Article pubs.acs.org/JPCB
Nanostructural Reorganization Manifests in Sui-Generis Density Trend of Imidazolium Acetate/Water Binary Mixtures Debostuti Ghoshdastidar and Sanjib Senapati* Department of Biotechnology, Bhupat and Jyoti Mehta School of Biosciences, Indian Institution of Technology Madras, Chennai 600 036, India S Supporting Information *
ABSTRACT: Ionic liquids (ILs) are emerging as a novel class of solvents in chemical and biochemical research. Their range of applications further expands when a small quantity of water is added. Thus, the past decade has seen extensive research on IL/ water binary mixtures. While the thermophysical properties of most of these mixtures exhibited the expected trend, few others have shown deviations from the general course. One such example is the increase in density of the 1-alkyl-3-methyl imidazolium acetate ([Rnmim][Ac])-based ILs with the addition of low to moderate concentrations of water. Although such a unique trend was observed for imidazolium cations of different tail lengths and also from independent experiments, the molecular basis of this unique behavior remains unknown. In this study, we examine the nanostructural reordering in [Rnmim][Ac] (n = 2−6) ILs due to added water by means of molecular dynamics simulations, and correlate the observed changes to the sui-generis density trend. Results suggest that the initial rise in density in these ILs mainly pertains to the water-induced increased spatial correlation among the polar components, where high basicity of the acetate anion plays a key role. At moderate water concentration, the density can rise further for ILs with longer cation tails due to hydrophobic clustering. Thus, while [emim][Ac]/water mixtures exhibit the density turnover at Xw = 0.5, [bmim][Ac] and [hmim][Ac] show the turnover at Xw = 0.7. The detailed understanding provided here could help the preparation of optimal IL/water binary mixtures for various biochemical applications.
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INTRODUCTION Over the past few decades, ionic liquids (ILs) have emerged as novel environmentally benign solvents of immense industrial significance.1,2 ILs are composed of voluminous, asymmetric ions that prevent the formation of stable crystal lattices, making them liquids at ambient temperatures. They contain both charged and hydrophobic moieties, and hence, unlike conventional molecular solvents, can dissolve a wide range of solutes of diverse polarities.1−5 Such versatile solvation abilities, along with the inherent low-toxicity, nonflammability, negligible vapor pressure, and high electrochemical and thermal stabilities, make ILs suitable “greener” alternatives to the widely used toxic organic solvents.6 Furthermore, through careful selection of cation−anion combinations, the chemical and physical properties of ILs can be modulated for task-specific applications.7 Hence, this class of “designer” solvents continues to find increasing use in an expanding number of areas, including polymer chemistry as self-assembly media;8 material chemistry for catalysis and synthesis;9 biotechnology for biocatalysis,10 biomolecular dissolution11−13 and stability;14,15 and electrochemistry for sensors, biosensors, and in electrochemical storage devices;16 etc. ILs are usually hygroscopic, and most of them can uptake a significant amount of water. The presence of water dramatically alters the solvation abilities of ILs. Of particular interest is the effect of water on the solubility and stability of biomolecules in © XXXX American Chemical Society
ILs. For example, while many proteins are barely soluble in neat ILs, addition of small quantities of water can assist protein solubility.15 IL/water binary mixtures also enhance enzyme structural and functional stabilities over neat ILs by preserving their hydration shells. Likewise, DNA can be preserved for long-term usage under ambient conditions in hydrated ILs.14 Similarly, the water content of hydrated ILs plays a crucial role in the dissolution and regeneration of cellulose from organic biomass.17 The wide applications of IL/water binary mixtures have prompted many researchers to determine the thermophysical properties of this versatile solvent medium. Beginning with the pioneering work of Seddon et al.,18 extensive research in this area has shown that the presence of water significantly alters the thermophysical properties of these binary mixtures. For example, densities and viscosities were found to decrease with increasing concentrations of water for the majority of the ILs.18−21 Surface tension also exhibited the expected trend of increase with addition of water for most IL/water binary mixtures.21−23 However, electrical conductivities of all IL/water mixtures show the expected trend of an increase at low water Special Issue: Biman Bagchi Festschrift Received: January 15, 2015 Revised: June 19, 2015
A
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The Journal of Physical Chemistry B mole fractions and a decrease at higher dilutions.21,24 Several groups also investigated the effect of component ions on the thermophysical properties of these systems. Rebelo and coworkers presented a detailed review on the surface tension of several IL/water binary mixtures. ILs containing small, hydrophilic ions were found to show a larger increase in surface tension with addition of water compared to hydrophobic ILs.22 Ohno and co-workers studied the dynamic phase transition behavior of IL/water binary mixtures comprising ammonium and phosphonium cations and various anions.25 The authors noted that the onset of phase transition was dependent on the hydrophilicity of the anions as well as their level of hydration. Likewise, the sign and magnitude of excess molar properties of IL/water binary mixtures were found to largely vary depending on the ion type.18,20,26,27 Interestingly, for some of the IL/water mixtures, certain thermophysical properties were found to deviate from the expected trends at low to moderate water mole fractions. For example, Liu et al. observed that the surface tension of imidazolium-based IL/water binary mixtures containing the [tetrafluoroborate] anion does not exhibit the expected extent of increase upon addition of low mole fractions of water.21 Seddon et al. have also noted deviations in the excess molar volumes of the same class of IL/water mixtures.18 The excess volume exhibited a very slow rise with the addition of water, up to a mole fraction (Xw) of 0.5. In another study by Rodriguez et al., molar volumes of other imidazolium IL/water binary mixtures, containing the [ethylsulfate] and [trifluoroacetate] anions, showed large negative deviations from the ideal behavior at low to moderate water concentrations.20 Recently, deviations were observed for the densities of the binary mixtures of [1-alkyl-3-methylimidazolium][acetate] ([Rnmim][Ac])/water.28−31 For example, Stevanovic et al. reported an increase in density from 1.0511 g cc−1 of neat [1-butyl-3methylimidazolium][acetate] ([bmim][Ac]) to 1.0621 g cc−1 of [bmim][Ac]/water binary mixture at Xw = 0.8.28 A similar phenomenon was observed for the binary mixtures of other ILs of the same family with different alkyl chain lengths. Thus, Maldonado et al. determined the densities of [1-ethyl-3methylimidazolium][acetate] ([emim][Ac])/water mixtures for the concentration range of Xw = 0−1 and observed an increase in density up to Xw = 0.61.29 Hall et al. studied the same system for concentrations of Xw ≥ 0.7 and found the experimental densities to exhibit clear deviations from theoretical predictions based on the mixing rule.30 Guan et al. also reported an increase in density of a series of [Rnmim][Ac]/water binary mixtures (n = 2−6) with addition of water in the IL-rich regime.31 For example, the densities of [1-hexyl-3-methyl imidazolium][acetate] ([hmim][Ac])/water systems increased from 1.0606 to 1.0656 g cc−1 for the studied concentration range of Xw = 0− 0.00135. However, the molecular basis of such a unique density trend is not known. In this study, we aim to unravel the nanostructural reorganization in [Rnmim][Ac] due to added water and connect the observed changes to their sui-generis density trend. Nanostructural organizations in neat ILs4,5,32−39 and their binary mixtures40−45 have been studied in the past by means of experiments and computer simulations. However, how these nanoscale changes manifest in the macroscopic observables, such as density or phase behavior, are scarcely studied.36,40,46,47 Hence, in this work, first we have presented the density turnover data available from experimental reports on [Rnmim][Ac]/water binary mixtures. Following that, we
have discussed results from a thorough investigation on the nanostructural organization of [emim][Ac]−, [bmim][Ac]−, and [hmim][Ac]−water mixtures. Finally, we have correlated the observed nanostructural changes with the available density data.
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SIMULATION DETAILS We performed all atom MD simulations of a series of pure ILs and their binary mixtures with water. Binary mixtures of three acetate-anion-based ILs comprising 1-alkyl-3-methylimidazolium ([Rnmim]+; n = 2, 4, 6) cations were studied. For the binary mixtures, the water mole fractions (Xw) were set at {0.05, 0.1, 0.3, 0.5, 0.7, 0.8, and 0.9}. To obtain the desired water mole fraction, the number of ion pairs in each system was fixed at 512 and the number of water molecules was adjusted. The compositions of the systems studied are presented in Table 1. Table 1. Summary of the IL/Water Binary Mixtures Studieda Xw
NIL
NWAT
%wtWAT
0.00 0.05 0.10 0.30 0.50 0.70 0.80 0.90
512 512 512 512 512 512 512 512
0 27 57 219 512 1195 2048 4608
0.0 0.5 1.0 5.0 10.0 20.0 30.0 50.0
a
Xw, NIL, NWAT, and %wtWAT are the mole fraction of water, number of IL ion pairs, number of water molecules, and percentage weight of water in the mixtures, respectively.
The atom notations used for IL are presented in Figure S1 (Supporting Information) to aid the discussion. The standard molecular mechanics potential energy function of the following form is used to describe the interatomic interactions in the systems E=
∑
K r(r − r0)2 +
bonds
+
∑
Kϕ 2
⎡A ij
∑⎢ i 0.7, water−water hydrogen bonding progressively replaces anion−water interactions. To illustrate the same, representative snapshots of hydrogenbonded networks from the simulated systems are shown as insets to Figure 3a, and the water coordination numbers of the acetate anion at the respective concentrations are presented in Table 2. At concentrations of Xw ≤ 0.3, the coordination
distribution indicate that water interacts more strongly with the hydrophilic [Ac]− anion than with the [bmim]+ cation. The site−site RDFs of OWAT−OAc exhibit the first minima at ∼3.0 Å, implying that water forms hydrogen bonds with the carboxylate oxygen of the anion. Up to Xw = 0.7, the peak intensities remain almost constant. A similar trend is observed for cation−water RDFs also. However, at Xw > 0.7, the peak intensities reduce consistently, signifying a more homogeneous distribution of the added water throughout the system. The observed transition in water distribution around IL ions was further probed by hydrogen bond analyses between anion and water. The geometric criteria of hydrogen bonds were used, where a radial distance of less than 3.5 Å between the donor and acceptor oxygen atoms and a donor−H−acceptor angle greater than 135° were reckoned.61 This criteria yielded 3.35 hydrogen bonds per molecule of water in a control simulation of TIP3P water, indicating a good match with the experimental value of 3.76 hydrogen bonds. Thus, both the geometric criteria for selection of hydrogen bonds and the water model were found to be suitable for analyses of hydrogen bonding interactions in the [bmim][Ac]/water mixtures. For all compositions, the number of hydrogen bonds formed per water molecule with another water or anion is presented in Figure 3a. Up to Xw = 0.7, hydrogen bonding between water
Table 2. Water Coordination Number (N) of Acetate Anion in [bmim][Ac]/Water Binary Mixtures at Different Water Mole Fractions (Xw)a Xw
N
Fbridged
Fbifurcated
0.05 0.10 0.30 0.50 0.70 0.80 0.90
0.09 0.19 0.74 1.69 3.38 4.35 5.61
0 0.016 0.130 0.406 0.292 0.154 0.077
0 0.006 0.031 0.163 0.662 0.820 0.910
a
Fbridged and Fbifurcated are the fraction of the total number of anions forming bridged and bifurcated hydrogen bonds, respectively, with water.
number progressively increases to 1, indicating that each anion interacts with a single water molecule. With further addition of water up to Xw = 0.5, almost every anion is coordinated by ∼2 water molecules. The illustrated hydrogen bond network at this concentration in Figure 3a-inset depicts the two carboxylate oxygens of acetate associated with one water molecule each, giving rise to anion−water bridges. The fraction of anions forming such bridged hydrogen bonds at different water mole fractions was computed and presented in Table 2. At this concentration, anion−water bridging is maximized, resulting in linear extension of the anion−water network. Upon further addition of water, the water coordination number of IL anions increases to ∼4 at Xw = 0.7. This implies that each carboxylate oxygen of acetate is now simultaneously hydrogen bonded to two water molecules, forming bifurcated anion−water interactions, as illustrated in Figure 3a (inset). A sharp increase in the formation of bifurcated interactions at Xw = 0.7 (Table 2) enables assimilation of more and more water into the anion− water network. Upon further dilution (Xw ≥ 0.8), anion−water bridged interactions diminish rapidly, and at Xw = 0.9, each anion is coordinated by ∼5.61 water molecules, similar to a water coordination number of 5.5−6.0 observed around [Ac]− anions in aqueous solutions at infinite dilution.62 At this concentration, water−water hydrogen bonding becomes stronger and a continuous water phase persists, breaking apart the polar IL network (Figure 3a, inset). Interestingly, such extensive interactions between anion and water induce structuring in the anion layer, as shown in Figure 3b. The broad distribution of anions in neat IL is progressively divided into two peaks, signifying local ordering among the [Ac]− anions in the presence of water. This local ordering is maximized at Xw = 0.7, indicated by the highest intensity of the first peak of the anion distribution (Figure 3b, inset). Anion Structuring Brings IL Cations Together. Waterinduced structuring in the anion layer results in enhanced spatial correlation among the polar head groups of the cation, as shown in Figure 4a. The center-of-mass distribution of the head groups indicates maximum structuring at 0.5 ≤ Xw ≤ 0.7.
Figure 3. (a) Number of water−[Ac]− (red) and water−water (black) hydrogen bonds as a function of Xw. Representative snapshots of the water−[Ac]− hydrogen-bonded network (gray dash) are shown for all compositions studied. Bridged and bifurcated anion−water hydrogen bonds are highlighted, respectively, at Xw = 0.5 and 0.7. Color scheme for atoms: cyan, C; red, O; white, water H. Anion H’s are omitted for clarity. Error bars are shown for the computed values. (b) Self RDFs of [Ac]− anions to show the distribution of all other anions around a central anion. Carboxylate carbon has been taken as the site of interest. The inset highlights a prominent anion structuring at Xw = 0.7. D
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connections between ring atoms. Evidently, the number of connections increases significantly in IL/water systems relative to neat IL, indicating increased proximity of cation head groups in the presence of water. A structuring in the anion layer can eventuate into improved packing among the cations only if the cation−anion interactions persist in the presence of water. As seen in Figure 4c, the cation headgroup−anion distributions are indeed unaffected by the addition of water up to Xw = 0.5. Notably, in neat [bmim][Ac], the most acidic hydrogen of the cation imidazolium ring (C2−H2, Figure S1, Supporting Information) formed bifurcated hydrogen bonds with the carboxylate oxygen (OAC) of two acetate anions simultaneously, apart from the prevalent single C2−H2···OAC hydrogen bonds. Such bifurcated hydrogen bonds have been observed in earlier studies on imidazolium-based ILs.64 Interestingly, the bifurcated hydrogen bonding interactions remained unaffected even in the presence of water up to Xw = 0.5, followed by a decrease at higher dilutions. Subsequently, with addition of water at Xw > 0.5, a surge in cation−water bifurcated H-bonds was observed. Cation−Cation Proximity Promotes Tail Aggregation. The persistent structuring of the polar IL domains promotes ordering of the hydrophobic alkyl tails of the cation. As shown in Figure 5, the center-of-mass distributions of the cation tails
Figure 5. Center-of-mass RDFs of the butyl chain of [bmim]+ showing the onset of cation tail aggregation at low to moderate water mole fractions (Xw ≤ 0.8).
Figure 4. (a) Center-of-mass RDFs of the imidazolium ring of [bmim] cation. The prepeak arises due to parallel packing of the vicinal imidazolium rings. (b) Snapshots of simulation boxes showing cationimidazolium rings (cyan). The ring atoms of cations that lie within a distance corresponding to the first RDF minimum from a central cation are connected with red lines to aid in visualization of the parallel arrangement between the rings. A predominance of red lines in Xw = 0.7 (right panel) indicates increased proximity of the cation head groups in the presence of water compared to Xw = 0.0 (left panel). (c) Site−site RDFs of [Ac]− around [bmim]+ as a function of Xw. The sites of interest include carboxylate oxygen of the anion (OANI) and C2 of the cationimidazolium ring. Atom notations for the cation are shown in Figure S1 (Supporting Information).
intensify with addition of water even up to Xw = 0.8, indicating aggregation of tail domains in the presence of water. Presumably, at high water concentrations, hydrophobic tails are driven together by increased van der Waals attractions. A similar structural enhancement of the nonpolar IL domain, as a result of improved spatial correlation in the polar domain, has been previously reported for imidazolium-based ILs containing longer alkyl tails.39,40 Thus, water induces concerted structuring of the polar and nonpolar domains of [bmim][Ac], which maximizes at 0.5 ≤ Xw ≤ 0.7. Notably, an earlier experimental study reported that in the same concentration regime [bmim][Ac]/water systems exhibit negative excess molar volume.28 A negative excess molar volume implies that the binary [bmim][Ac]/water systems, in this concentration range, are more densely packed than the individual components. Upon further addition of water, anions are completely solvated with a water coordination number of ∼5.66, similar to the value of 5.5−6.0 reported for [Ac]− anions in aqueous solutions at infinite dilution.62 Hence, at Xw = 0.9, water completely screens anion−cation interactions, leading to disintegration of both polar and nonpolar domains. This is reflected in the diminished peak intensities of all ion−ion distributions at this concentration (Figures 2−5).
The presence of a prepeak at ∼4 Å signifies a parallel arrangement of the vicinal imidazolium rings, as defined earlier by Bowron et al. for a similar IL, [emim][Ac].63 These authors and others have shown that strongly coordinating anions, like [Ac]−, [NO3]−, and halides, induce stacking among the imidazolium rings of cations.39,40 In the presence of water, the highly basic acetate anion further reinforces packing of the cation head groups. For a vivid description of this headgroup structuring with addition of water, snapshots of the simulated systems depicting the imidazolium rings are presented in Figure 4b. Cation rings lying within a distance corresponding to the first RDF minimum from a central ring are indicated by E
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The Journal of Physical Chemistry B Longer Cation Tails Accomplish More Ordered Nanostructuring. To investigate the effect of IL cation tail on water-induced nanostructuring, we simulated binary mixtures of [Rnmim][Ac] ILs containing a smaller [emim] and a longer [hmim] cation for the same compositions studied above. Both systems exhibited increased anion−anion structuring and enhanced cation headgroup proximity with addition of water (Figures S2 and S3, Supporting Information). Notably, at all concentrations, the highest anion−anion ordering was observed for the binary systems of [hmim][Ac], followed by [bmim][Ac] and [emim][Ac], as evident from the relative peak intensities (compare Figure 3b and Figure S2, Supporting Information). Aggregation of the alkyl tails also followed the same trend, with the hexyl tail exhibiting the highest structuring (compare Figure 5 and Figure S4, Supporting Information). The [emim] cation with the short ethyl group barely shows any distinct trend of tail aggregation with increasing water concentration, unlike [bmim] and [hmim] cations. Nanostructuring in neat ILs usually takes place through the segregation of the polar domain of the anion and cation headgroup and the nonpolar domain of the cation tail.4,5,32−34,37,38 Interestingly, as our results show, nanostructuring is strengthened in the presence of water, particularly for ILs with long hydrophobic tails. This could be due to somewhat increased mobility of these relatively larger moieties in the less viscous water and their aversion to groups of opposite polarity. For a vivid illustration of the differential tail aggregation in the three systems, snapshots of the simulation box at representative concentrations are presented in Figure 6. Following a representation proposed by Lopes et al., the connections within and between moieties in the polar domain of the IL are colored red and those within the nonpolar domain are colored gray.35 While the shorter ethyl groups barely cluster either in the absence or presence of water, the butyl chains form large interspersed islands and the hexyl chains aggregate into a larger microphase in water. Most notable differences in cation tail aggregation with increasing chain length were observed at water concentrations of Xw > 0.5. Beyond this concentration, aggregation between alkyl tails of the [hmim] cation was enhanced, while the reverse was observed for [emim]+ (Figure 5 and Figure S4, Supporting Information). Since aggregation could result in a heterogeneous distribution of polar and nonpolar domains, as shown in Figure 6, we examined the density distributions of the tail groups across the simulation box. Domains of aggregated tails separated by the polar domain, comprised of anion, cation head groups, and water, exhibited distinct peaks interspersed with deep valleys (Figure 7). As the figure shows, at Xw ≤ 0.5, the density profiles of the tails are similar in [emim][Ac] and [hmim][Ac] systems, indicating that no effective tail aggregation has started. At 0.5 < Xw ≤ 0.8, the tail domains in [hmim][Ac] strongly aggregate, while those in [emim][Ac] are homogeneously dispersed. At Xw = 0.9, the aggregated tail domains diffuse out completely, even in the [hmim][Ac] system, signifying a more homogeneous distribution of polar and nonpolar domains. Ordered Nanostructuring Manifests in Density Turnover. The characteristic nanostructuring exhibited by the [Rnmim][Ac]/water mixtures directly manifests in their density behavior. Both [emim][Ac]/water and [hmim][Ac]/water mixtures exhibit a similar trend of initial density rise followed by a decline (Figure 8), similar to [bmim][Ac]/water mixtures. The simulated density trends of these two mixtures are also in
Figure 6. Snapshots of equilibrated simulation boxes showing a heterogeneous distribution of polar and nonpolar domains in binary mixtures of [Rnmim][Ac] ILs (n = 2, 4, 6) at different compositions. (a, b) [emim][Ac]−, (c, d) [bmim][Ac]−, and (e, f) [hmim][Ac]− water systems at Xw = 0.0 (left panel) and Xw = 0.7 (right panel). Red connections represent the network of polar componentswater, IL anion, and cation headgroupand the gray connections represent the network of nonpolar tail groups.
very good agreement with experimental reports. Experimental densities of [emim][Ac]/water binary mixtures reported by Maldonado et al. increased up to Xw ∼ 0.6.29 A significant rise in density was observed in [hmim][Ac]/water systems as well, for the concentration range measured by Guan et al. [ρ (g cc−1) = 1.0606−1.0656 for Xw = 0−0.00135, at 298 K].31 The effect of nanostructuring on the density trend can be shown more clearly by computing Δρthe difference between the densities of a binary mixture and the corresponding neat IL (Figure 9). For all concentrations, the highest Δρ are exhibited by [hmim][Ac] followed by [bmim][Ac] and [emim][Ac]. This order is similar to the order of nanostructuring exhibited by these IL/water mixtures. The [hmim][Ac]/water systems with the highest nanostructuring show the most prominent density increase, followed by [bmim][Ac] and [emim][Ac].
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DISCUSSION The density change exhibited by different ILs of the [Rnmim][Ac] family can be clearly demarcated into two regimes (Figure 9). For concentrations up to Xw = 0.5, Δρ increases for all systems. This can be accounted for by the increased spatial correlation of the polar components, where water at lower concentrations mediates anion−anion structuring and consequently cation headgroup−headgroup proximity, as water at this concentration does not screen the IL cation headgroup−anion electrostatic interaction completely. The small differences in density increment among the different F
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Figure 9. Relative increase in the density of [emim][Ac] (solid line), [bmim][Ac] (dashed line), and [hmim][Ac] (dotted line) with the addition of water. Δρ indicates the difference between the densities of a binary mixture and the corresponding neat IL.
alkyl tail length, it is primarily structuring of the polar IL domain that causes the initial density rise with addition of water. The high basicity of the carboxylate group of the acetate anion makes such extensive hydrogen bonding possible, which leads to improved structuring. This is in agreement with earlier studies where increased interactions between acetate anion and water resulted in unusual trends in density and other thermophysical properties of [emim][Ac]/water mixtures.30,65 Interestingly, a similar rise in density was observed in binary mixtures of another imidazolium-based IL containing glycinate anion.47 However, the rise in density was less notable in this system, presumably due to lower basicity of the anion. This could also explain why such a density rise was not observed for imidazolium-based ILs containing weakly basic anions.18−21,53−60 In the second regime of Xw > 0.5, a turnover in density is exhibited by all systems. However, the concentration at which density turnover occurs and its magnitude show significant chain length dependence. While the densities of [emim][Ac]/ water systems start decreasing beyond Xw = 0.5, for ILs with longer tails, the densities continue to rise up to Xw = 0.7. Interestingly, a similar turnover in structuring of an IL with a longer alkyl tail, [omim][NO3], was observed at an even higher water concentration of Xw ∼ 0.75−0.8.40 Moreover, the decrease in densities at higher dilutions becomes less prominent with increasing tail length. Clearly, this is due to the nanostructural reordering imposed by aggregation of longer alkyl tails. This implies that, at concentrations of Xw > 0.5, the persistence of the IL polar network is determined by the competition between the breakdown of this network by the intruding water and the strengthening of the nonpolar network by tail aggregation. For a more vivid illustration, snapshots of the polar and nonpolar IL network in the three systems are presented in Figures S5 and S6 (Supporting Information), respectively. The polar network disintegrates in the presence of large concentrations of water, irrespective of the alkyl chain length (Figure S5, Supporting Information). The short alkyl chain of [emim] cation fails to aggregate and separates out along with the disintegrating polar groups (Figure S6, Supporting Information). However, cations with longer alkyl tails continue to aggregate, thereby resisting a complete breakdown of the structure. Hence, the longer the alkyl chain, the greater the nanostructural ordering and more intense
Figure 7. 2-D number density (n) profiles of cation alkyl tail groups in (a) [emim][Ac]− and (b) [hmim][Ac]−water mixtures at representative water mole fractions as a function of simulation box length. Number densities have been averaged across the xy-plane of the simulation box.
Figure 8. Comparison of simulated (open circles) and experimental (closed circles) densities for [emim][Ac]/water and simulated (closed triangles) densities for [hmim][Ac]/water binary mixtures. Experimental data for [emim][Ac]/water are taken from ref 29. Error bars are shown for the computed density values.
ILs in Figure 9 could be due to the cooperativity between the polar and nonpolar domains, as will be clear in the next paragraph. Hence, it can be reasserted that, irrespective of the G
DOI: 10.1021/acs.jpcb.5b00433 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry B the density turnover, as shown by [omim]+ followed by [hmim]+ and [bmim]+. The spatial heterogeneity in IL/water systems can influence the dynamical behavior of these binary mixtures. Selfdiffusivities of constituent ions and water are commonly determined as a useful measure of translational motions in IL/ water binary mixtures. Hence, the diffusion coefficients of the polar cation headgroup, anion, and water as well as the nonpolar tail of the [Rnmim][Ac]/water binary mixtures were computed (Figure 10). The changes in the dynamics with
good match with experimental data on the structure and dynamics of neat ILs.66−68 However, here we have followed the more conventional protocol for charge assignment, since water in IL/water binary mixtures could screen the charges on IL ions, especially at moderate to high concentrations. This is evident from the reproducibility of the experimental order of diffusivity of the IL ions at these concentrations, as shown in Figure 10. Another widely used protocol to modulate pair interactions in ILs and reproduce correct dynamics is to carry out high-temperature simulations.39−41 Thus, to ensure that the nanostructural changes observed in this study were not artifacts of overestimated interactions, we also simulated all [Rnmim][Ac]/water mixtures at an elevated temperature. The systems were heated up to 350 K and equilibrated under constant temperature−pressure conditions for density determination, followed by an additional 5 ns run in the NVE ensemble for dynamical analyses. The densities of the neat IL/water binary mixtures determined at 350 K showed a similar trend of initial increase with addition of water, as shown in Figure S7 (Supporting Information), strengthening our findings. The selfdiffusivities of the IL ions at this temperature (data not shown) also showed a similar trend as in Figure 10 and matched well with the experimentally determined order of 1 × 10−11 m2 s−1.
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CONCLUSIONS In this study, we have primarily focused on the effect of the imidazolium cation tail length on the nanostructural reorganization and dynamics of [Rnmim][Ac]/water binary mixtures, and their consequent influence on the anomalous densities of this IL family. We observed that the density change exhibited by different ILs of this family could be clearly demarcated into two regimes (Figure 9). While the increased structuring among the polar components (cation headgroup, anion, and water) led to the initial rise in density in the first regime (Xw ≤ 0.5), the role of the alkyl tail became evident in the second regime of higher water concentrations. As a result, the density of [emim][Ac]/water binary mixtures showed a turnover at Xw = 0.5, whereas, for longer-tailed [bmim] and [hmim] cations, the turnover was shifted to Xw > 0.7 (see Figures 1, 8, and 9). As evident from Figures 3 and 4, the spatial correlation in the polar domain maximizes at Xw = 0.5, beyond which the nanostructuring continues to grow only in cations with longer alkyl chains due to water-induced tail aggregation (Figures 5−7 and Figures S4−S6, Supporting Information). This differential nanostructuring caused by varying cation tail lengths could explain the different turnover points in the measured densities of [emim][Ac]/water and [bmim][Ac]/water binary mixtures.28,29 Acetate anion−water interactions were found to play a key role in structuring the polar domain of the binary mixtures in agreement with earlier studies.30,65 An in-depth analysis showed that the pattern of these interactions changed with water concentration, which intricately controlled not only the initial rise but also the subsequent turnover in density of [Rnmim][Ac]/water binary mixtures. When present in low concentrations, isolated water molecules occupy locations between adjacent anions and form water-mediated hydrogen bonded anion bridges (Figure 3a). As more and more hydrogen bonded bridges are formed (Table 2), the spatial correlation among the IL anions (and consequently among the cation head groups) increases, leading to a steady rise in density up to Xw ∼ 0.5. Upon further dilution, the incoming water disrupts the watermediated anion bridges and increasingly forms bifurcated
Figure 10. Self-diffusion coefficients of the polar components: (a) cation headgroup, (b) anion, and (c) water and (d) the nonpolar tail as a function of water mole fraction in [emim][Ac]− (black), [bmim][Ac]− (red), and [hmim][Ac]−water (green) binary mixtures. The error bars for the computed diffusion coefficients are shown. Insets highlight a decrease in the dynamics of IL components at Xw ∼ 0.3−0.5 (a, b, d) and the retarding effect of the alkyl chain on water and tail group diffusion even at Xw ≥ 0.7 (c, d). The simulated diffusion coefficient of pure TIP3P water, (5.38 ± 0.34) × 10−9 m2 s−1, showed excellent match with the original report.69
addition of water could be clearly demarcated into two regimes: a sluggish change in diffusion corresponding to a glassy state at low water concentrations (Xw ≤ 0.5), followed by an exponential rise due to a breakdown of the ion network by the percolating water phase at high water concentrations. Interestingly, at Xw ∼ 0.3−0.5, the dynamics of the IL componentsespecially the imidazolium headgroup and acetateshowed an observable decrease relative to neat IL (insets to Figure 10a,b). Quite evidently, the decreased dynamics is a manifestation of the increased structuring among the polar moieties of IL induced by the added water at these concentrations. The decrease in dynamics was more prominent in ILs with longer tails. This occurs due to increasing aggregation of the cation tails in the presence of water resulting in their retarded dynamics even at higher dilutions (inset to Figure 10d). It is worth mentioning here that the computed diffusion coefficients of the IL ions at water concentrations of Xw ≤ 0.5 appear to be an order of magnitude smaller than the available experimental value for neat [emim][Ac].63 This is because the existing force field models for ILs overestimate the charges on the ions, thereby leading to increased electrostatic interactions and decreased dynamics. Recently, new force field parameters for ILs based on refined charges have been found to show a H
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anion−water hydrogen bonds, leading to decreased anion structuring and a decline in the density. However, this density turnover is shifted to higher concentration in longer-tailed cations, due to water-induced tail aggregation, as described above. The dynamics of these mixtures also exhibited a similar turnover at moderate water concentration.
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1 giving the molecular structure and atomic notations of the [R n mim] cation; Figures S2−S4 showing the distributions of IL anions, cation head groups, and cation tails in the [Rnmim][Ac]/water binary mixtures. Figures S5 and S6 presenting snapshots of the simulation boxes highlighting the polar and nonpolar network in [Rnmim][Ac]/water binary mixtures at representative water concentrations. Figure S7 showing the densities of [Rnmim][Ac]/water binary mixtures at 350 K. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcb.5b00433.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +91-44-22574122. Fax: +91-44-22574102. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The computational facilities provided by High Performance Computing Centre, IIT Madras, are gratefully acknowledged. REFERENCES
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