Yang Wu*,
Xiaoxue Ma,
Yao Li,
Wei Guan*,
Jian Tong and
Na Hu
College of Chemistry, Key Laboratory of Green Synthesis and Preparative Chemistry of Advanced Materials, Liaoning University, Shenyang, 110036, China. E-mail: wuyoung@hotmail.com
First published on 6th January 2014
The effects of water on the structures of the amino acid ionic liquid (IL) 1-ethyl-3-methylimidazolium glycine ([emim][Gly]) are explored by a classical simulation method. The density and surface tension of the [emim][Gly]–H2O mixture are experimentally studied by a standard addition method. Simulation and experiment show that the density of the [emim][Gly]–H2O mixture reaches the maximum at 2–4 mass% water. Different analysis tools, including radial distribution, an interstice model, and molecular intrinsic characteristic contours are used to describe the structural modifications of the [emim][Gly] and ionic aggregates as a function of the solution concentration. At xw < 0.33 (mole fraction), the isolated water and dimer are located at the interstices formed between the ions, do not modify the network of ILs, and slightly strengthen the interactions between the cation and anion. Consequently, a turnover in the evolution of the IL structures and properties ensues. At 0.33 < xw < 0.50, the formation of relatively large water clusters, such as trimers and tetramers, leads to interstice distention, breakage of the cation–anion network, and gradual loosening of the interactions. With a further increased water concentration, a bicontinuous phase is generated and ionic clusters disperse in a continuous water phase. The size and morphology of the water aggregates are evaluated and analyzed by several statistical functions.
Water is regarded as inimical to pure ILs, because the presence of trace amounts of water can drastically affect the physicochemical properties of ILs, such as density, viscosity, conductivity, and surface characters. This effect enables significant modifications in the rate and selectivity of chemical reactions carried out using these ILs. In the case of CO2 capture in a flue gas using ILs, Brennecke et al.12 found that large amounts of water decrease CO2 solubility. For membrane applications using acetate-based ILs, the presence of water significantly alters membrane performance.13 Meanwhile, several reports have recently shown that during biopolymer dissolution, pure ILs almost cannot dissolve proteins without denaturation. ILs containing a small amount of water are reportedly superior at dissolving and preserving proteins, in which water is an excellent partner of ILs.14,15
The component ions of ILs strongly interact with water through ion–dipole interactions. Analysis of the hydrated states of these ions elucidates the physicochemical properties of IL–water mixtures. Based on the work of Dupont et al.,16 numerous researchers have experimentally and theoretically investigated the interactions between ILs and water. Some research techniques include IR and Raman spectroscopy,17–19 NMR,20 SFG spectroscopy,21 X-ray crystallography,22 fluorescence,23 thermodynamics study,24,25 DFT,17,25,26 and MD.27–31 They found that water molecules are solitarily dispersed throughout the ILs at low water concentrations.17,20 With a slightly increased water concentration, the dimeric association of water molecules is observed in the mixture.18,28 With a further increased water concentration, the association of the water molecules increases and spreads throughout the ILs. Subsequently, small ionic clusters are formed, and a second continuous phase is ultimately formed effectively.19,30
Miscibility is important in considering the properties of IL–water mixtures. ILs with hydrophilic ions, such as halide (i.e. [Cl]− or [Br]−), carboxylate ([RCOO]−), or amino acid anions ([AA]−) are generally mostly miscible with water. Meanwhile, ILs with highly fluorinated and charge-delocalized anions such as [Tf2N]− and [PF6]− tend to form hydrophobic ILs and are immiscible with water. However, previous work has shown that even extremely “hydrophobic” ILs have important solubilities with water32 that may be due to ionic characteristics. The ions in ILs cannot be closely packed, and many interstices (or holes) between ions exist, which may be related to the macroscopic properties based on interstice theory.33–36 Thus, upon incorporation into ILs, the water molecules are initially positioned in the interstices of ILs, and hydrated ions are formed. Even a small amount of water can dramatically influence the liquid properties of ILs without any reaction occurring, as mentioned in ref. 15.
This study aims to elucidate the relationship between the number of water molecules and local molecular structure of 1-ethyl-3-methylimidazolium glycine ([emim][Gly]). Combined with the standard addition method (SAM), we identified an important factor that has not been clearly considered in theoretical analysis, i.e. the number of water molecules that would initially break the structure of the [emim][Gly]. The calculated thermodynamics characteristics of [emim][Gly] by molecular dynamics (MD) simulations are compared with experiments related to interstice theory. The molecular shapes and frontier electron densities of Yang37,38 are applied to fit the volume data of water and prove the effect of the interstice volume of ILs on the [emim][Gly] properties. Finally, the microscopic structure of the [emim][Gly], water aggregation, and the connection between them are elaborately explored.
Scheme 1 Schematic structure and atom-type notations of the 1-ethyl-3-methylimidazolium cation ([emim]+) and glycine anion ([Gly]−) in the AMBER force field. |
We prepared eight model systems containing solutions with xw = 0, 0.25, 0.33, 0.5, 0.67, 0.75, 0.89, and 0.92 (Table 1). The initial system geometries were generated by randomly inserting [emim][Gly] and water molecules into the simulation cell and then allowing the system to relax. This step was followed by equilibrating the system for 1 ns within the NPT ensemble at 298 K and 0.1 MPa, which were controlled by the Berendsen method.45 After the equilibration, the total intermolecular energies and densities were monitored until a steady state was reached, for example, Fig. S2† in the ESI shows the evolution of the pure [emim][Gly] system, as a function of time. Then, the next 1 ns trajectory within NPT was carried out to obtain the density and molar volume. Finally, considering the specific viscosity of the ILs, the production stage was continued for 5 ns under the NVT ensemble. During the production stage, configurations of the simulation box were recorded every 0.2 ps for the subsequent structural analysis. Cubic periodic boundary conditions were used to simulate the bulk liquid with the long-range electrostatic interactions, computed using the Ewald summation.
System | xw | xIL | Nw | NIL | Box (Å3) |
---|---|---|---|---|---|
a xw is the water molar fraction, and NIL and Nw are the numbers of ionic-liquid pairs and water molecules in each simulation box. | |||||
Pure IL | 0.0 | 1.0 | 0 | 192 | 36.90 |
3:1 | 0.25 | 0.75 | 64 | 192 | 37.00 |
2:1 | 0.33 | 0.67 | 96 | 192 | 37.26 |
1:1 | 0.50 | 0.50 | 192 | 192 | 37.91 |
1:2 | 0.67 | 0.33 | 192 | 96 | 30.98 |
1:3 | 0.75 | 0.25 | 192 | 64 | 27.84 |
1:8 | 0.89 | 0.11 | 192 | 24 | 22.67 |
1:12 | 0.92 | 0.08 | 192 | 16 | 21.29 |
Vinterstice = 1.3582(kbT/γ)3/2 |
An Anton Paar DMA 4500 oscillating U-tube densitometer was used to measure the density of the samples. The temperature in the cell was regulated to ±0.01 K with a solid state thermostat. Before the measurement, the apparatus was calibrated once a day with dry air and double-distilled, freshly degassed water. The value of the density of pure water was then measured by the calibrated apparatus at 298.15 ± 0.01 K and agreed well with the literature, within the experimental error.47 Finally, the densities of [emim][Gly] with different water contents were measured at the same temperature.
By the use of the tensiometer of the forced bubble method (DPAW type produced by Sang Li Electronic Co.), the surface tension of water was measured at 298.15 ± 0.01 K and was in good agreement with the literature, within an experimental error of ±0.1 mJ m−2.47 Then, the values of the surface tension of the samples were measured by the same method at 298.15 ± 0.01 K.
A comparison between the thermodynamics properties derived from experiment and those derived from the MD simulations were then conducted. We first benchmarked the force fields used for the [emim][Gly] in this paper. From constant-pressure MD simulations, the density (ρ) and molar volume (Vm) of pure [emim][Gly] were calculated and are listed in Table 3. The predicted ρ and Vm are 1.1960 g cm−3 and 154.88 cm3 mol−1 at 300 K, respectively, which agree well with the experimental density (1.1721 g cm−3) and molar volume (158.04 cm3 mol−1) by SAM at 298.15 K.46
System | xw | wH2Ob | ρ | Vm | VEm | Uinter | ΔHvap | c |
---|---|---|---|---|---|---|---|---|
a The simulation temperature was 300 K.b wH2O is the water mass fraction.c Experimental data by standard addition method. | ||||||||
Pure | 0.0 | 0.0 | 1.1721c | 158.04c | 150.36c | |||
1.1960 | 154.88 | 0.0 | −534.448 | 137.92 | 874.4 | |||
3:1 | 0.25 | 3.137% | 1.1993 | 119.60 | −1.167 | −418.856 | 122.08 | 999.9 |
2:1 | 0.33 | 4.567% | 1.1968 | 108.66 | −1.178 | −357.156 | 92.30 | 826.5 |
1:1 | 0.50 | 8.856% | 1.1897 | 85.42 | −1.225 | −296.741 | 99.72 | 1144.0 |
1:2 | 0.67 | 16.478% | 1.1865 | 61.68 | −1.754 | −204.706 | 75.52 | 1183.9 |
1:3 | 0.75 | 22.572% | 1.1787 | 50.74 | −1.778 | −169.298 | 72.04 | 1370.5 |
1:8 | 0.89 | 44.015% | 1.1268 | 32.30 | −1.112 | −93.7040 | 52.31 | 1486.4 |
1:12 | 0.92 | 52.734% | 1.1048 | 28.40 | −0.916 | −74.9789 | 45.55 | 1516.0 |
The densities of the mixture of H2O and [emim][Gly] are obtained from the MD simulations at different mole fractions of H2O and xw, as shown in Fig. 1 and Table 3. The densities are approximately decreased by the presence of water and strongly depend on the molar fraction of the water added. Interestingly, the densities of the [emim][Gly]–water mixture change to be slightly larger until xw = 0.25 (wH2O ≈ 3.137%), as listed in Table 3, and this phenomenon has been found in our previous experiments. For other typical ILs, such as [emim][NTf2] and [bmim][BF4], a slightly increasing trend has also been observed28 but has not attracted considerable attention. The interstice model provides a coherent explanation for this phenomenon. Given the large size and asymmetric shape of cations and anions in ILs, the ions may not be closely packed and many interstices may exist between ions. Thus, with small amounts of water, the intruding water preferred to enter the interstice through strong hydrogen bonds, which inevitably resulted in the slight increase in density and difficulty removing trace water from ILs. The specific water states, especially the number of water molecules in the interstice, are discussed in the following sections.
The non-linear behavior of densities clearly demonstrates the non-ideality of the [emim][Gly]–H2O mixtures, and this non-ideality can be measured through the excess thermodynamic properties. The inset in Fig. 1 shows VEm as a function of the mole fraction of water. Notably, the negative deviations from the ideal behavior may be attributed to the attractive interactions between [emim][Gly] and H2O. Although no experimental data are available for [emim][Gly], Torres48 measured the positive deviation in the VEm of [bmim][BF4] mixture, whereas Brennecke25 obtained the negative deviation for [emim][EtSO4]. We can observe that the plots of excess molar volume in Fig. 1 also reveal structural changes in moving from the IL-rich region to the water-rich region. Similar to the hydrophilic characteristic of [emim][Gly], VEm considerably decreases to about 1.2 cm3 mol−1 for xw = 0.25 (wH2O = 3.137%) and gently decreases until xw = 0.50 (wH2O = 8.856%). Then, VEm continuously decreases and then considerably increases beyond about xw > 0.80 (wH2O = 44.015%).
ΔHvap = 0.01121(γV2/3mN1/3) + 2.4 |
Fig. 2 Plot of surface tension vs. amount of water in [emim][Gly] by SAM at 298.15 K, where r is the correlation coefficients and s is the standard deviations. |
The enthalpy of vaporization and the cohesion (c) of the particles in the liquid phase are related to the change in internal energy, Uinter, which can be directly extracted from the MD simulations. Our estimations of ΔHvap and c are calculated as:
ΔHvap = RT − (Uinter − xwUionpair) |
c = −(Uinter − xwUionpair)/Vm |
The ΔHvap of the pure [emim][Gly] is similar to those of typical ILs. For example, the experimental vaporization enthalpy of [emim][BF4], [emim][Tf2N], and [bmim][NO3] are 157, 134, and 163 kJ mol−1, respectively.51 These values are much higher than those of ordinary molecular solvents because of the strong electrostatic ionic interactions. The calculated ΔHvap energies decrease in an approximately parabolic manner with xw, but interestingly, the energies are relatively small as xw approaches 0.33. Such a special behavior can also be found for the cohesive energy densities, as listed in Table 3. We consider that the structural organization of the [emim][Gly]–H2O mixture must exhibit special points at a relatively low water content, as clearly explained by the microscopic structures described below. At 300 K, the cohesive energy densities are 874.4 J cm−3 for pure [emim][Gly], 761 J cm−3 for [emim][PF6], and 912.7 J cm−3 for [bmim][BF4].52,53 By contrast, the c values of the heavy hydrocarbons hexadecane and naphthalene are 268 and 410 J cm−3, respectively.54 The extremely high c value of [emim][Gly] mainly results from electrostatic interactions and explains why these liquids have such low volatility. Additionally, with an increased water fraction, the c values increase stepwise with the only exception mentioned above.
Previous studies have analyzed the liquid structure by examining site–site pair RDFs. Fig. 3b and c show the RDFs between the oxygen and nitrogen atoms of [Gly]− and different hydrogen atoms of [emim]+, respectively. The hydrogen atoms of the cation are found to preferably distribute around the carbonyl O atom rather than the nitrogen atom in the anion. As shown in Fig. 3, the order of the activities of different H atoms in the cation is H2 > H4 > H1 = HC, which is consistent with the ab initio optimized geometry of the gas-phase ionic pairs for [emim][Gly].50
The cation–water and anion–water center-of-mass RDFs show that for a given solution, the first peak corresponding to the anion–water distances is higher and occurs at smaller separations than those observed for the cation–water counterparts. This finding suggests that water preferentially associates with [Gly]− rather than [emim]+, which is also illustrated by the site–site RDFs shown in Fig. S5.† For anion–water and cation–water, the first peaks gradually weaken with an increased water content, which agrees with the results for other ILs.28–30 At a low water content (xw = 0.25 and 0.33), the first and second peaks of the anion–water RDFs are almost unaffected, whereas for xw = 0.5, the first peak is significantly lower and the second peak gradually broadens, shift to a larger distance, and eventually disappears. Additionally, in the case of the water–ion interactions (Fig. 4 and S4†), the intensities almost decrease with an increased solution dilution. This does not necessarily indicate that the interactions between water and ions are weakened by solution dilution. The observed intensity shifts are controlled by the concentration and heterogeneous distribution of each species in solution. For [emim][Gly]-rich solutions, the water molecules tend to accumulate near the glycine anions, giving rise to the highest first and second peaks of the RDFs in Fig. 4, and the solutions become more heterogeneous. With further dilution, the water molecules start to accumulate more homogeneously, thereby contributing to the observation of progressively less intense first peaks and vanished second peaks.57
For xw = 0.5, the RDF between the water molecules starts to show two pronounced peaks at 2.75 and 6.85 Å. Additionally, at rshell = 4.55 Å, N for the water–water first solvation shell is 1.83. The snapshot in Fig. 6 also shows not only water dimers, but also trimers at xw = 0.5. At xw > 0.5, the water network tends to be gradually observed and eventually forms a second continuous microphase. The second peaks for the water–water RDFs are broadened and shifted to larger distances compared with those at xw < 0.5. At xw > 0.9, the less intense first peak and the vanished second peak indicate that the water molecules start to accumulate more homogeneously and begin to show structural features resembling those of pure water (Fig. 6).
Combined with the above analysis, we conclude that a turnover at about xw ≈ 0.33 occurs. Before this point, the added water enters the interstice of [emim][Gly] and primarily exists as isolated water and dimer, and no larger water aggregations are observed. Based on the interstice model, the average volume of the interstice of [emim][Gly] can be obtained (Vinterstice = 28.99 Å3), because of its surface tension (γ = 53.5 × 10−3 N m−1) at 298.15 K. This finding leads to the question, “How many can water molecules are located in these interstices and initially break the structure of [emim][Gly]?”. Based on the molecular intrinsic characteristic contour and water cluster structures,37,38 the calculated volumes of isolated water and dimer are about 13.55 and 31.37 Å3, respectively. Interestingly, the water monomer and dimer can comfortably be located in the interstices formed between [emim]+ and [Gly]−, and the water trimer in such interstices leads to crowding, which exactly explains the results of the analysis above. At this turnover point, the isolated and dimer molecules enter the interstices, which nearly do not change the IL network, and then slightly strengthen the interaction between the cation and anion. Thereafter, the gradual formation of relatively large water clusters distends the interstices and breaks the network of ILs. Although the activity of water in the interstices can be relatively lower, the effect on the IL properties is considerable.14
The P(n) probability for different solution compositions is shown in Fig. 8. At xw < 0.33, the molecules are isolated or belong to dimer clusters. At around xw = 0.5, another turnover occurs from a regime with water monomers, dimers, and trimers to a regime where larger aggregates become increasingly likely. At xw < 0.9, Fig. 6 and 8 clearly show that the water clusters span most of the possible size range. At xw > 0.9, almost all water molecules belong to a single large aggregate spanning the entire simulation box and gradually forms a second continuous phase.41,57 The connectivity index Cw can be used to further investigate the nature of water clusters surrounding the IL ions, such as spherical and compact droplets, linearly connected chains, or some intermediate simulations.41 Notably, the dimer clusters are excluded from the overall average because they are trivially linear. At xw < 0.5, the Cw index is always extremely close to unity, and the linear nature of the water cluster is clearly observed, as shown in Fig. 6 and 8. At xw = 0.5, 0.67, and 0.75, the calculated Cw values are 1.03607, 1.02186, and 1.0817, respectively. This finding indicates that after the two-water clusters located in the interstices, the water molecules highly favor to extension by the linear types through narrow slits in ILs. With further increased water content, i.e., at xw = 0.89 and 0.92, the Cw index gradually increases (1.149 and 1.285, respectively). This finding suggests that although the second continuous microphase of water is progressively formed, this microphase differs from that of the pure water system (as in bulk water, the Cw index nearly equals 2).
A particularly interesting result is that the maximum density of the [emim][Gly]–H2O solution is observed when the content of water xw < 0.33. The physical origins of this phenomenon are evaluated within the context of the interstice model, molecular intrinsic characteristic contour, microstructural RDF analyses, and statistical functions of the size and morphology of the water aggregates. Since the average volume of the interstices in the [emim][Gly] corresponds to the volume of a dimeric water molecule, at the low water content (xw < 0.33), the water molecules exist as isolated monomers and dimers in the interstices and no large aggregations of water molecules are observed. Although the monomers and dimers of water molecules cannot significantly modify the network of ILs, they are likely to promote the interactions between the cations and anions in the liquid. Consequently, changes in the specific properties of [emim][Gly], such as density, excess molar volume, and heat of vaporization, are observed in experimental evaluations and in MD simulations. At 0.33 < xw < 0.50, relatively larger clusters of water molecules (such as trimers), that are not located in the interstices, can be observed. The cation–anion network of the [emim][Gly] IL starts to break apart and subsequently, the cation–anion interactions gradually become weak. With a further dilution of the solution, the [emim]+–[Gly]− network is largely disrupted, generating ion clusters dispersed in a continuous water phase.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47410k |
This journal is © The Royal Society of Chemistry 2014 |