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Ion-pairing and aggregation of ionic liquid-neutralized polyoxometalate salts in aqueous solutions
Fluid Phase Equilibria 425 (2016) 31e39
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Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
Ion-pairing and aggregation of ionic liquid-neutralized polyoxometalate salts in aqueous solutions Yuan Mei a, Wei Huang a, Zhen Yang b, Jun Wang a, Xiaoning Yang a, * a State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China b College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 March 2016 Received in revised form 6 May 2016 Accepted 6 May 2016 Available online 9 May 2016
The ion-pairing interaction for the ionic liquids (ILs)-based polyoxometalate (POM) salts in the aqueous solution is critical to the chemistry of the ILs-POM hybrid. Herein, for the first time, molecular dynamics simulations have been performed to investigate the aqueous solutions of the representative a-PW12O3 40 Keggin anion (PW3), neutralized by three different types of alkyl-functionalized imidazole-typed IL cations (1-alkyl-3-methylimidazolium cations, alkyl: n-ethyl, n-butyl, and n-octyl). The simulations provide a direct visual observation on the aggregate morphology for the POM anions in aqueous solution. Meanwhile, distinctive ion-pairing interaction mechanism and interfacial solvation behavior have been revealed. Our simulation results indicate that the alkyl-functionalized imidazole cations can affect the effective interaction between the charged POM anions, that is, the alkyl chain length in imidazolium cation could be applied to regulate the aggregation behavior among the negative POM anions. The unique ion-pairing phenomena can be ascribed to the variable modulating effects from neutralizing ILtyped cations, which not only acts as the screening counterions of electrostatic repulsion, but also induces the alkyl chain-anion interaction and overcomes the anion-anion repulsion. © 2016 Published by Elsevier B.V.
Keywords: Polyoxometalate (POM) Ionic liquids (ILs) Ion-pairing interaction Aggregation Molecular simulation
1. Introduction Polyoxometalates (POMs), also called isopolyanions/heteropolyanions, are an extremely diverse family of metal-oxygen clusters composed of early transition metals in high oxidation states (mainly VⅤ, MoⅥ, or WⅥ) [1]. They have several types of chemical structures and the Keggin-type heteropolyanion is the most representative POM salt because of its easy generation, strong acidity, and thermal stability [2e4]. Due to the remarkably tunable properties of POMs, there are widespread applications in catalysis, medicine, as well as electronic and magnetic materials [2,5e7]. On the other hand, ionic liquids (ILs) are known as a class of organic salts, which melt at relatively low temperatures (<100 C) [8e10]. Ionic liquids are generally used as novel reaction media because of their unique properties [9,11]. Recently, the IL-based polyoxometalate salts, that pair the POM anions and the functionalized IL cations, have gained considerable attention [12e15]. The new hybrids improve the chemical and
* Corresponding author. E-mail address: [email protected] (X. Yang). http://dx.doi.org/10.1016/j.fluid.2016.05.006 0378-3812/© 2016 Published by Elsevier B.V.
physical properties of the POM unit without changing its structure. Wang et al. [16] synthesized a series of POM-based salt catalysts by combining Keggin POM-anions with IL-cations functionalized by different alkyl groups. The POM-related ILs have high melting temperature (above 250 C), and thus they are not the conventional ionic liquids [16]. They could be used as the homogeneous catalysts with high catalytic efficiency and convenient recovery. It should be noted that most of these POM-related catalytic reactions usually occur in solution media [13,15,16], thus, it is very necessary to understand how these POM anions are solvated, and how these anions interact with other ions, including with IL-cations, in solvation mode. In general, the ion-pairing formation plays an important role on the physicochemical properties in the POM chemistry [3,17]. Especially, the extent and geometry of ion pairing for the POMs and their counterions might significantly influence the electron transfer for catalytic reactions of the IL-based polyoxometalate salt catalysts [18]. A detailed understanding of the ion pairing interaction and aggregate morphology requires microscopic pictures of the POM anions and their counterions in solution. Molecular dynamics (MD) simulation has been proposed as a useful tool to provide a molecular-level mechanism of solvation and
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ion pairing of POM anions in the presence of counterions [7,19]. Recent MD simulation [2] revealed that Keggin a-PW12O3 40 anions can form oligomers through direct contact with hydrophilic counterions. Brodbeck et al. [20] have simulated PW12O3 40 anions surrounded by amphiphilic cationic dendrimers in trichloromethane solution. Previous simulations have suggested that POM-POM interaction can be counterion-dependent. However, to our best knowledge, very few computational study [21] has been reported for the ion-pairing interactions of the IL-based polyoxometalate salts. Especially, the amphiphilic nature of alkyl-functionalized IL cations might induce special ion-pairing interaction and lead to unusual aggregation to POM anions. In this work, we selected the Keggin POM anion phospho3 tungstic acid (PW12O3 for short) as a repre40 , denoted as PW sentative polyoxoanion. Based on all-atomic MD simulation, we characterized the solvation behavior of PW3 neutralized by three different alkyl-functionalized imidazole-typed IL cations (1-ethyl3-methyl imidazole [C2mim]þ,1-butyl-3-methyl imidazole þ [C4mim] and 1-octyl-3-methyl imidazole [C8mim]þ) in aqueous solution. This study provided a comprehensive analysis of the ion pairing interactions for the highly charged polyoxometalate anions combined with the imidazolium cations in IL-based POM salt systems.
2. Simulation methods 2.1. Potential models The structures of the simulated PW3 anion and the three [Cnmim]þ cations (n ¼ 2, 4, 8) have been illustrated in Fig. 1. In the simulation system, the potential energy is described as the function by a sum of bond, angle, dihedral deformation energies, and the nonbonded interactions including electrostatic and van der Waals potentials in the following Amber form, [2,5,22].
Utotal ¼
X bonds
2
kb ðr r0 Þ þ
X angles
kq ðq q0 Þ þ 2
X kf ð1 2
2.2. Simulation details The MD simulations were first carried out in the canonical (NVT) ensemble with temperature of 300K using the Lammps package [26]. All the systems were simulated with periodic boundary conditions, using a cutoff value of 12 Å for the non-bonded L-J interactions. The electrostatic interactions were calculated using the particle mesh Ewald summation method [27]. The whole ILs-based POM complex structures were placed into the simulation cell. The water molecules were randomly inserted into the cubic box. The equations of motion were integrated with a time step of 1 fs. The simulation trajectories of all the atoms were dumped with a time interval of 1 ps for the subsequent analysis. The detailed system information is listed in Table 1, wherein, different concentration conditions were considered. The infinite dilution with only one POM anion was labeled as 1-PW3, and the other two concentrations were labeled as 8-PW3 and 27-PW3, respectively. Each system was run for at least 30 ns. The statistical averages were calculated during the last 5 ns from each simulation. 3. Results and discussion 3.1. Anion-anion interactions Fig. 2 shows the radial distribution functions (RDFs, g(r)) between the PW3 anions with a well-defined peak at ca. 11 Å. On account of the distance of 5.3 Å [24] between the central
dihedrals
" !6 X qi qj R*ij 2εij þ cosðn4 gÞÞ þ 4pε0 Rij Rij i
parameters of the PW12O3 40 anion were taken from the work by pez et al. [24], which is based on the Amber force field and has Lo been extensively applied for the simulation POM anions [2,7]. The Lennard-Jones (L-J) potential and atomic charges proposed by Liu et al. [8] were used to model the alkyl-functionalized ionic liquid (ILs) cations and this model is also based on the Amber force field with modifications. In addition, the atomic charge for 1-octyl-3methyl-imidazole cation (denoted as [C8mim]þ) is from Li et al. [25]. The corresponding L-J potential parameters and atomic þ charges for PW12O3 40 anion and [Cnmim] cations (n ¼ 2,4,8) were listed in Table S1 (supplementary materials).
Table 1 Characteristics of the simulated systems.
(1)
where the potential parameters have their usual meanings. The van der Waals (vdW) interactions between different particles were calculated by the Lorentz-Berthelot mixing rule. The potential for water was the SPC/E model [23]. The Keggin anions were treated as rigid particles throughout this work. The all-atom force field
System
Nsolute
1-PW3
1 PW3 1 PW3 1 PW3 8 PW3 8 PW3 8 PW3 27 PW3 27 PW3 27 PW3
8-PW3
27-PW3
3 [C2mim]þ 3 [C4mim]þ 3 [C8mim]þ 24 [C2mim]þ 24 [C4mim]þ 24 [C8mim]þ 81 [C2mim]þ 81 [C4mim]þ 81 [C8mim]þ
Nwater
Box size/Å
7873 7841 7667 24374 24306 24314 24357 24300 24302
60.9 61.0 60.1 90.2 90.1 90.0 90.0 90.0 90.0
60.9 61.0 60.1 90.0 90.0 90.0 90.0 90.0 90.0
Time/ns
60.9 61.0 60.1 90.1 90.0 90.0 90.0 90.0 90.0
35 35 35 30 30 30 30 30 30
þ Fig. 1. Schematic representation of the simulated PW3 anion (a-PW12O3 40 ) and the three alkyl-functionalized imidazole-typed IL ([Cnmim] ) cations (n ¼ 2,4,8) considered in this work. The yellow balls correspond to the phosphorus, and the green balls correspond to the tungsten, and the terminal oxygen (Ot), type 1 bridging oxygen (Ob1) and type 2 bridging oxygen (Ob2) have been specified in the POM structure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. RDFs between the central P atom of PW3 anion and average coordination numbers of PWs in aqueous solutions with [C2mim]þ as counterions (upper left), and the corresponding effective pair potential of mean force, PMF (below left), obtained by integration of the RDFs. The typical snapshots of 8-PW3 and 27-PW3 simulation boxes, POM anions aggregate through the direct contact, and form noncovalent dimers (upper Ⅱ) or even trimers (below Ⅱ) in aqueous solution.
phosphorous atoms and the surface terminal oxygen atom in the PW3 anion, thus the RDF peak represents a direct contact between POM anions. The corresponding snapshots further demonstrate that the PW3 anions with the [C2mim]þ counterions usually aggregate through the direct contact, and form noncovalent dimers or even trimers. Similar result has been reported by Chaumont et al. [2] for the POM anions in aqueous solution with other types of counterions. The average coordination number of the PW3 anions is 0.1 and 0.6, respectively, within 12 Å for the 8-PW3 and 27PW3 systems with the [C2mim]þ counterions. Although the larger system can lead to enhanced aggregation, the two systems show qualitatively consistent behavior. To better characterize the effective interaction between the PW3 anions, the effective potential of mean force (PMF) between the PW3 anions in aqueous solution was computed from the RDFs [28], WðrÞ=ðkTÞ ¼ ln gðrÞ. For the 27-PW3 system, the PMF well (WðrÞ) at the direct contact is larger than 2 kT. This suggests that the ion pairs of POMs in the presence of [C2mim]þ counterions could be stable in the higher concentration condition because the interaction free energy is negative and larger than twice the thermal energy [6]. However, the presence of repulsive barrier in the separation of 14 Å implies the anions also show feature of dispersion in aqueous solution. For the system of [PW][C4mim]3 with the alkyl chain of cation being butyl group, weak RDF peaks can be observed below 12 Å (Fig. 3). This is consistent with the corresponding snapshots where no obvious aggregation can be formed. Meanwhile, the small RDF peak at 11.5 Å in the 27-PW3 system corresponds to only 0.02 PW3 anions on the average coordination number, illustrating that, for the [PW][C4mim]3 systems, the anions are mostly monomers and appear well dispersed in aqueous solution. The PW3 anions mainly exhibit repulsive interaction between themselves, as seen from the PMFs (left bottom in Fig. 3), which are quite different from that for the [PW][C2mim]3. This result demonstrates that increasing the length of alkyl chain in the imidazolium cations can suppress the aggregation of PW3 anions and improve their dispersion in aqueous solution. However, PW3 aggregates reappear in the aqueous solution of [PW][C8mim]3 (snapshots in Fig. 4). From the RDF for the 8-PW3 system (left upper, Fig. 4), the most remarkable feature is the
presence of two undefined wide peaks between 11.0 and 15.5 Å. This implies the formation of special molecular arrangements among PW3 anions. As seen in the snapshots, the anions not only have direct contact, but also form larger aggregates mediated by the bridging [C8mim]þ cations, which could be due to the electrostatic attraction of cations and anions. On average, only 0.05 PW3 anions can be found around each one within the radial distance of 11.0 Å, however, the coordination number could attain 0.3 within ca. 15.5 Å. In the large [PW][C8mim]3 system with 27 PW3 anions, intensive peak can be observed from the RDF curve. There also exist one defined sharp RDF peak at ~11 Å and one wide peak at ~15 Å. The average coordination number, corresponding to first RDF peak, shows each PW3 is surrounded by 0.5 PW3 anions, indicating enhanced direct contact between PW3 anions. The second peak shows that the average coordination number of PW3 anions is ~1.0 within ca. 15 Å, further showing superior aggregation, as compared with the 8-PW3 system. The pairing aggregation of anions is much stronger, as compared with what is observed for the other two [PW][Cnmim]3 systems. Consequently, for the [PW][C8mim]3 system, PW3 anions not only form dimers through direct contact, meanwhile, each PW3 monomer or dimer is coordinated to several [C8mim]þ neutralizing cations, and thus generating larger aggregates mediated by the cations. As shown in the snapshot of [PW][C8mim]3 system, the PW3 anions are located at the interior of the aggregate structure with certain hydration. This spatial arrangement for the anions and cations in the [PW][C8mim]3 system somewhat resembles the dendrimer-encapsulated Keggin ions reported previously [20], wherein the polyoxometalate anion is surrounded by amphiphilic cationic dendrimers. This unique structure formed by the PW3 and [C8mim]þ ions may represent the pseudo-liquid nature of POM catalysts. This result suggests that with further increasing the alkyl chain length in the cation, a microphase structure of POM could be formed. For the [PW][C8mim]3 system, the PMFs have the larger valley, with WðrÞ > 2kT, indicating a stable contact between the PW3 anions. The second minima in the PMF curves corresponds to the larger and loose aggregate networks in the form of PW3 monomer or dimer mediated by the long chain [C8mim]þ cations. In short, our simulation demonstrates an interesting phenomenon
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Fig. 3. RDFs between the central P atom of PW3 anion and average coordination numbers of PWs in aqueous solutions with [C4mim]þ as counterions (upper left), and the corresponding effective pair potential of mean force, PMF (below left), obtained by integration of the RDFs. The typical snapshots of 8-PW3 and 27-PW3 simulation boxes, the anions are mostly monomers (right) and appear well dispersed in aqueous solution.
Fig. 4. RDFs between the central P atom of PW3 anion and average coordination numbers of PWs in aqueous solutions with [C8mim]þ as counterions (upper left), and the corresponding effective pair potential of mean force, PMF (below left), obtained by integration of the RDFs. The typical snapshots of 8-PW3 and 27-PW3 simulation boxes, POM anions generates larger aggregates mediated (right) in aqueous solution.
for the aggregation/dispersion of the imidazole-typed ionic liquidbased POM salts: aggregation for PW3 anions with [C2mim]þ counterion, dispersion with the [C4mim]þ case, but aggregation again with the [C8mim]þ. 3.2. Anion-cation interactions In order to understand the unique ion-pairing behavior for the POM anions, it is necessary to explore the anion-cation interaction, which can be characterized by the RDFs of cation-anion. As shown in Fig. 5, with the [C4mim]þ counterions, there is no significant peak in the RDFs (Fig. 5(b)), representing weak interaction between
the PW3 anion and the [C4mim]þ counterions. Although previous DFT computation [21] has shown that enhanced electrostatic attraction exists between POM anion and [C4mim]þ cation, solvation effect might reduce their interaction. In the situation of [C2mim]þ counterions, a broad peak (around 8 Å) can be observed in the cation-anion RDF curves. This broad distribution may signify both the direct and the solvent-mediated contacts. In addition, the coordination number of [C2mim]þ cations around the POM anion are 0.6 and 2.1 within the separation of 12 Å, respectively, for the two concentrations. This coordination number is obviously larger that with [C4mim]þ counterions. From Fig. 5(a) and (b), it is speculated that the aggregation/
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Fig. 5. RDFs between the POM anion and the IL-typed cations and the average coordination numbers of cations around the anions, obtained by integration of the RDFs. (a) [PW][C2mim]3 system; (b) [PW][C4mim]3 system; (c) [PW][C8mim]3 system.
dispersion of POM ions is correlated with the anion-cation interaction. According to our result, the good dispersion of the POM anions in the [PW][C4mim]3 system is associated with the weak contact degree between the PW3 anion and [C4mim]þ cation, whereas the enhanced anion-cation interaction can bring about the obvious attraction among the POM anions, as seen in the [PW] [C2mim]3 system. However, it is interesting to note that with the [C8mim]þ counterions (Fig. 5(c)), even stronger RDF peaks appear between anions and cations. The average coordination number up to 12 Å for cations around the PW anion are 1.4 (8-PW3 system) and 3.0 cations (27-PW3 system), respectively. Thus, in this situation, the larger aggregate structures in the [PW][C8mim]3 system can also be ascribed to enriched molecular arrangement of [C8mim]þ cations around the PW3 anions. The interaction picture between the anion and the cation can be clearly depicted by three-dimensional (3D) spatial distributions (Fig. 6), where the 3D densities of cations around the center of mass of POM anion within the radial distance of 10 Å were plotted. In the
35
[PW][C2mim]3 and [PW][C8mim]3 systems, the high density regions of surrounding countering cations show that there are indeed obvious interaction between the POM anions and the imidazoletyped IL cations. Comparatively, the [C8mim]þ cations have more densified distribution around the anion. However, for the [PW] [C4mim]3, only lower and sparse spatial density regions are observed for the cations around the PW3 anion, suggesting weak interaction between them. The 3D spatial distributions are consistent with the preceding RDF results in Fig. 5. According to the above results, the variation of alkyl chain length in imidazolium cation could be applied to regulate the aggregation behavior among the negative POM anions. For the alkylfunctionalized imidazole-typed IL cations in aqueous solution, as shown in the previous research [32], an increase in the length of hydrophobic hydrocarbon unit not only enhances the hydrophobicity of cation, but also promotes the vdW interaction arising from the alkyl chains of ILs. For the [PW][C2mim]3 systems, the IL cation is modestly amphiphilic with weak vdW interaction from alkyl chains. The [C2mim]þ has definite electrostatic affinity toward the PW3, which could effectively screens the electrostatic repulsion between PW3 anions and prompt their aggregation. In the case of [C4mim]þ cations, the increase in hydrocarbon units decreases the electrostatic attraction between IL cation and POM anion, possibly owing to the hydrophobicity and steric effect. This reducing affinity between PW3 and [C4mim]þ will weaken the screening function of cation on the repulsion between POM anions and impede the aggregation. However, in the case of [C8mim]þ cations with a further increase in the length of alkyl chain, the increased hydrocarbon-hydrocarbon interaction between the alkyl groups will produce interlinking structure among the alkyl-functionalized imidazole-typed cations. It is suspected that the crosslinking structure will lead to strong vdW and Coulombic attraction toward the POM anions, as a result, PW3 monomers could be attractively trapped inside, forming huge aggregates of POM anions, as shown in the snapshots of Fig. 4. The increased hydrophobic interaction from the alkyl-chain groups has also been found in the selfassembly thermodynamics of alkylimidazolium surfactants [33,34], where the Gibbs free energy of micelle formation becomes more negative as the alkyl chain length increases.
3.3. Interfacial hydration behavior Fig. 7 presents the RDFs of water molecules around PW3 anions. The observable peaks in the RDF curves exhibit distinct four hydration layers, which might represent distinct accessibility of solvent molecules around POM anions [24]. For both [PW][C2mim]3 and [PW][C8mim]3 systems at dilution condition, no obvious
Fig. 6. Three-dimensional density distributions of [Cnmim]þ around the PW3 anion in aqueous solution obtained from MD simulations, (a) [C2mim]þ, (b) [C4mim]þ, and (c) [C8mim]þ cations (P, W and O atoms are represented by yellow, green and red balls, respectively). In each case, the red and blue represent high density and low density, respectively, from the center of PW3 anions within a distance of 10 Å. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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For the [PW][C4mim]3 system, the hydration around the oxygen atoms in the PW3 anions produces larger RDF peaks, consistent with the enhanced solvation interaction around the anions, as shown in Fig. 8. The Ob1eOw RDF curve shows two high hydration peaks in the positions of 3 Å and 5 Å. The first one is due to the bridging oxygen Ob1, and the second one indicates the influence of the terminal oxygen Ot. The first peak in the RDF of Ob1eHw suggests that the hydrogen atoms of water mostly point to the bridging oxygens Ob1, probably forming hydrogen bonds (HBs). A similar feature has been observed for the Ot-water RDFs. It is interesting to note that the existence of three obvious peaks in the Ob2eOw RDF, implying that water molecules can permeate the internal structure of the PW3 anions. Meanwhile, the RDF of the corresponding HwOb2 indicates that the H atoms of water molecules orient outward (from the Ow atoms) relative to the bridging oxygens (Ob2), probably as a result of the steric effect. 3.4. Hydrogen-bonding around POM anions
Fig. 7. RDFs of water molecules from the center of PW3 anions in different simulation system. (a) RDFs for [PW][C2mim]3 system. (b) RDFs for [PW][C4mim]3 system. (c) RDFs for [PW][C8mim]3 system. Each systems divided into four hydration layers based on the hydration minimum positions.
hydration layer can be identified between the Ob1 and Ob2 sites. Comparatively, the hydration RDFs (Fig. 7(b)) with [C4mim]þ cations display more close and intensive peaks. For instance, the first position of hydration layer is located at around 5 Å from the center of PW anion, which is obvious shorter than those in the other two systems, as well as other previously reported system [6,24]. The behavior represents the high penetration of water molecules toward the internal oxygen atoms (Ob1 and Ob2) in the Keggion anion. Therefore, with the [C4mim]þ counterions, we conclude that the intensity of solvation shell around PW3 anions becomes enhanced, which is also able to provide a resistance to the aggregation of the POM anions. This intensified solvation could be related to the weak affinity between the [C4mim]þ cation and the PW3 anion. In Fig. 8, more detailed interfacial hydration can be shown with the RDFs between each oxo site of the PW3 anions and the neighboring water molecules. In general, for the PW[C2mim]3 and PW[C8mim]3 systems, water molecules mainly localize around the bridging oxygens (Ob1) and terminal oxygens (Ot). Comparatively, weak peaks in the RDFs of Ob2eH2O indicate that the internal oxygen Ob2 is not easily accessible in the two systems. It should be emphasized that the relatively weak hydration peaks in the [PW] [C8mim]3 system could be owing to the special crosslinking network generated by hydrophobic alkyl groups in the [C8mim]þ cations.
Fig. 9 shows the probability distribution for the average number of HBs formed between the specific sites on the PW3 anions and the surrounding water molecules for the 8-PW3 systems, wherein the geometric criteria [24,29e31] were used. As shown in Fig. 9, for the PW3 anion with the [C4mim]þ anion, the average number of formed HBs is ~21 for each anion, whereas the average HB number is only ~10 for the [C2mim]þ and [C8mim]þ systems. This result is consistent with the enhanced hydration behavior around the PW3 anion in the [PW][C4mim]3 system (see Figs. 7 and 8). Fig. 9 also provides the information on how many HB numbers were formed for each oxo site. For all the simulated systems, the number of HBs between the bridging oxygen Ob1 and the neighboring water molecules is generally larger than other oxygen atoms. For instance, the average 15 HBs have been formed with the Ob1 atoms in the [PW][C4mim]3 system, whereas only ~4 HBs are formed for Ot atoms. This is probably due to strenuous movement of the outermost water molecules, which reduce the formation of HB. It is also found that no HB has been detected for the bridging oxygen Ob2 in the [PW][C4mim]3 system, possibly owing to the special orientation of water molecules near the location of Ob2 oxygen atoms. Two types of time correlation functions: the continuous time correlation function (SHB ðtÞ), and the intermittent time correlation function (CHB ðtÞ), were used to analyze the dynamics behavior of HBs. SHB ðtÞ could offer the accurate HB lifetime and CHB ðtÞ just characterizes the structural relaxation of HBs. They were calculated by the following equations [30,31], respectively:
Hij ð0ÞHij ðtÞ SHB ðtÞ ¼ Hij ð0Þ CHB ðtÞ ¼
hij ð0Þhij ðtÞ hij ð0Þ
(2)
(3)
where Hij ðtÞ ¼ 1 if the HB continuously exists between an HB (ij) pair during the time interval 0-t and Hij ðtÞ ¼ 0 otherwise. hij ðtÞ ¼ 1 if there is an HB between a particular HB (ij) pair at time t, and hij ðtÞ ¼ 0 otherwise. Fig. 10 illustrates the behavior of SHB ðtÞ and CHB ðtÞ for different oxo sites. As expected, SHB ðtÞ shows much faster decay thanCHB ðtÞ, because CHB ðtÞ allows reformation after the breaking of HBs. Meanwhile, the two correlation functions for Ob1 show the slowest declination, whereas those for Ob2 have the fastest deterioration. This agrees well with the fact that in the POM anion Ob2 is generally less accessible. Although the outermost Ot atoms are easily
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Fig. 8. RDFs between three oxo sites (Ob1, Ob2, Ot) of the PW3 anions and the neighboring water molecules for 27-PW3 systems with [C2mim]þ(left), [C4mim]þ(middle) and [C8mim]þ(right) cations.
Fig. 9. Probability distribution of the average number of hydrogen bonds on each type of the oxygens and total oxygens in PW3 anion for the 8-PW3 systems [C2mim]þ(left), [C4mim]þ(middle) and [C8mim]þ(right) cations.
accessible by water molecules, the decay of the HB correlation function is intermediate, corresponding to the moderate HB stability. This is consistent with relative less number for the Ot-H2O HBs. Among the three systems, both SHB ðtÞ and CHB ðtÞ curves with the [C4mim]þ cations generally decay with the slowest rate. This provides a further support that the HBs in the [PW][C4mim]3 system is the most stable. In order to quantify the lifetimes for these HBs, the SHB ðtÞ curves have been fitted by the three-weighed exponential function:
SHB ðtÞ ¼ A$expð t=ta Þ þ B$expð t=tb Þ þ C$expð t=tc Þ (4) where A, B and C are tunable parameters ðA þ B þ C ¼ 1Þ, while ta , tb and tc are time constants. From the fitting function, tHB S can be calculated [30] and they are listed in Table S2 in the supplementary material. It is clearly observed that the HB lifetime for the [PW] [C4mim]3 system is longer than other two systems. It was also
found that the average HB lifetime tHB S for Ob1eH2O is generally larger those for Ob2eH2O and OteH2O HBs. Additionally, the whole HB lifetime with the imidazolium cation is larger than the reported value using Naþ as the counterions [24], due to the effect of [Cnmim]þ. 4. Conclusions In this work, MD simulations have been carried out to study the ion pairing interaction and aggregation for the a-PW12O3 40 Keggin anions (PW3), neutralized by three types of alkyl-functionalized imidazole-typed IL cations, with different lengths of alkyl chain, in aqueous solution. Different ion pairing behavior, including anion-anion and anion-cation, for the IL-based POM salt systems was revealed. With the [C2mim]þ counterions, the PW3 anions demonstrate common contact, mostly forming dimers or trimers with each other, where [C2mim]þ could be considered as glue to screen the Coulombic repulsion between the PW3 anions. When the alkyl
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functionalized ionic liquid cations as the counterions. According to our simulations, the hydrophobic nature from the alkyl-group in the [Cnmim]þ cations is critical to produce the effect. It could be speculated that, in non-polar solvents, new ion-pairing interaction and aggregation will appear, which might help to form biphasic POM catalytic system, as reported in previous experimental reports [13e16]. Acknowledgements This work was supported by the National Natural Science Foundation of China under Grants 21136005, Research funding from State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201404), and A PAPD Project of Jiangsu Higher Education Institution. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.fluid.2016.05.006. References
Fig. 10. The continuous time correlation function SHB(t) for Ob1 e water, Ob2 e water and Ot e water. The inset shows the corresponding intermittent time correlation function CHB(t) for Ob1 e water, Ob2 e water and Ot e water.
group in the cations becomes butyl group ([C4mim]þ), the PW3 anions and the [C4mim]þ cations are well dispersed in the solvent, without the formation of ion aggregates. In this case, an enhanced solvation structure has been observed around the POM anion. However, obvious different ion-pairing behavior has been observed for the ILs-based POM salts using the long-chain [C8mim]þ counterions. The PW3 anions not only form dimers and trimmers through direct association, but also arrange themselves as larger aggregate network, loosely surrounded or mediated by the [C8mim]þ cations. The observed phenomena can be explained as the unique nature of the alkyl-functionalized imidazole-typed IL cations, in which the electrostatic force from the headgroup and the vdW interaction from alkyl chain groups provide the joint function. This molecular mechanism for our heteropolyacid ionic liquids has never been observed for POM anions neutralized by other simple counterions [2,7], wherein only electrostatic attraction exists between anions and cations. Our molecular simulation provides a potential scheme to regulate the supramolecular dispersion/ aggregate chemistry for the POM anions by using the alkyl-
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Contents lists available at ScienceDirect
Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
Ion-pairing and aggregation of ionic liquid-neutralized polyoxometalate salts in aqueous solutions Yuan Mei a, Wei Huang a, Zhen Yang b, Jun Wang a, Xiaoning Yang a, * a State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China b College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 March 2016 Received in revised form 6 May 2016 Accepted 6 May 2016 Available online 9 May 2016
The ion-pairing interaction for the ionic liquids (ILs)-based polyoxometalate (POM) salts in the aqueous solution is critical to the chemistry of the ILs-POM hybrid. Herein, for the first time, molecular dynamics simulations have been performed to investigate the aqueous solutions of the representative a-PW12O3 40 Keggin anion (PW3), neutralized by three different types of alkyl-functionalized imidazole-typed IL cations (1-alkyl-3-methylimidazolium cations, alkyl: n-ethyl, n-butyl, and n-octyl). The simulations provide a direct visual observation on the aggregate morphology for the POM anions in aqueous solution. Meanwhile, distinctive ion-pairing interaction mechanism and interfacial solvation behavior have been revealed. Our simulation results indicate that the alkyl-functionalized imidazole cations can affect the effective interaction between the charged POM anions, that is, the alkyl chain length in imidazolium cation could be applied to regulate the aggregation behavior among the negative POM anions. The unique ion-pairing phenomena can be ascribed to the variable modulating effects from neutralizing ILtyped cations, which not only acts as the screening counterions of electrostatic repulsion, but also induces the alkyl chain-anion interaction and overcomes the anion-anion repulsion. © 2016 Published by Elsevier B.V.
Keywords: Polyoxometalate (POM) Ionic liquids (ILs) Ion-pairing interaction Aggregation Molecular simulation
1. Introduction Polyoxometalates (POMs), also called isopolyanions/heteropolyanions, are an extremely diverse family of metal-oxygen clusters composed of early transition metals in high oxidation states (mainly VⅤ, MoⅥ, or WⅥ) [1]. They have several types of chemical structures and the Keggin-type heteropolyanion is the most representative POM salt because of its easy generation, strong acidity, and thermal stability [2e4]. Due to the remarkably tunable properties of POMs, there are widespread applications in catalysis, medicine, as well as electronic and magnetic materials [2,5e7]. On the other hand, ionic liquids (ILs) are known as a class of organic salts, which melt at relatively low temperatures (<100 C) [8e10]. Ionic liquids are generally used as novel reaction media because of their unique properties [9,11]. Recently, the IL-based polyoxometalate salts, that pair the POM anions and the functionalized IL cations, have gained considerable attention [12e15]. The new hybrids improve the chemical and
* Corresponding author. E-mail address: [email protected] (X. Yang). http://dx.doi.org/10.1016/j.fluid.2016.05.006 0378-3812/© 2016 Published by Elsevier B.V.
physical properties of the POM unit without changing its structure. Wang et al. [16] synthesized a series of POM-based salt catalysts by combining Keggin POM-anions with IL-cations functionalized by different alkyl groups. The POM-related ILs have high melting temperature (above 250 C), and thus they are not the conventional ionic liquids [16]. They could be used as the homogeneous catalysts with high catalytic efficiency and convenient recovery. It should be noted that most of these POM-related catalytic reactions usually occur in solution media [13,15,16], thus, it is very necessary to understand how these POM anions are solvated, and how these anions interact with other ions, including with IL-cations, in solvation mode. In general, the ion-pairing formation plays an important role on the physicochemical properties in the POM chemistry [3,17]. Especially, the extent and geometry of ion pairing for the POMs and their counterions might significantly influence the electron transfer for catalytic reactions of the IL-based polyoxometalate salt catalysts [18]. A detailed understanding of the ion pairing interaction and aggregate morphology requires microscopic pictures of the POM anions and their counterions in solution. Molecular dynamics (MD) simulation has been proposed as a useful tool to provide a molecular-level mechanism of solvation and
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ion pairing of POM anions in the presence of counterions [7,19]. Recent MD simulation [2] revealed that Keggin a-PW12O3 40 anions can form oligomers through direct contact with hydrophilic counterions. Brodbeck et al. [20] have simulated PW12O3 40 anions surrounded by amphiphilic cationic dendrimers in trichloromethane solution. Previous simulations have suggested that POM-POM interaction can be counterion-dependent. However, to our best knowledge, very few computational study [21] has been reported for the ion-pairing interactions of the IL-based polyoxometalate salts. Especially, the amphiphilic nature of alkyl-functionalized IL cations might induce special ion-pairing interaction and lead to unusual aggregation to POM anions. In this work, we selected the Keggin POM anion phospho3 tungstic acid (PW12O3 for short) as a repre40 , denoted as PW sentative polyoxoanion. Based on all-atomic MD simulation, we characterized the solvation behavior of PW3 neutralized by three different alkyl-functionalized imidazole-typed IL cations (1-ethyl3-methyl imidazole [C2mim]þ,1-butyl-3-methyl imidazole þ [C4mim] and 1-octyl-3-methyl imidazole [C8mim]þ) in aqueous solution. This study provided a comprehensive analysis of the ion pairing interactions for the highly charged polyoxometalate anions combined with the imidazolium cations in IL-based POM salt systems.
2. Simulation methods 2.1. Potential models The structures of the simulated PW3 anion and the three [Cnmim]þ cations (n ¼ 2, 4, 8) have been illustrated in Fig. 1. In the simulation system, the potential energy is described as the function by a sum of bond, angle, dihedral deformation energies, and the nonbonded interactions including electrostatic and van der Waals potentials in the following Amber form, [2,5,22].
Utotal ¼
X bonds
2
kb ðr r0 Þ þ
X angles
kq ðq q0 Þ þ 2
X kf ð1 2
2.2. Simulation details The MD simulations were first carried out in the canonical (NVT) ensemble with temperature of 300K using the Lammps package [26]. All the systems were simulated with periodic boundary conditions, using a cutoff value of 12 Å for the non-bonded L-J interactions. The electrostatic interactions were calculated using the particle mesh Ewald summation method [27]. The whole ILs-based POM complex structures were placed into the simulation cell. The water molecules were randomly inserted into the cubic box. The equations of motion were integrated with a time step of 1 fs. The simulation trajectories of all the atoms were dumped with a time interval of 1 ps for the subsequent analysis. The detailed system information is listed in Table 1, wherein, different concentration conditions were considered. The infinite dilution with only one POM anion was labeled as 1-PW3, and the other two concentrations were labeled as 8-PW3 and 27-PW3, respectively. Each system was run for at least 30 ns. The statistical averages were calculated during the last 5 ns from each simulation. 3. Results and discussion 3.1. Anion-anion interactions Fig. 2 shows the radial distribution functions (RDFs, g(r)) between the PW3 anions with a well-defined peak at ca. 11 Å. On account of the distance of 5.3 Å [24] between the central
dihedrals
" !6 X qi qj R*ij 2εij þ cosðn4 gÞÞ þ 4pε0 Rij Rij i
parameters of the PW12O3 40 anion were taken from the work by pez et al. [24], which is based on the Amber force field and has Lo been extensively applied for the simulation POM anions [2,7]. The Lennard-Jones (L-J) potential and atomic charges proposed by Liu et al. [8] were used to model the alkyl-functionalized ionic liquid (ILs) cations and this model is also based on the Amber force field with modifications. In addition, the atomic charge for 1-octyl-3methyl-imidazole cation (denoted as [C8mim]þ) is from Li et al. [25]. The corresponding L-J potential parameters and atomic þ charges for PW12O3 40 anion and [Cnmim] cations (n ¼ 2,4,8) were listed in Table S1 (supplementary materials).
Table 1 Characteristics of the simulated systems.
(1)
where the potential parameters have their usual meanings. The van der Waals (vdW) interactions between different particles were calculated by the Lorentz-Berthelot mixing rule. The potential for water was the SPC/E model [23]. The Keggin anions were treated as rigid particles throughout this work. The all-atom force field
System
Nsolute
1-PW3
1 PW3 1 PW3 1 PW3 8 PW3 8 PW3 8 PW3 27 PW3 27 PW3 27 PW3
8-PW3
27-PW3
3 [C2mim]þ 3 [C4mim]þ 3 [C8mim]þ 24 [C2mim]þ 24 [C4mim]þ 24 [C8mim]þ 81 [C2mim]þ 81 [C4mim]þ 81 [C8mim]þ
Nwater
Box size/Å
7873 7841 7667 24374 24306 24314 24357 24300 24302
60.9 61.0 60.1 90.2 90.1 90.0 90.0 90.0 90.0
60.9 61.0 60.1 90.0 90.0 90.0 90.0 90.0 90.0
Time/ns
60.9 61.0 60.1 90.1 90.0 90.0 90.0 90.0 90.0
35 35 35 30 30 30 30 30 30
þ Fig. 1. Schematic representation of the simulated PW3 anion (a-PW12O3 40 ) and the three alkyl-functionalized imidazole-typed IL ([Cnmim] ) cations (n ¼ 2,4,8) considered in this work. The yellow balls correspond to the phosphorus, and the green balls correspond to the tungsten, and the terminal oxygen (Ot), type 1 bridging oxygen (Ob1) and type 2 bridging oxygen (Ob2) have been specified in the POM structure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. RDFs between the central P atom of PW3 anion and average coordination numbers of PWs in aqueous solutions with [C2mim]þ as counterions (upper left), and the corresponding effective pair potential of mean force, PMF (below left), obtained by integration of the RDFs. The typical snapshots of 8-PW3 and 27-PW3 simulation boxes, POM anions aggregate through the direct contact, and form noncovalent dimers (upper Ⅱ) or even trimers (below Ⅱ) in aqueous solution.
phosphorous atoms and the surface terminal oxygen atom in the PW3 anion, thus the RDF peak represents a direct contact between POM anions. The corresponding snapshots further demonstrate that the PW3 anions with the [C2mim]þ counterions usually aggregate through the direct contact, and form noncovalent dimers or even trimers. Similar result has been reported by Chaumont et al. [2] for the POM anions in aqueous solution with other types of counterions. The average coordination number of the PW3 anions is 0.1 and 0.6, respectively, within 12 Å for the 8-PW3 and 27PW3 systems with the [C2mim]þ counterions. Although the larger system can lead to enhanced aggregation, the two systems show qualitatively consistent behavior. To better characterize the effective interaction between the PW3 anions, the effective potential of mean force (PMF) between the PW3 anions in aqueous solution was computed from the RDFs [28], WðrÞ=ðkTÞ ¼ ln gðrÞ. For the 27-PW3 system, the PMF well (WðrÞ) at the direct contact is larger than 2 kT. This suggests that the ion pairs of POMs in the presence of [C2mim]þ counterions could be stable in the higher concentration condition because the interaction free energy is negative and larger than twice the thermal energy [6]. However, the presence of repulsive barrier in the separation of 14 Å implies the anions also show feature of dispersion in aqueous solution. For the system of [PW][C4mim]3 with the alkyl chain of cation being butyl group, weak RDF peaks can be observed below 12 Å (Fig. 3). This is consistent with the corresponding snapshots where no obvious aggregation can be formed. Meanwhile, the small RDF peak at 11.5 Å in the 27-PW3 system corresponds to only 0.02 PW3 anions on the average coordination number, illustrating that, for the [PW][C4mim]3 systems, the anions are mostly monomers and appear well dispersed in aqueous solution. The PW3 anions mainly exhibit repulsive interaction between themselves, as seen from the PMFs (left bottom in Fig. 3), which are quite different from that for the [PW][C2mim]3. This result demonstrates that increasing the length of alkyl chain in the imidazolium cations can suppress the aggregation of PW3 anions and improve their dispersion in aqueous solution. However, PW3 aggregates reappear in the aqueous solution of [PW][C8mim]3 (snapshots in Fig. 4). From the RDF for the 8-PW3 system (left upper, Fig. 4), the most remarkable feature is the
presence of two undefined wide peaks between 11.0 and 15.5 Å. This implies the formation of special molecular arrangements among PW3 anions. As seen in the snapshots, the anions not only have direct contact, but also form larger aggregates mediated by the bridging [C8mim]þ cations, which could be due to the electrostatic attraction of cations and anions. On average, only 0.05 PW3 anions can be found around each one within the radial distance of 11.0 Å, however, the coordination number could attain 0.3 within ca. 15.5 Å. In the large [PW][C8mim]3 system with 27 PW3 anions, intensive peak can be observed from the RDF curve. There also exist one defined sharp RDF peak at ~11 Å and one wide peak at ~15 Å. The average coordination number, corresponding to first RDF peak, shows each PW3 is surrounded by 0.5 PW3 anions, indicating enhanced direct contact between PW3 anions. The second peak shows that the average coordination number of PW3 anions is ~1.0 within ca. 15 Å, further showing superior aggregation, as compared with the 8-PW3 system. The pairing aggregation of anions is much stronger, as compared with what is observed for the other two [PW][Cnmim]3 systems. Consequently, for the [PW][C8mim]3 system, PW3 anions not only form dimers through direct contact, meanwhile, each PW3 monomer or dimer is coordinated to several [C8mim]þ neutralizing cations, and thus generating larger aggregates mediated by the cations. As shown in the snapshot of [PW][C8mim]3 system, the PW3 anions are located at the interior of the aggregate structure with certain hydration. This spatial arrangement for the anions and cations in the [PW][C8mim]3 system somewhat resembles the dendrimer-encapsulated Keggin ions reported previously [20], wherein the polyoxometalate anion is surrounded by amphiphilic cationic dendrimers. This unique structure formed by the PW3 and [C8mim]þ ions may represent the pseudo-liquid nature of POM catalysts. This result suggests that with further increasing the alkyl chain length in the cation, a microphase structure of POM could be formed. For the [PW][C8mim]3 system, the PMFs have the larger valley, with WðrÞ > 2kT, indicating a stable contact between the PW3 anions. The second minima in the PMF curves corresponds to the larger and loose aggregate networks in the form of PW3 monomer or dimer mediated by the long chain [C8mim]þ cations. In short, our simulation demonstrates an interesting phenomenon
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Fig. 3. RDFs between the central P atom of PW3 anion and average coordination numbers of PWs in aqueous solutions with [C4mim]þ as counterions (upper left), and the corresponding effective pair potential of mean force, PMF (below left), obtained by integration of the RDFs. The typical snapshots of 8-PW3 and 27-PW3 simulation boxes, the anions are mostly monomers (right) and appear well dispersed in aqueous solution.
Fig. 4. RDFs between the central P atom of PW3 anion and average coordination numbers of PWs in aqueous solutions with [C8mim]þ as counterions (upper left), and the corresponding effective pair potential of mean force, PMF (below left), obtained by integration of the RDFs. The typical snapshots of 8-PW3 and 27-PW3 simulation boxes, POM anions generates larger aggregates mediated (right) in aqueous solution.
for the aggregation/dispersion of the imidazole-typed ionic liquidbased POM salts: aggregation for PW3 anions with [C2mim]þ counterion, dispersion with the [C4mim]þ case, but aggregation again with the [C8mim]þ. 3.2. Anion-cation interactions In order to understand the unique ion-pairing behavior for the POM anions, it is necessary to explore the anion-cation interaction, which can be characterized by the RDFs of cation-anion. As shown in Fig. 5, with the [C4mim]þ counterions, there is no significant peak in the RDFs (Fig. 5(b)), representing weak interaction between
the PW3 anion and the [C4mim]þ counterions. Although previous DFT computation [21] has shown that enhanced electrostatic attraction exists between POM anion and [C4mim]þ cation, solvation effect might reduce their interaction. In the situation of [C2mim]þ counterions, a broad peak (around 8 Å) can be observed in the cation-anion RDF curves. This broad distribution may signify both the direct and the solvent-mediated contacts. In addition, the coordination number of [C2mim]þ cations around the POM anion are 0.6 and 2.1 within the separation of 12 Å, respectively, for the two concentrations. This coordination number is obviously larger that with [C4mim]þ counterions. From Fig. 5(a) and (b), it is speculated that the aggregation/
Y. Mei et al. / Fluid Phase Equilibria 425 (2016) 31e39
Fig. 5. RDFs between the POM anion and the IL-typed cations and the average coordination numbers of cations around the anions, obtained by integration of the RDFs. (a) [PW][C2mim]3 system; (b) [PW][C4mim]3 system; (c) [PW][C8mim]3 system.
dispersion of POM ions is correlated with the anion-cation interaction. According to our result, the good dispersion of the POM anions in the [PW][C4mim]3 system is associated with the weak contact degree between the PW3 anion and [C4mim]þ cation, whereas the enhanced anion-cation interaction can bring about the obvious attraction among the POM anions, as seen in the [PW] [C2mim]3 system. However, it is interesting to note that with the [C8mim]þ counterions (Fig. 5(c)), even stronger RDF peaks appear between anions and cations. The average coordination number up to 12 Å for cations around the PW anion are 1.4 (8-PW3 system) and 3.0 cations (27-PW3 system), respectively. Thus, in this situation, the larger aggregate structures in the [PW][C8mim]3 system can also be ascribed to enriched molecular arrangement of [C8mim]þ cations around the PW3 anions. The interaction picture between the anion and the cation can be clearly depicted by three-dimensional (3D) spatial distributions (Fig. 6), where the 3D densities of cations around the center of mass of POM anion within the radial distance of 10 Å were plotted. In the
35
[PW][C2mim]3 and [PW][C8mim]3 systems, the high density regions of surrounding countering cations show that there are indeed obvious interaction between the POM anions and the imidazoletyped IL cations. Comparatively, the [C8mim]þ cations have more densified distribution around the anion. However, for the [PW] [C4mim]3, only lower and sparse spatial density regions are observed for the cations around the PW3 anion, suggesting weak interaction between them. The 3D spatial distributions are consistent with the preceding RDF results in Fig. 5. According to the above results, the variation of alkyl chain length in imidazolium cation could be applied to regulate the aggregation behavior among the negative POM anions. For the alkylfunctionalized imidazole-typed IL cations in aqueous solution, as shown in the previous research [32], an increase in the length of hydrophobic hydrocarbon unit not only enhances the hydrophobicity of cation, but also promotes the vdW interaction arising from the alkyl chains of ILs. For the [PW][C2mim]3 systems, the IL cation is modestly amphiphilic with weak vdW interaction from alkyl chains. The [C2mim]þ has definite electrostatic affinity toward the PW3, which could effectively screens the electrostatic repulsion between PW3 anions and prompt their aggregation. In the case of [C4mim]þ cations, the increase in hydrocarbon units decreases the electrostatic attraction between IL cation and POM anion, possibly owing to the hydrophobicity and steric effect. This reducing affinity between PW3 and [C4mim]þ will weaken the screening function of cation on the repulsion between POM anions and impede the aggregation. However, in the case of [C8mim]þ cations with a further increase in the length of alkyl chain, the increased hydrocarbon-hydrocarbon interaction between the alkyl groups will produce interlinking structure among the alkyl-functionalized imidazole-typed cations. It is suspected that the crosslinking structure will lead to strong vdW and Coulombic attraction toward the POM anions, as a result, PW3 monomers could be attractively trapped inside, forming huge aggregates of POM anions, as shown in the snapshots of Fig. 4. The increased hydrophobic interaction from the alkyl-chain groups has also been found in the selfassembly thermodynamics of alkylimidazolium surfactants [33,34], where the Gibbs free energy of micelle formation becomes more negative as the alkyl chain length increases.
3.3. Interfacial hydration behavior Fig. 7 presents the RDFs of water molecules around PW3 anions. The observable peaks in the RDF curves exhibit distinct four hydration layers, which might represent distinct accessibility of solvent molecules around POM anions [24]. For both [PW][C2mim]3 and [PW][C8mim]3 systems at dilution condition, no obvious
Fig. 6. Three-dimensional density distributions of [Cnmim]þ around the PW3 anion in aqueous solution obtained from MD simulations, (a) [C2mim]þ, (b) [C4mim]þ, and (c) [C8mim]þ cations (P, W and O atoms are represented by yellow, green and red balls, respectively). In each case, the red and blue represent high density and low density, respectively, from the center of PW3 anions within a distance of 10 Å. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Y. Mei et al. / Fluid Phase Equilibria 425 (2016) 31e39
For the [PW][C4mim]3 system, the hydration around the oxygen atoms in the PW3 anions produces larger RDF peaks, consistent with the enhanced solvation interaction around the anions, as shown in Fig. 8. The Ob1eOw RDF curve shows two high hydration peaks in the positions of 3 Å and 5 Å. The first one is due to the bridging oxygen Ob1, and the second one indicates the influence of the terminal oxygen Ot. The first peak in the RDF of Ob1eHw suggests that the hydrogen atoms of water mostly point to the bridging oxygens Ob1, probably forming hydrogen bonds (HBs). A similar feature has been observed for the Ot-water RDFs. It is interesting to note that the existence of three obvious peaks in the Ob2eOw RDF, implying that water molecules can permeate the internal structure of the PW3 anions. Meanwhile, the RDF of the corresponding HwOb2 indicates that the H atoms of water molecules orient outward (from the Ow atoms) relative to the bridging oxygens (Ob2), probably as a result of the steric effect. 3.4. Hydrogen-bonding around POM anions
Fig. 7. RDFs of water molecules from the center of PW3 anions in different simulation system. (a) RDFs for [PW][C2mim]3 system. (b) RDFs for [PW][C4mim]3 system. (c) RDFs for [PW][C8mim]3 system. Each systems divided into four hydration layers based on the hydration minimum positions.
hydration layer can be identified between the Ob1 and Ob2 sites. Comparatively, the hydration RDFs (Fig. 7(b)) with [C4mim]þ cations display more close and intensive peaks. For instance, the first position of hydration layer is located at around 5 Å from the center of PW anion, which is obvious shorter than those in the other two systems, as well as other previously reported system [6,24]. The behavior represents the high penetration of water molecules toward the internal oxygen atoms (Ob1 and Ob2) in the Keggion anion. Therefore, with the [C4mim]þ counterions, we conclude that the intensity of solvation shell around PW3 anions becomes enhanced, which is also able to provide a resistance to the aggregation of the POM anions. This intensified solvation could be related to the weak affinity between the [C4mim]þ cation and the PW3 anion. In Fig. 8, more detailed interfacial hydration can be shown with the RDFs between each oxo site of the PW3 anions and the neighboring water molecules. In general, for the PW[C2mim]3 and PW[C8mim]3 systems, water molecules mainly localize around the bridging oxygens (Ob1) and terminal oxygens (Ot). Comparatively, weak peaks in the RDFs of Ob2eH2O indicate that the internal oxygen Ob2 is not easily accessible in the two systems. It should be emphasized that the relatively weak hydration peaks in the [PW] [C8mim]3 system could be owing to the special crosslinking network generated by hydrophobic alkyl groups in the [C8mim]þ cations.
Fig. 9 shows the probability distribution for the average number of HBs formed between the specific sites on the PW3 anions and the surrounding water molecules for the 8-PW3 systems, wherein the geometric criteria [24,29e31] were used. As shown in Fig. 9, for the PW3 anion with the [C4mim]þ anion, the average number of formed HBs is ~21 for each anion, whereas the average HB number is only ~10 for the [C2mim]þ and [C8mim]þ systems. This result is consistent with the enhanced hydration behavior around the PW3 anion in the [PW][C4mim]3 system (see Figs. 7 and 8). Fig. 9 also provides the information on how many HB numbers were formed for each oxo site. For all the simulated systems, the number of HBs between the bridging oxygen Ob1 and the neighboring water molecules is generally larger than other oxygen atoms. For instance, the average 15 HBs have been formed with the Ob1 atoms in the [PW][C4mim]3 system, whereas only ~4 HBs are formed for Ot atoms. This is probably due to strenuous movement of the outermost water molecules, which reduce the formation of HB. It is also found that no HB has been detected for the bridging oxygen Ob2 in the [PW][C4mim]3 system, possibly owing to the special orientation of water molecules near the location of Ob2 oxygen atoms. Two types of time correlation functions: the continuous time correlation function (SHB ðtÞ), and the intermittent time correlation function (CHB ðtÞ), were used to analyze the dynamics behavior of HBs. SHB ðtÞ could offer the accurate HB lifetime and CHB ðtÞ just characterizes the structural relaxation of HBs. They were calculated by the following equations [30,31], respectively:
Hij ð0ÞHij ðtÞ SHB ðtÞ ¼ Hij ð0Þ CHB ðtÞ ¼
hij ð0Þhij ðtÞ hij ð0Þ
(2)
(3)
where Hij ðtÞ ¼ 1 if the HB continuously exists between an HB (ij) pair during the time interval 0-t and Hij ðtÞ ¼ 0 otherwise. hij ðtÞ ¼ 1 if there is an HB between a particular HB (ij) pair at time t, and hij ðtÞ ¼ 0 otherwise. Fig. 10 illustrates the behavior of SHB ðtÞ and CHB ðtÞ for different oxo sites. As expected, SHB ðtÞ shows much faster decay thanCHB ðtÞ, because CHB ðtÞ allows reformation after the breaking of HBs. Meanwhile, the two correlation functions for Ob1 show the slowest declination, whereas those for Ob2 have the fastest deterioration. This agrees well with the fact that in the POM anion Ob2 is generally less accessible. Although the outermost Ot atoms are easily
Y. Mei et al. / Fluid Phase Equilibria 425 (2016) 31e39
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Fig. 8. RDFs between three oxo sites (Ob1, Ob2, Ot) of the PW3 anions and the neighboring water molecules for 27-PW3 systems with [C2mim]þ(left), [C4mim]þ(middle) and [C8mim]þ(right) cations.
Fig. 9. Probability distribution of the average number of hydrogen bonds on each type of the oxygens and total oxygens in PW3 anion for the 8-PW3 systems [C2mim]þ(left), [C4mim]þ(middle) and [C8mim]þ(right) cations.
accessible by water molecules, the decay of the HB correlation function is intermediate, corresponding to the moderate HB stability. This is consistent with relative less number for the Ot-H2O HBs. Among the three systems, both SHB ðtÞ and CHB ðtÞ curves with the [C4mim]þ cations generally decay with the slowest rate. This provides a further support that the HBs in the [PW][C4mim]3 system is the most stable. In order to quantify the lifetimes for these HBs, the SHB ðtÞ curves have been fitted by the three-weighed exponential function:
SHB ðtÞ ¼ A$expð t=ta Þ þ B$expð t=tb Þ þ C$expð t=tc Þ (4) where A, B and C are tunable parameters ðA þ B þ C ¼ 1Þ, while ta , tb and tc are time constants. From the fitting function, tHB S can be calculated [30] and they are listed in Table S2 in the supplementary material. It is clearly observed that the HB lifetime for the [PW] [C4mim]3 system is longer than other two systems. It was also
found that the average HB lifetime tHB S for Ob1eH2O is generally larger those for Ob2eH2O and OteH2O HBs. Additionally, the whole HB lifetime with the imidazolium cation is larger than the reported value using Naþ as the counterions [24], due to the effect of [Cnmim]þ. 4. Conclusions In this work, MD simulations have been carried out to study the ion pairing interaction and aggregation for the a-PW12O3 40 Keggin anions (PW3), neutralized by three types of alkyl-functionalized imidazole-typed IL cations, with different lengths of alkyl chain, in aqueous solution. Different ion pairing behavior, including anion-anion and anion-cation, for the IL-based POM salt systems was revealed. With the [C2mim]þ counterions, the PW3 anions demonstrate common contact, mostly forming dimers or trimers with each other, where [C2mim]þ could be considered as glue to screen the Coulombic repulsion between the PW3 anions. When the alkyl
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functionalized ionic liquid cations as the counterions. According to our simulations, the hydrophobic nature from the alkyl-group in the [Cnmim]þ cations is critical to produce the effect. It could be speculated that, in non-polar solvents, new ion-pairing interaction and aggregation will appear, which might help to form biphasic POM catalytic system, as reported in previous experimental reports [13e16]. Acknowledgements This work was supported by the National Natural Science Foundation of China under Grants 21136005, Research funding from State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201404), and A PAPD Project of Jiangsu Higher Education Institution. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.fluid.2016.05.006. References
Fig. 10. The continuous time correlation function SHB(t) for Ob1 e water, Ob2 e water and Ot e water. The inset shows the corresponding intermittent time correlation function CHB(t) for Ob1 e water, Ob2 e water and Ot e water.
group in the cations becomes butyl group ([C4mim]þ), the PW3 anions and the [C4mim]þ cations are well dispersed in the solvent, without the formation of ion aggregates. In this case, an enhanced solvation structure has been observed around the POM anion. However, obvious different ion-pairing behavior has been observed for the ILs-based POM salts using the long-chain [C8mim]þ counterions. The PW3 anions not only form dimers and trimmers through direct association, but also arrange themselves as larger aggregate network, loosely surrounded or mediated by the [C8mim]þ cations. The observed phenomena can be explained as the unique nature of the alkyl-functionalized imidazole-typed IL cations, in which the electrostatic force from the headgroup and the vdW interaction from alkyl chain groups provide the joint function. This molecular mechanism for our heteropolyacid ionic liquids has never been observed for POM anions neutralized by other simple counterions [2,7], wherein only electrostatic attraction exists between anions and cations. Our molecular simulation provides a potential scheme to regulate the supramolecular dispersion/ aggregate chemistry for the POM anions by using the alkyl-
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