8 58.3

1:2:0.06 1:2:0.06

60.8 61.1

1:2 1:1 1:2 1:2 1:2:3 1:2:2 1:2:1 1:2:1:1 1:4 1:2 1:2 1:2 1:2 1:2:12 1:2:24 1:4 1:4 2:1 2:1 2:1 2:1 1:1 1:8 1:1 1:4 1:4 1:2 2:1 2:1 2:1 4:1 2:1 2:1 2:1 2:1 2:1

70.5 43.6 48.2 53.8 42.4 42.6 43.5 44.6 46.4 46.8 50.7 35.5 40.9 45.0 47.3 39.8 51.8 51.8 53.7 57.0 57.2 61.0 48.6 63.7 38.8 43.3 33.4 41.2 41.3 42.6 42.6 42.9 43.0 43.3 43.8 44.1

Figure 1. Effect of HBDs on the surface tension of ChCl-/ChBrbased (a), PEG-based (b), and ZnCl2-/FeCl3-/MgCl2-based (c) DESs. The comparison of surface tension between measured DESs and reported values (d).

The slight discrepancy of our data (ChCl:glycerol, 57.8 mN· m−1 and ChCl:EG, 49.4 mN·m−1) with the reported values (ChCl:glycerol, 57.24 mN·m−1 and ChCl:EG, 48.91 mN·m−1) might be due to several factors. First, the temperature is 20 °C in this work, while it is 25 °C in the work by Mjalli. Second, our method is the Wilhelmy plate method, differing from the du Nouy ring method by Mjalli.17 ChCl-based DESs own a higher value of surface tension than that of ChBr-based DESs on average at 20 °C (Figure 1a). The lowest surface tension for ChCl-based DESs is 47.4 mN·m−1 (i.e., ChCl:LA), and the highest surface tension of ChBr-based DESs is 48.2 mN·m−1 (i.e., ChBr:EG). While fixing the same HBA of ChCl (Figure 1a), the order of surface tension is ChCl:phytic acid > ChCl:glycerol:MgCl2·6H2O ≈ ChCl:glycerol:ZnCl2 > ChCl:glycerol:FeCl3·6H2O > ChCl:glycerol > ChCl:ZnCl2 > ChCl:EG:FeCl3·6H2O > ChCl:EG:ZnCl2 > ChCl:EG > ChCl:LA. It seems that the presence of FeCl3· 6H2O (i.e., ChCl:glycerol:FeCl3·6H2O, 58.3 mN·m−1), MgCl2· 6H2O (i.e., ChCl:glycerol:MgCl2·6H2O, 61.1 mN·m−1), and ZnCl2 (i.e., ChCl:glycerol:ZnCl2, 60.8 mN·m−1) slightly increases the surface tension when compared to ChCl:glycerol (57.8 mN·m−1). The surface tension of ChBr:EG and ChBr:LA at 20 °C is 48.2 mN·m−1 and 43.6 mN·m−1 (Figure 1a), respectively. The difference in surface tension is related to the nature of component, mole ratio, electrostatic interaction, van der Waals interaction, and intermolecular hydrogenbonding interaction.15 The strength of the hydrogen bond for DESs could be indicated by solvatochromic probes,25,26 neutron total scattering,27 molecular dynamics,28,29 DFT calculations,30,31 and IR spectra.23,24 It might also be the reason for the order of surface tension for TEAC-based (TEAC:acetic acid > TEAC:propionic acid), betaine-based (betaine:formic acid > betaine:LA > betaine:EG), and LiTf2Nbased (LiTf2N:glycerol > LiTf2N:NMA) DESs (Table 1). The order of surface tension for PEG-based DESs (Figure 1b) is PEG:oxalic acid (44.1 mN·m−1) > PEG:thiourea (43.8 mN·m−1) > PEG:glutaric acid (43.3 mN·m−1) > PEG:urea (43.0 mN·m−1) ≈ PEG:malonic acid (42.9 mN·m−1) > PEG:citric acid (42.6 mN·m−1) ≈ PEG:benzamide (42.6

based DESs, respectively. For DESs varying in HBD, ChCl:phytic acid (70.5 mN·m−1) owns the highest surface tension, followed by MgCl2·6H2O:glycerol (63.7 mN·m−1). PEG:propionic acid owns the lowest surface tension of 33.4 mN·m−1, followed by ZnCl2:LA (39.8 mN·m−1). Figure 1d indicates that our experimental results are consistent with previous results from other groups.9,12,17,18 12743

DOI: 10.1021/acs.iecr.9b00867 Ind. Eng. Chem. Res. 2019, 58, 12741−12750

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Industrial & Engineering Chemistry Research mN·m−1) > PEG:LA 41.3 mN·m−1) ≈ PEG:acetamide (41.2 mN·m−1). Surface tension of inorganic salts-based DESs at 20 °C is presented in Figure 1c. ZnCl2:LA owns the lowest surface tension, which is possibly due to the weak hydrogen-bonding interaction between ZnCl2 and LA. The weaker hydrogenbonding interaction usually leads to a lower surface tension, while the hydrogen-bonding network would enhance the surface tension (such as 72.8 mN·m−1 for pure water at 20 °C with stronger hydrogen bonds is higher than that of most common liquids). Surface tension of ZnCl2-based DESs is ordered as ZnCl2:EG (51.8 mN·m−1) ≈ ZnCl2:ChCl (51.5 mN·m−1) ≫ ZnCl2:LA (39.8 mN·m−1). The negligible difference in the surface tension between ZnCl2:EG (51.8 mN·m−1) and ZnCl2:ChCl (51.5 mN·m−1) might be ascribed to the similar strength in hydrogen-bonding interaction between the components in DESs. For FeCl3-based DESs (Figure 1c), the surface tension is ordered as FeCl3· 6H2O:glycerol (61.0 mN·m−1) > FeCl3·6H2O:urea (57.2 mN·m−1) ≈ FeCl3·6H2O:EG (57.0 mN·m−1) > FeCl3· 6H2O:oxlalic acid (53.7 mN·m−1) > FeCl3·6H2O:LA (51.8 mN·m−1). 3.2. Effect of HBA on the Surface Tension. Figure 2 shows the effect of HBA on the surface tension of DESs at 20

could be listed as MgCl2·6H2O:glycerol (63.7 mN·m−1) > FeCl3·6H2O:glycerol (61.0 mN·m−1) > ChCl:glycerol (57.8 mN·m−1) > ChI:glycerol (53.8 mN·m−1) > LiTf2N:glycerol (43.3 mN·m−1). It indicates that the crystal water in glycerolbased DESs (i.e., MgCl2·6H2O:glycerol 63.7 mN·m−1 and FeCl3·6H2O:glycerol 61.0 mN·m−1) slightly alters high surface tension when compared to that of ChCl:glycerol (57.8 mN· m−1). The surface tension of LiTf2N:glycerol (43.3 mN·m−1) might be useful for the application of DESs as the electrolytes of lithium-ion batteries.32,33 It means that the presence of inorganic salt influences the surface tension of DESs. Moreover, the higher surface tension of ChCl:glycerol is larger than that of ChI:glycerol, implying that the hydrogen-bonding interaction between ChCl and glycerol is stronger than that between ChI and glycerol. It is understandable because the Cl anion with a smaller atom radius more easily forms a hydrogen bond than an I anion. The surface tension of EG-based DESs decreases in the order of ZnCl2:EG (51.8 mN·m−1) > ChCl:EG (49.4 mN· m−1) ≈ MgCl2·6H2O:EG (48.6 mN·m−1) > ChBr:EG (48.3 mN·m−1) > betaine:EG (46.4 mN·m−1). Similar to the PEGbased DESs, the range of surface tension for EG-based DESs is also very small, ca. 5.4 mN·m−1. However, the maximal change of surface tension for EG-based DESs (5.4 mN·m−1) is higher than that for PEG-based DESs (2.9 mN·m−1). One of the possible reasons might be due to the weaker hydrogen-bonding ability of PEG (after polymerization) than EG (before polymerization). Thus, PEG is inert to the other components in DESs, while EG is more sensitive. Similarly, the higher tendency of hydrogen-bonding ability of EG (than PEG) would also mean a higher averaged surface tension of EG (than PEG). It could be corroborated by the higher surface tension of EG-based DESs (in Figures 2a and 2b) than that of PEGbased DESs (in Figure 1b). The surface tension of ChCl:EG (49.4 mN·m−1) is higher than that of ChBr:EG (48.3 mN·m−1) (Figure 2a). One of the explanations is the stronger hydrogen-bonding ability of the Cl anion in ChCl:EG than the Br anion in ChBr:EG. The Cl anion owns a smaller radium and a higher electronegativity than the Br anion, thus owning a higher hydrogen-bonding formation ability. Apart from the hydrogen-bonding ability, the surface tension of the component is also the key factor influencing the overall surface tension of DESs. The surface tension of DESs is determined by the nature, categories, and concentration of surface molecules and the interaction between HBA and HBD, among HBA, or among HBD. Propionic acid-based, LA-based, and oxalic acid-based DESs are selected as the acid-containing DESs for investigating the effect of HBA on the surface tension of DESs (Figure 2c). TEAC:propionic acid owns a higher surface tension than PEG:propionic acid. The surface tension of LA-based DESs is ordered as FeCl3·6H2O:LA (51.8 mN·m−1) > ChCl:LA (47.4 mN·m−1) > betaine:LA (46.8 mN·m−1) > ChBr:LA (43.6 mN· m−1) > PEG:LA (41.3 mN·m−1) > ZnCl2:LA (39.8 mN·m−1). The presence of inorganic components could either increase (FeCl3·6H2O:LA) or decrease (ZnCl2:LA) the surface tension of LA-based DESs. We had thought that the presence of crystal water in FeCl3·6H2O:LA would increase its surface tension. However, when we measure the surface tension of FeCl3:LA, we find that FeCl3:LA is too viscous and that its surface tension could not be measured by our method. That is to say, the presence of crystal water in inorganic salt (e.g., FeCl3· 6H 2 O:LA) would decrease the surface tension (e.g.,

Figure 2. Effect of HBA on the surface tension of alcohol-based DESs, i.e., EG-based (a) and glycerol-based DESs (b). Effect of HBA on the surface tension of carboxylic acid (propionic acid, LA-based and oxalic acid)-based DESs (c). Effect of HBA on the surface tension of ureabased DESs (d).

°C. Figures 2a and 2b present the surface tension of EG-based and glycerol-based DESs with different HBA. Figure 2c displays the surface tension of carboxylic acid-based (including propionic acid-based, LA-based, and oxalic acid-based) DESs. Figure 2d provides the effect of HBA on the surface tension of urea-based DESs. For all the DESs varying in HBA, the highest surface tension is MgCl2·6H2O:glycerol (63.7 mN·m−1), followed by FeCl3·6H2O:glycerol (61.0 mN·m−1). PEG:propionic acid owns the lowest surface tension of 33.4 mN·m−1, followed by ZnCl2:LA (39.8 mN·m−1). Glycerol-based DESs own a higher value of surface tension than EG-based DESs (Figures 2a and 2b). The lowest surface tension for glycerol-based DESs is 43.3 mN·m−1 (i.e., LiTf2N:glycerol). Surface tension of glycerol-based DESs 12744

DOI: 10.1021/acs.iecr.9b00867 Ind. Eng. Chem. Res. 2019, 58, 12741−12750

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Industrial & Engineering Chemistry Research

Figure 3. Effect of water, organic solvents, inorganic salts, components, and mole ratio on the surface tension of ChCl:LA, betaine:LA, and ChCl:glycerol. The defaulted temperature is 20 °C. The solvent of salts (e.g., KCl) is water. The molar concentration of KCl at 20 °C is ca. 4.0 M. The surface tension of DESs+phenol mixtures without data could not be measured by our method due to their solid states.

(Figure 3c). It suggests that the presence of DES/solvent mixtures would significantly decrease the surface tension of water. Surface tension of ChCl:LA:ethanol increases significantly before the mole fraction of Xsolvent = 0.2 but not so obviously for ChCl:LA. The effect of the mole ratio on both binary (Figure 3a) and ternary (Figure 3c) DESs is not obvious. The local minima of surface tension at the concentration of Xsolvent = 0.9 are also not changed for the above DESs varying in mole ratio. The effect of common solvent on the surface tension of betaine:LA and ChCl:glycerol is given in Figures 3d and 3e. The mole fraction of 0.9 is also the local minimal point for the surface tension of the betaine:LA+H2O mixture. For the ChCl:glycerol+H2O mixture, the mole fraction of 0.9 is also the turning point of surface tension although it is not the local minimal point. This tendency is suitable for the DESs-related inorganic salts solution and ternary DESs/H2O mixture (Figures S1−S3) too. For other solvents (e.g., ethanol, isopropyl alcohol, EtAc, acetone), the surface tension of DESs generally decreases with the concentration of solvents. This phenomenon was also reported by Edler for the ChCl:malonic acid+H2O system.10 Note that addition of EtAc and acetone into the ChCl:glycerol would lead to phase separation, thus the corresponding surface tension is not displayed in the figure. ChCl:LA (1:2, 47.4 mN·m−1) owns a slightly higher surface tension than ChCl:LA (1:4, 44.4 mN·m−1, Table 1). Increasing the mole ratio of phenol in betaine:LA:phenol or ChCl:LA:phenol slightly decreases the surface tension (Table 1). For example, ChCl:LA (1:2), ChCl:LA:phenol (1:2:1), ChCl:LA:phenol (1:2:2), and ChCl:LA:phenol (1:2:3) own the surface tension of 47.4 mN·m−1, 43.9 mN·m−1, 43.5 mN·m−1, and 42.5 mN·m−1, respectively. The presence of water in ternary DESs ChCl:LA:phenol (1:2:1, 43.9 mN·m−1) and betaine:LA:phenol (1:2:1, 43.5 mN·m−1) slightly increases the surface tension (i.e., 44.3 mN·m−1 for ChCl:LA:phenol:H2O (1:2:1:1) and 44.6 mN·m−1 for ChCl:LA:phenol:H2O (1:2:1:1)). Addition of more water into glucose:LA:H2O (1:2:12, 45.0 mN·m−1)

FeCl3:LA). Similarly, the surface tension of ChCl:glycerol:FeCl3 is 59.9 mN·m−1, higher than that of ChCl:glycerol:FeCl3·6H2O (58.3 mN·m−1). It again corroborates that crystal water in salt of DESs would make the surface tension of DESs lower. The order of surface tension for oxalic acid-based DESs is FeCl3·6H2O:oxalic acid (53.6 mN·m−1) > PEG:oxalic acid (44.1 mN·m−1). The higher hydrogen-bonding ability of Cl (in FeCl3·6H2O:oxalic acid) than PEG (in PEG:oxalic acid) contributes to the higher surface tension of FeCl3·6H2O:oxalic acid. The surface tension of FeCl3·6H2O:urea (57.2 mN·m−1) is higher than that of PEG:urea (43.0 mN·m−1), further corroborating this hypothesis (Figure 2d). 3.3. The Surface Tension of DESs+Other Solvents. We select ChCl:LA (1:2), ChCl:glycerol (1:2), and betaine:LA (1:2) as the representative DESs to investigate the surface tension of DESs+other solvents. Figure 3a shows the effect of water and organic solvents on the surface tension of ChCh:LA (1:2) at 20 °C. There is an obvious local minimum of surface tension at XH2O = 0.9 (mole fraction) for the system of ChCh:LA (1:2)+water. The presence of a small amount of DES significantly decreases the surface tension of water while having a very small effect on the surface tension of ethanol, acetone, isopropyl alcohol, and EtAc. However, adding other solvents (i.e., acetone, ethanol, isopropyl alcohol, EtAc) into ChCh:LA (1:2) leads to the decrease in surface tension of DES. We select ChCl:LA+water, ChCl:LA+1.5 M KCl, and ChCl:LA+saturated KCl to study the effect of salt on the surface tension. Results show that inorganic salt solution has a very limited effect on the surface tension of DES (Figure 3b). All of the above three systems own local minimum of surface tension at the concentration of Xsolvent = 0.9. It shows that the presence of a small amount of DES would significantly decrease the surface tension of water and inorganic salt solution. Surface tension of ChCl:LA:ethanol and ChCl:LA:phenol also has the dividing point at the mole fraction of Xsolvent = 0.9 12745

DOI: 10.1021/acs.iecr.9b00867 Ind. Eng. Chem. Res. 2019, 58, 12741−12750

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force and induced force. Further efforts are still needed to elucidate other factors influencing surface tension. ChCl:glycerol is selected to analyze the mechanism affecting surface tension of water/DES mixtures in detail. Figure 3f shows the effect of water on the surface tension of ChCl:glycerol. The surface tension of pure water is 72.8 mN·m−1 at 20 °C, thus we conceive that the addition of water into the ChCl:glycerol (57.8 mN·m−1 at 20 °C) would enhance the surface tension of ChCl:glycerol. Unexpected, the surface tension of ChCl:glycerol water solution is almost unchanged in the mole concentration range of 0 ∼ ca. 0.8 (Figure 3f). It implies that in this region of concentration, the hydrogen-bonding interaction between water and DESs is similar to that among water and among DESs because the surface tension is influenced by the overall hydrogen-bonding interaction. As expected, the surface tension increases with the mole fraction of ca. 0.8 ∼ ca. 1, indicating that XH2O = 0.9 (i.e., mass fraction of WH2O= 63 wt %) is the dividing point for the hydrogen-bonding alternation among the DESs/water system. It is highly possible that the chemical structure of DESs in this point is totally disrupted, which also indicates that ChCl:glycerol loses the nature of DESs and dissociates into the water solution of ChCl and glycerol. The whole conjecture of interaction between ChCl:glycerol and water was consistent with the conclusion drawn by previous reports.26,27,29,35 Edler et al. proposed that there was a dividing water concentration (51 wt %) to disrupt the retained structure of DESs in spite of using a different DES ChCl:urea.27 They also observed a minimal surface tension for the water+ChCl:malonic acid system.20 Toner et al. found that 35.8 wt % water was the maximal water concentration that retains the eutectic property of DESs ChCl:glycerol (1:2).29 However, the dividing water mass concentration here for surface tension of ChCl:glycerol at 63 wt % water concentration is higher than 35.8 wt % reported by Toner.29 The discrepancy might be due to the difference in their studying methods (theory), temperature (50 °C), and physical properties (bulk). 3.5. Effect of Temperature on the Surface Tension. We also select two systems (ChCl:LA (1:2)+water and ChCl:LA (1:2)+acetone) to investigate the effect of temperature on surface tension as a function of mole fraction of solvent. Figure 5a gives us a hint that the increase of 10 °C temperature from 20 to 30 °C can negligibly reduce the surface tension of ChCl:LA+water and ChCl:LA+acetone. Increasing the temperature does not change the local minimal point of surface tension at XH2O = 0.9. There is only limited difference in surface tension of ChCl:LA+H2O between the curve at 20 and 30 °C. It should be noted that this is the first observation of local minimal points for the surface tension of the DESs +H2O system. It might be due to the critical micelle concentration (CMC) of DES in water near XH2O = 0.9. Although CMC = 0.9 could be more determined, it still provided us a hint that more micelles would be formed when XH2O ≪ 0.9. It also means that the alternation of temperature could negligibly affect the CMC of the DESs+H2O system. Moreover, the variation of temperature also does not alter the decreasing tendency of surface tension for ChCl:LA+acetone. Specifically, surface tension of ChCl:LA+acetone at 20 °C decreases when the mole fraction of acetone increases. Similarly, at a higher temperature of 30 °C, the surface tension

also increases the surface tension (i.e., glucose:LA:H2O (1:2:24), 47.3 mN·m−1, Table 1). 3.4. Explanation of the Water Effect on Surface Tension by IR. IR spectra are the common route to study intermolecular hydrogen bonds.24,34 We use the IR absorption peak of D2O (νD2O) around 2520 cm−1 to represent the overall hydrogen-bonding interaction between water and DESs because νD 2O can reflect the overall hydrogen-bonding interaction, while other absorption peaks can only stand for the partial hydrogen-bonding interaction. A lower value of νD2O indicates a stronger hydrogen-bonding interaction between water and DESs and vice versa. For the systems of DES (ChCl:glycerol, ChCl:LA, and betaine:LA)+H2O, there is a turning point around the mole fraction of 0.9 (Figure 3f). The IR peak position of D2O (νD2O) in the D2O/DES mixtures as a function of mole fraction of D2O (XD2O) is measured. Results show that the changing tendency of νD2O is similar to that of surface tension (Figure 4)

Figure 4. IR peak position of D2O in the D2O/DESs mixture as a function of mole fraction of D2O. The IR peak position at XD2O = 0.1 is hard to distinguish.

but not so perfectly matched. The correlation coefficients between νD2O and the corresponding surface tension of D2O/ DESs mixtures are higher than 0.89 (0.98 for ChCl:glycerol, 0.89 for ChCl:LA, and 0.93 for betaine:LA). It is the first time that direct evidence is provided that surface tension of the water/DESs mixture is positively correlated to the strength of hydrogen bonds.15 The hydrogen-bonding interaction can also explain the surface tension for different systems of water/DESs. The order of surface tension can generally be listed as ChCl:glycerol +H2O > ChCl:LA+H2O > betaine:LA+H2O (Figure 3f). Correspondingly, the value of νD2O roughly owns the order of ChCl:glycerol+D2O < ChCl:LA+D2O < betaine:LA+D2O (Figure 4), meaning the hydrogen-bonding strength order of ChCl:glycerol+D2O > ChCl:LA+D2O > betaine:LA+D2O. The order of surface tension is positively correlated with that of hydrogen-bonding strength. The discrepancy might be due to the hydrogen-bonding interaction among D2O and among DESs or other forms of interactions, such as van der Waals 12746

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Figure 5. Effect of temperature on the surface tension of DESs+water and DESs+acetone mixtures as a function of mole fraction of solvents (a). Surface tension of some typical DESs as a function of temperature from 20 to 60 °C with the interval of 10 °C (b). The fitted line uses the Kelvin temperature as the variable x. The fitted equation is listed: ChCl:glycerol (1:2, y = 73.33−0.052x, R2 = 0.87), ChCl:LA (1:2, y = 72.97−0.086x, R2 = 0.93), betaine:LA (1:2, y = 71.01−0.083x, R2 = 0.89), glucose:LA:H2O (1:2:24, y = 74.67−0.093x, R2 = 0.90), PEG:propionic acid (1:2, y = 30.38−0.155x, R2 = 0.94), TEAC:propionic acid (1:2, y = 59.96−0.053x, R2 = 0.99), LiTf2N:NMA (1:4, y = 56.53−0.069x, R2 = 0.98), ChCl:glycerol:FeCl3 (1:2:0.06, y = 176.40−0.403x, R2 = 0.92), ChCl:LA (1:4, y = 60.42−0.055x, R2 = 0.99), LiTf2N:glycerol (1:4, y = 53.89− 0.036x, R2 = 0.92), ChCl:LA:phenol (1:2:2, y = 60.06−0.056x, R2 = 0.98), TEAC:acetic acid (1:2:2, y = 50.84−0.033x, R2 = 0.92), ChCl:LA:phenol (1:2:3, y = 61.02−0.065x, R2 = 0.89), ChCl:LA:phenol:H2O (1:2:1:1, y = 59.25−0.051x, R2 = 0.96), betaine:LA:phenol (1:2:2, y = 54.5−0.040x, R2 = 0.88), betaine:LA:phenol (1:2:3, y = 58.7−0.055x, R2 = 0.97), betaine:LA:phenol:H2O (1:2:1:1, y = 63.9−0.065x, R2 = 0.95), FeCl3·6H2O:LA (2:1, y = 127.66−0.261x, R2 = 0.98), betaine:LA:phenol (1:2:1, y = 64.43−0.067x, R2 = 0.99), ChCl:LA:phenol (1:2:1, y = 49.30− 0.018x, R2 = 0.96), ChCl:EG (1:2, y = 57.33−0.030x, R2 = 0.99), glucose:LA (1:5, y = 59.43−0.055x, R2 = 0.95), glucose:LA (1:7, y = 60.54− 0.060x, R2 = 0.95), glucose:LA (1:9, y = 59.01−0.055x, R2 = 0.93), glucose:LA (1:5, y = 70.53−0.072x, R2 = 0.99).

bromide:glycerol (1:14).18 Abbott’s group plotted the curve of surface tension of glycerol-based DESs vs temperature to find a linear relationship.13 The decreasing rate of surface tension as a function of temperature could be expressed by the slope a in the fitted equation y = ax + b. The value of slope is always negative due to the fact that increasing the temperature decreases the surface tension. A more negative value of the slope suggests that the surface tension is more sensitive to the temperature. Some representative DESs are selected for a detailed discussion (Figure 5b). The slope of LiTf2N:glycerol (−0.036) is higher than that of LiTf2N:NMA (−0.069). It indicates that both LiTf2N:glycerol and LiTf2N:NMA decrease the surface tension. Moreover, the decreasing rate of surface tension perturbed by temperature for LiTf2N:NMA is more sensitive than that of LiTf2N:glycerol. Apart from the effect of HBD on the decreasing rate of Li-based DESs, the influence of the mole ratio on that of ChCl-based DESs (e.g., ChCl:LA) is also analyzed. The slope of ChCl:LA (1:2) is −0.086, lower than that (−0.055) of ChCl:LA (1:4), implying a higher extent of the temperature-dependent effect on surface tension of DESs when decreasing the ratio of LA. Comparison between PEG:propionic acid (1:2, slope = −0.155) and TEAC:propionic acid (1:2, slope = −0.053) could conclude that HBA has an obvious effect on the slope. However, for LA-based DESs, the effect of HBA on the slope is very limited, i.e., ChCl:LA (1:2, slope = −0.086) and betaine:LA (1:2, slope =

of the ChCl:LA+acetone mixture is also reduced when adding more acetone into ChCl:LA. It indicates that the temperature would not alter the surface tension tendency as a function of concentration. However, in this case of the ChCl:LA+acetone system, no CMC is found because there is no dividing point at the whole concentration. It implies that ChCl:LA tends to form a micelle in water when the mole fraction of DES is near 0.1, while there is no micelle existing in acetone. The possible reason might be the presence of a hydrophobic group and a hydrophilic group for water in ChCl:LA, but all the groups in ChCl:LA are soluble in acetone. Figure 5b shows the effect of temperature on the surface tension of DESs from 20 to 60 °C with the intervals of 10 °C. Results show that the relationship between surface tension of DESs and temperature could be linearly fitted with the equation y = ax + b, where y, a, x, and b are the surface tension, slope, temperature, intercept, respectively. Mjalli et al. also found that the surface tension of ammonium- and phosphonium-based DESs was linearly correlated with the temperature.17 Ayoub et al. also reported that the surface tension of phosphonium-based DESs was decreased linearly with the temperature.15 Six glycerol-based DESs reduced their surface tension linearly with increasing the temperature as reported by Hayyan, such as y = 80.67−0.0883x for methyl triphenyl phosphonium bromide:glycerol (1:3), y = 89.17− 0.1205x for benzyl triphenyl phosphonium chloride:glycerol (1:16), and y = 83.58−0.1409x for allyltriphenyl phosphonium 12747

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i ∂S y jij ∂γ zyz = −jjj zzz j z k ∂A {T , p , nB k ∂T { A , p , nB

−0.083). Adding a trace of FeCl3 into ChCl:glycerol changes the slope extensively (−0.052 for ChCl:glycerol (1:2) and −0.403 for ChCl:glycerol:FeCl3 (1:2:0.06)). The effect of water is very important due to the high hygroscopicity of DESs.24 The result shows that the presence of water in DESs makes the surface tension more sensitive to temperature, e.g., the comparison between ChCl:LA:phenol:H2O (1:2:1:1, slope = −0.051) and ChCl:LA:phenol (1:2:1, slope = −0.018). The surface tension of DESs is the lowest at 60 °C in the temperature range 20−60 °C. It is understandable because low temperature (20 °C) can hardly break the cohesive energy of the surface molecules, thus leading to a higher surface tension. Instead, there is enough energy to disrupt the cohesive forces at relatively high temperature (60 °C, Scheme 2). Higher Scheme 2. Proposed Mechanism for the Effect of Temperature on the Surface Tensiona

temperature would decrease the hydrogen-bonding interaction among DESs or between DESs and other solvents, implying a lower tension of surface for the investigated system. Hayyan et al. also explained that the increase of temperature would destroy the intermolecular interaction of DESs (e.g., hydrogenbonding interaction), hence a lower surface tension.18 The basic equation of thermodynamics (eq 1) suggests that the Gibbs free energy (G) could be expressed as the function of temperature (T with the unit of Kelvin), pressure (p), concentration (nB), and surface area (A), in which S, V, μB, and γ represent entropy, volume, chemical potential, and surface tension, respectively. Eq 2 could be deduced from eq 1. (eq 3) would obtain negative

( ∂∂Tγ )A ,p,n

(eq

B

B

4), which means that increasing temperature would decrease the surface tension. Data in Figure 5b are consistent with eq 4.

■

( ∂∂AS )T ,p,n during the ∂γ is also nearly temperature investigated, the value of ( ∂T ) A ,p,n

Owing to the nearly constant value of

B

B

∑ μBdnB + γdA B

i ∂γ y Hγ = γ + TS γ = γ − T jjj zzz k ∂T { A , p , nB

(5)

(6)

ASSOCIATED CONTENT

S Supporting Information *

constantly negative from 20 to 60 °C. It suggests that surface tension is negatively correlated to temperature.36 As expected in Figure 5b, surface tension of all the DESs investigated could be well linearly fitted with the temperature. dG = −SdT + Vdp +

i ∂γ y S γ = −jjj zzz k ∂T { A , p , nB

(4)

ij ∂γ yz jj zz ChCl:glycerol:MgCl2· 6H2O ≈ ChCl:glycerol:ZnCl2 > ChCl:glycerol:FeCl3·6H2O > ChCl:glycerol > ChCl:ZnCl2> ChCl:EG:FeCl3·6H2O > ChCl:EG:ZnCl 2 > ChCl:EG > ChCl:LA; ZnCl 2 :EG > ZnCl2:ChCl ≫ ZnCl2:LA; FeCl3·6H2O:glycerol > FeCl3· 6H2O:urea ≈ FeCl3·6H2O:EG > FeCl3·6H2O:oxlalic acid > FeCl3·6H2O:LA; PEG:oxalic acid > PEG:thiourea > PEG:glutaric acid > PEG:urea ≈ PEG:malonic acid > PEG:citric acid ≈ PEG:benzamide > PEG:LA ≈ PEG:acetamide. The effect of HBA follows the order MgCl2·6H2O:glycerol (63.7 mN·m−1) > FeCl3·6H2O:glycerol (61.0 mN·m−1) > ChCl:glycerol (57.8 mN·m−1) > ChI:glycerol (53.8 mN·m−1) > LiTf2N:glycerol (43.3 mN·m−1); ZnCl2:EG > ChCl:EG ≈ MgCl2·6H2O:EG > ChBr:EG > betaine:EG; FeCl3·6H2O:LA > ChBr:LA > PEG:LA > ZnCl2:LA; FeCl3·6H2O:urea > PEG:urea. The presence of water in DESs increases the surface tension of DESs extensively when the water mole fraction is higher than 0.9. However, for other solvents the surface tension of DESs decreases continuously as a function of solvent concentration. The effect of mole ratio on the surface tension is very limited. Increasing temperature from 20 to 60 °C would decrease the surface tension of DESs.

The representative DES ChCl:glycerol shows a lower surface tension at higher temperature (a) and a higher surface tension at lower temperature (b). HBA and HBD represent the hydrogen-bonding acceptors and hydrogen-bonding donors, respectively. More flames indicate a higher temperature.

( ∂∂AS )T ,p,n

ij ∂S yz jj zz >0 k ∂A {T , p , nB

(2)

The surface tension of DESs equals to the surface free energy per unit area. Thus, the entropy of surface formation Sγ and enthalpy of surface formation Hγ could be deduced by eqs 5 and 6, respectively.15,36,37 Results show that Sγ is constant for a specific DES in a certain temperature range (Table S1 and Figure S4). The order of Sγ for different DESs is glucose: LA:H2O > ChCl:LA > betaine:LA > ChCl:glycerol. However, Hγ decreases with the temperature (Table S1 and Figure S5). ChCl:glycerol owns the highest value of Hγ among all the DESs investigated at a fixed temperature.

a

Positive

Article

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b00867. Figure S1, effect of water and KCl solution on surface tension of betaine:LA; Figure S2, effect of water on surface tension of betaine:LA:ethanol and betaine:LA:phenol; Figure S3, effect of water on surface tension of ChCl:glycerol:LA; Figure S4, entropy of surface

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formation Sγ for DESs; Figure S5, enthalpy of surface formation Hγ for DESs; and Table S1, effect of temperature on surface tension of DESs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Y.C., E-mail: [email protected]. Phone: +86-316-2188211. Fax: +86-316-2112462. *L.F., E-mail: [email protected]. Phone: +86-3162188211. Fax: +86-316-2112462. *T.M., E-mail: [email protected]. Phone: +86-10-62514925. Fax: +86-10-62516444. ORCID

Tiancheng Mu: 0000-0001-8931-6113 Author Contributions §

Y.C. and W.C. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21773307) for financial support.

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REFERENCES

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