Poly(vinylidene fluoride

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General Research

Separation Efficiency of CO2 in Ionic Liquids/Poly(vinylidene fluoride) Composite Membrane: A Molecular Dynamics Study Tao Song, Xiaochun Zhang, Yonggang Li, Kun Jiang, Suojiang Zhang, Xiangmei Cui, and Lu Bai Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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Industrial & Engineering Chemistry Research

Separation Efficiency of CO2 in Ionic Liquids/Poly(vinylidene fluoride) Composite Membrane: A Molecular Dynamics Study Tao Songa,b, Xiaochun Zhangb,*, Yonggang Lid, Kun Jiangb,c, Suojiang Zhangb,*, Xiangmei Cuia,*, Lu Baib aCollege

of chemical engineering, Qinghai University, Xining 810016, P. R. China

bBeijing

Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green Process

and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China cCollege

of Chemical and Engineering, University of Chinese Academy of Sciences, Beijing 100049,

P. R. China dCollege of Chemistry and Environmental Engineering, Baise University, Baise 533000, P. R. China

ACS Paragon Plus Environment

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

ABSTRACT Ionic liquids (ILs)/polymer composite membranes show great potentials for CO2 separation, the main challenge is to select the appropriate combination of ILs and polymer in a limited time and cost. In this work, the microstructure interactions and dynamic properties of a series of systems of 1-butyl-3-methylimidazolium hexafluorophosphate

([bmim][PF6]),

1-butyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) and 1-butyl-3-methylimidazolium tetracyanoborate ([bmim][B(CN)4] composited with poly(vinylidene fluoride) (PVDF) with increasing ionic liquids (ILs) content were studied by molecular dynamics simulations for understanding the composite membrane at molecular level. The radial distribution functions show that ILs aggregation is formed in the PVDF matrix, and the aggregation region continuously expands and finally becomes ionic channels with increased ILs content to 50wt%. The weakest cations-anions interaction of [bmim][B(CN)4] and the strongest interaction of PVDF-[B(CN)4] as well as the weakest aggregation of [bmim][B(CN)4] are favorable for forming the continuous ionic channels in PVDF matrix for CO2 diffusion. The increased self-diffusion coefficients of PVDF after the addition of ILs, which originates from the strong interaction of ILsPVDF and the broken hydrogen bond network among PVDF chains, facilitate the transportation of CO2 among PVDF chains. Moreover, the free energy of solvation, Henry’s law constant and self-diffusion coefficients for CO2 in three ILs/PVDF systems suggest that [bmim][B(CN)4]/PVDF composite membrane possesses better CO2 separation performance. Keywords: ionic liquids, PVDF, composite membrane, carbon dioxide, molecular dynamics simulation


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1. INTRODUCTION The greenhouse effect, which is mainly caused by the increasing emission of carbon dioxide (CO2) due to the burning of fossil fuels, has become a global concern issue.1-3 Carbon capture, utilization and storage (CCUS) is considered as one of the most efficient ways to mitigate greenhouse gas emissions, in which CO2 capture and separation is the core of entire technology.4-6 Since 1999, Blanchard et al.7 firstly reported that supercritical CO2 is highly soluble in ionic liquids (ILs), while ILs do not dissolve in supercritical CO2. A large number of conventional and functionalized ILs have been synthesized for absorbing CO2 due to their unique physicochemical properties such as negligible vapor pressure, good thermal stability and designability.8-15 It was found that ILs show high solubility for CO2 relative to CH4, H2 and N2.16-26 ILs have been proposed as one of the promising alternatives for CO2 capture and separation. Nevertheless, there are some drawbacks that cannot be ignored for pure ILs relative to current industrial solvents, such as high viscosity and cost. Recently, ILs/polymer composite membranes, formed by incorporating ILs into the polymer matrix, have been investigated for CO2 separation applications.27-36 The ILs/polymer composite membranes show more advantages in contrast to pure ILs. The high viscosity of ILs can be alleviated due to the short gas diffusion paths in membrane units.27 In addition, ILs can be immobilized because of the rigidity of polymer chains as well as the strong intermolecular interaction between ILs and polymer chains. Thus, the amount and loss of ILs could be reduced.37 In 2009, Hong et al.28 effectively incorporated 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) in the poly(vinylidene fluoride-hexafluoropropyl) (PVDF-HFP) as CO2 separation membrane. The CO2 permeability of 400 barrer and CO2/N2 selectivity of 60 were obtained when the ratio of ILs to PVDF-HFP is equal to 2:1, which surpasses the “2008 Robeson upper bound”. Since then, a lot of researchers have focused on separating CO2 by ILs/polymer composite membranes. For instance, Chen et al.33 fabricated the 1-ethyl-3-methylimidazolium tetracyanoborate ([emim][B(CN)4])/PVDF composite membranes and investigated the effect of ILs content on CO2 separation performance. It was found that the CO2 permeability increases with increasing ILs



content, and the [emim][B(CN)4]/PVDF (2/1) membrane has a CO2 permeability of 1778 barrer with a CO2/N2 selectivity of 41.1. Estahbanati et al.35 reported that the CO2 permeability and CO2/N2 selectivity reach to 190 barrer and 105.6 in Poly(ether-bamide-6) (Pebax 1657)/1-butyl-3-methylimidazolium tetrafluorobotate ([bmim][BF4]) composite membrane with 50% ILs. Mannan et al.29 investigated the separation of CO2 from gas mixtures by using polyethersulfone (PES) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]) composite membrane, and found that the CO2/CH4 separation performance surpasses the “2008 Robeson Upper Bound”. Lots of composite membranes show improved CO2 permeability and selectivity relative to pure polymer membranes, but the CO2 separation performance depends on the combination of the ILs and polymer. However, it is very hard to select the appropriate ILs/polymer combination from various kinds of ILs and polymers only by experiment. As we all know, even if one polymer is used, there are millions of ILs could be considered for the fabrication of new ILs/polymer composite membranes. Therefore, it is necessary to study the interaction mechanism of composite membranes by the computational method. Molecular dynamics (MD) simulations, being a very useful tool to study interaction mechanism, have been widely adopted in many aspects,38-44 and there are a few works related to ILs/polymer composite membranes.45-51 Sun et al.46 investigated the microstructure and transport properties of [bmim][BF4]/Nafion composite membrane through all-atom MD and coarse-grained MD simulations, which is mainly used to develop fuel cells. They found that ILs are able to form ionic clusters in Nafion, and ionic clusters in ILs/Nafion are wider than water clusters in water/Nafion. Costa et al.47 studied the polymer electrolytes composed of 1,3-dimethylimidazolium hexafluorophosphate ([dmim][PF6])/[bmim][PF6] and poly(ethylene oxide) (PEO) through MD simulations. It was reported that ILs are well dispersed in PEO, and the distribution of PEO and anions around cations are complementary. Siqueira et al.48, 49 performed

MD

simulations

of

the

butyltrimethylammonium

bis(trifluoromethylsulfonyl)imide ([N4111][Tf2N]), PEO and their mixtures with CO2/SO2. It was found that the solubility of CO2/SO2 in [N4111][Tf2N]/PEO is between


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pure PEO and pure ILs (less). The increased viscosity was found with the addition of ILs into PEO, which may be attributed to the effect of ILs on the gauche population of O-C-C-O dihedral in PEO. Abedini et al.45 studied the solubility of CO2 and CH4 in the ionic

polyimides(i-PIs)+1-butyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) composite by MD and grand canonical Monte Carlo (GCMC) simulations. They found that the exposed surface area of the adsorbent is very sensitive to the gas solubility, and the location of CO2 and CH4 around i-PIs are significantly altered after the addition of ILs. Although there are some works on the interaction mechanism of ILs/polymer membranes, molecular level understanding of the relationship between the structure of the ILs/polymer composite and the CO2 separation performance is still lacking. Furthermore, to the best of our knowledge, few simulation works focus on the research of CO2 in the composite membrane composed of different kinds of ILs with the same polymer until now. Therefore, in this work, we carried out all-atom MD simulations of CO2 in the systems of PVDF with three imidazolium-based ILs at different concentration. The ILs are [bmim][PF6], [bmim][Tf2N] and [bmim][B(CN)4]. The reason for choosing these systems is that their absorption properties toward CO2 have been systematically studied by experiments.31,

33

The effect of anions on the

microstructure, interactions and dynamic properties of three kinds of ILs/PVDF composite membranes were thoroughly investigated by analyzing the radial distribution functions (RDFs), coordination numbers, spatial distribution functions (SDFs), hydrogen bond (HB) interaction and self-diffusion coefficients. Then, the thermodynamic and dynamic properties of CO2 as well as the interaction of CO2 with three different ILs/PVDF systems were studied. The free energy of solvation, Henry’s law constant, self-diffusion coefficients for CO2 in three ILs/PVDF systems and RDFs between CO2 and ILs/PVDF composite were calculated. Finally, the conclusions were summarized. Overall, our main purpose is to get a deep understanding of the interaction mechanism of the composite membranes and provide a guidance for the design of ILs/polymer membrane with superior CO2 separation performance. 2. COMPUTATIONAL DETAILS



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2.1. ILs-PVDF systems All-atom force fields were used in the simulations. The force field parameters based on AMBER model for [bmim]+, [PF6]-, [Tf2N]- were taken from Liu’s work,52 while those for [B(CN)4]- and PVDF were obtained from Koller et al.53 and Lachet et al.54, respectively. The chemical structure of [bmim]+, [PF6]-, [Tf2N]-, [B(CN)4]- and PVDF are shown in Figure 1. One of the atom types in [bmim]+ is named for further analysis in the following part. All MD simulations were carried out for the mixtures of PVDF and three kinds of ILs at different concentration by using Gromacs 5.1.4 package.55,

56

The ILs are

[bmim][PF6], [bmim][Tf2N] and [bmim][B(CN)4]. For PVDF, 150 chains having each 20 carbon atoms polymerized by 10 monomers of vinylidene fluoride were considered. The weight percentage of ILs are 0%, 10%, 20%, 30%, 40%, 50% and 100% (pure). The number of ionic pairs for simulated ILs/PVDF systems were set according to the weight percentage of ILs, which are 0, 39, 88, 150, 232, 349, 349, respectively. Details of simulation systems are presented in Table 1. To make sure that the 150 chains having each 20 carbon atoms polymerized by 10 monomers is sufficient, the systems of [bmim][PF6]/PVDF with increasing ILs content of 10%, 20%, 30%, 40% and 50% were simulated, in which 80 chains having each 70 carbon atoms (C70) polymerized by 35 monomers of vinylidene fluoride for PVDF were studied. The center-of-mass RDFs of cations-anions, cations-cations and anions-anions, site-site RDFs between F atoms and H atoms in PVDF were calculated. The calculated results are compared with those of [bmim][PF6]/PVDF systems containing 20 carbon atoms (C20) for PVDF. As shown in Figure S1-3 in the Supporting Information, the all RDFs between C20 system and C70 system are almost identical (including the shapes, positions and intensities of all peaks), which means that the same conclusions can be obtained regardless of C20 or C70 systems, and C20 is sufficient. In addition, the long-chain polymers move slowly and it could be not easy for them to cross the possible potential barrier. Finally, 20 carbon atoms for PVDF were chosen in all simulations. The initial configurations of different ILs/PVDF composite systems were randomly generated by using the Packmol package57 with three-dimensional periodic


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boundary conditions. The long-range electrostatic interactions were disposed of using Particle mesh Ewald58, 59 method with a cutoff radius of 1.2 nm, which value was also adopted for non-bonded interactions calculation. Lennard-Jones (LJ) potential was shifted by a constant such that it was zero at the cut-off and the LJ tail correction was added to the energy and pressure. Lorenz-Berthelot (LB) mixing rule was used for dealing with LJ parameters. The temperature was maintained through Nose-Hoover thermostat60 with a coupling constant of 0.2 ps. The pressure of 1 bar was controlled by Parrinello-Rahman barostat61 with a coupling constant of 2.0 ps. All the covalent bonds were restricted by LINCS algorithm.62 In order to remove the possible overlapping structures, the energy minimization was firstly performed for initial configurations with the steepest descent algorithm. The energy minimization stopped until the minimum forces were below 100 kJ mol-1 nm-1. Then the Canonical Ensemble (NVT) was adopted to equilibrate system for 100 ps at a temperature of 493 K. The final configuration after NVT was chosen as an input for another simulation under Isobaric-Isothermal Ensemble (NpT) with a simulated annealing for the sake of crossing possible potential barriers. Each configuration was heated to 650 K for 20 ns, then cooled down to 493 K in 10 ns and equilibrated for another 20 ns. Finally, simulated annealing was followed by 50 ns production run under NpT at 493 K and 1 bar, and the trajectories were recorded every 1 ps for result analysis. All the calculated properties were obtained from block averages, and statistical uncertainty was estimated by the standard deviation. 2.2. ILs-PVDF-CO2 systems To calculate the dynamic properties of CO2 in ILs/PVDF systems and the interaction of CO2 with ILs/PVDF composite, 50 CO2 molecules were inserted into three different equilibrated ILs/PVDF systems with 50% ILs. The force field parameters of CO2 were taken from Shi et al.17 The configurations of ILs-PVDF-CO2 were equilibrated for 50 ns, then followed with 50 ns production run under NpT. The trajectories of production run were collected every 500 steps with a time step of 2 fs for further analysis. Free energy of solvation was calculated by inserting one CO2 molecule into three



different equilibrated ILs/PVDF systems with 50% ILs and the configurations were equilibrated further for 5 ns under NpT at 493 K and 1 bar. In these calculations, the Bennett acceptance ratio method was adopted.63 The free energy of solvation was calculated by the equation, 𝜆=1

∆𝐺𝑠𝑜𝑙 =

∫ 〈 ∂𝜆 〉𝑑𝜆 ∂𝐻

𝜆=0

where λ is the coupling parameter, H is the Hamiltonian of the system, angle bracket is the average from trajectories, and λ=0 and λ=1 denote the fully solvated CO2 molecule and the neat systems, respectively. The continuous-two-step approach was used for the calculation of the free energy of solvation. Firstly, for calculating the van der Waals contribution to the free energy of solvation, the LJ interaction between CO2 and ILs/PVDF was slowly turned off with a step of 0.05 from λ=0 to λ=1. At the same time, the Coulomb interaction between them was continuously existent (λ=0) until thoroughly turning off the LJ interaction. Secondly, the Coulomb contribution to the free energy of solvation was calculated after the computation of van der Waals contribution (the λ was set to a constant of 1 for LJ contribution during this time), and the step was also 0.05 between λ=0 to λ=1 for electrostatic portion of the solvation free energy calculation. A detailed changing processes of the free energy of solvation for CO2 in three ILs/PVDF systems with ILs content of 50% as a function of coupling parameter λ are displayed in Figure S4 in the Supporting Information. The number of steps was 2000000 with a time step of 2 fs. The temperature and pressure were maintained constant through stochastic dynamics integration scheme. 3. RESULTS AND DISCUSSION 3.1. ILs-PVDF systems 3.1.1. Microstructure of ILs/PVDF systems The microstructure of ILs/polymer composite membrane is very important, which is directly related to the performance of the membrane. Figure 2 shows the snapshots of ILs/PVDF composite systems with increasing ILs concentration after 100 ns under NpT at 493 K and 1 bar. It is obvious that ILs aggregate to form ionic channel and the ILs content has a significant effect on the size and shape of channels. The small and


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scattered ionic channels are formed at low ILs content, while the dispersive ionic channels are interconnected and form continuous ionic channels with gradually increasing ILs content. Previous studies also found the formation of ILs aggregation regions in ILs/polymer composite membranes through experiment and MD simulations,36, 46, 64 including those membranes of ILs confined in PVDF matrix.33, 65, 66 In comparison with the tubular water channel in water/polymer system, the ionic channel is bigger and wider in ILs/polymer composite, which can be attributed to the strong interaction between F atoms in PVDF and imidazolium ring in cations of ILs.65 Sun et al.46 reported that ILs penetrate into Nafion due to the strong electrostatic interaction between sulfonate groups in Nafion and cations, thus forming the wider ILs aggregation regions in the Nafion matrix. Dai et al.36 fabricated a series of [bmim][BF4]/Nafion composite membranes for CO2 separation by experiment and found that the ILs content has a great effect on the CO2 separation efficiency. The CO2 permeability of pure Nafion membrane is extremely low, while the incorporation of ILs can significantly enhance CO2 permeability without sacrificing much CO2/N2 selectivity, especially at high ILs content. The small-angle X-ray scattering results indicate that the addition of ILs changes the nanostructure of membranes, and high ILs content leads to the formation of continuous ILs nano-channels in ILs/Nafion membranes, thus significantly enhancing CO2 permeability. Overall, the formation of continuous ionic channels in ILs/polymer composite membrane has a positive effect on the CO2 separation from gas mixtures. On the other hand, it is obvious to observe from Figure 2 that the ability to form continuous ionic channel of different ILs in the PVDF matrix are different. For instance, when the ILs content is 50%, the distribution of continuous ionic channels in [bmim][PF6]/PVDF is most concentrated, while the continuous ionic channels are almost

regularly

dispersed

in

[bmim][B(CN)4]/PVDF,

which

suggest

that

[bmim][B(CN)4]/PVDF composite membrane can better separate CO2 from gas mixtures relative to another two ILs/PVDF systems. In order to further confirm the results, the RDFs were calculated and discussed at the following part. 3.1.2. RDFs analysis



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Interactions of cations with anions Radial distribution functions (RDFs) provide the possible distribution of other particles around a given particle along whole trajectories, which gives significant comprehension of the microstructure. Figure 3 presents the center-of-mass RDFs of cations-anions

in

[bmim][PF6]/PVDF,

[bmim][Tf2N]/PVDF

and

[bmim][B(CN)4]/PVDF with different concentration of ILs. As shown in Figure 3, the positions of the first peaks of cations-anions RDFs do not shift with the increase of ILs content for three systems, which is possibly caused by the strong cations-anions electrostatic interaction. Whereas, the first peak heights are significantly enhanced when ILs are confined in the PVDF matrix. The lower ILs content has a higher peak intensity. The phenomenon of the enhancement of RDFs peak indicates that ILs are separated by PVDF chains, forming nonuniform aggregation regions of ILs in the PVDF matrix. These results are in accordance with the obtained microstructure of ILs/PVDF systems in Figure 2 and other works.19, 21, 39, 41, 46 In addition, as shown in Figure 3, when the composite system has the same ILs content (e.g. 10%), the first peak of [bmim][PF6]/PVDF is the highest, while that for [bmim][B(CN)4]/PVDF is the lowest, indicating that the degree of ILs aggregation in [bmim][B(CN)4]/PVDF is the weakest among the studied ILs/PVDF systems. Consequently, [bmim][B(CN)4] can better disperse in the PVDF matrix, which is beneficial to form continuous ionic channels for the diffusion of CO2. Zhou et al.39 found the heights of all RDFs peaks for water-water in different water/ILs systems obey the order of water/[bmim][Tf2N] > water/[bmim][BF4] > water/1-butyl-3-methylimidazolium acetate ([bmim][Ac]) and concluded that the degree of water aggregation in water/[bmim][Ac] is the weakest. Interactions of F with H in PVDF In order to study the effect of ILs on the interaction among PVDF chains, the sitesite RDFs between F atoms and H atoms in PVDF for all simulated ILs/PVDF systems were analyzed. As shown in Figure 4, in three kinds of studied ILs/PVDF systems, the F-H RDFs own a sharp peak at 0.26 nm and the first minimum is around 0.3 nm. However, the intensity of first peak increases with the increase of ILs content, which means the formation of PVDF aggregation regions in ILs/PVDF systems. The higher


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ILs content leads to the more heterogeneous distribution of PVDF, which is in accordance with the microstructure of ILs/PVDF systems in Figure 2. Furthermore, the corresponding coordination numbers of F-H at the first solvation shell were also calculated and presented in table 2. The coordination numbers were obtained by the 𝑟

equation of N(r) = ∫0𝜌𝑔(𝑟)4π𝑟2𝑑𝑟, which represents the number of H atoms within a sphere of radius r centered on F atoms. As seen in Table 2, the three ILs/PVDF systems show the same trend. The number of F-H gradually decreases with increasing ILs content, indicating that the interaction among PVDF chains is weakened with the addition of ILs, which is most likely due to the shielding effect of ILs.13, 45 On the other hand,

the

order

of

the

coordination

number

is

[bmim][PF6]/PVDF

>

[bmim][Tf2N]/PVDF > [bmim][B(CN)4]/PVDF when the ILs content is the same. For example, the coordination numbers are 4.19, 4.05 and 4.01 for [bmim][PF6]/PVDF, [bmim][Tf2N]/PVDF and [bmim][B(CN)4]]/PVDF with the addition of 40% ILs, respectively. It seems that the difference for these numbers are not so significant, which could be due to the weaker PVDF-ILs interaction as shown in Figure 5, but they also can be used for evaluating the effect of ILs addition on the interaction among PVDF chains. The results reveal that the interaction among PVDF chains is most affected by [bmim][B(CN)4], demonstrating the best dispersibility of [bmim][B(CN)4] in the PVDF matrix, which agrees well with the results obtained from Figure 3 and the microstructure shown in Figure 2. Interactions of PVDF with anions To investigate what causes the difference of ILs aggregation degree among three ILs/PVDF systems, the site-site RDFs of PVDF-anions and the corresponding coordination numbers at the first solvation shell were analyzed for all ILs/PVDF systems with ILs content of 50%. The CPVDF, Panion, Nanion and Banion atoms were chosen to represent the site on PVDF, [PF6]-, [Tf2N]- and [B(CN)4]-, respectively. As shown in Figure 5, the RDFs of C-B has the highest first peak height at 0.63 nm, while the lowest RDFs peak height is observed for C-P appearing at 0.54 nm, which clearly indicates the interaction between [B(CN)4]- anion and PVDF is the strongest. The PVDF-anions



coordination numbers for three ILs/PVDF systems were calculated, which are 2.02, 2.99, 3.14 for C-P, C-N and C-B, respectively. The number of anions surrounding PVDF follows the order of [bmim][B(CN)4]/PVDF > [bmim][Tf2N]/PVDF > [bmim][PF6]/PVDF, which agrees well with the order of RDFs peak intensity shown in Figure 5, further implying that PVDF interact strongly with [B(CN)4]- anion. Therefore, the RDFs of PVDF-anions and corresponding coordination numbers indicate that the order of interaction of PVDF-anions is [B(CN)4]-PVDF > [Tf2N]-PVDF > [PF6]-PVDF. To confirm what causes the distinction of PVDF-anions interaction among three studied ILs/PVDF systems, the cohesive energy density of three ILs was calculated based on the methodology proposed by Liu et al.52 The values of cohesive energy density are 766.4, 569.1 and 172.8 J/cm3 for [bmim][PF6], [bmim][Tf2N] and [bmim][B(CN)4], respectively. On the other hand, cohesive energy density is related to the interaction between cations and anions. The stronger cations-anions interaction always leads to the larger cohesive energy density. The results mean that the cations-anions interaction of [bmim][B(CN)4] is the weakest. Consequently, it can be concluded that the weaker cations-antions interaction leads to the stronger polymer-anions interaction and the weaker ILs aggregation in the polymer matrix. Interactions of cations-cations and anions-anions To reveal the effect of PVDF on the interaction of cations-cations and anionsanions, the center-of-mass RDFs of cations-cations and anions-anions of three ILs/PVDF systems with 50% and 100% ILs were calculated and presented in Figure 6. For cations-cations, the three composite systems show different behavior, as shown in Figure 6(a), (c) and (e). The position of the first peak for [bmim][PF6] is almost not altered when the same number of ILs pairs are confined in the PVDF matrix, while it shifts from 0.94 nm to 0.97 nm for [bmim][Tf2N], and the maximum displacement is observed for [bmim][B(CN)4] with the first peak position shifting from 0.81 nm to 0.85 nm. The same phenomena are also found for anions-anions, as shown in Figure 6(b), (d) and (f). The position of the first peak for [bmim][PF6] is unchanged with a constant value of 0.72 nm, while it transfers from 0.82 nm to 0.85 nm for [bmim][Tf2N]. The [bmim][B(CN)4] has the maximum displacement of first peak position shifting from


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0.96 nm to 1.0 nm, which is due to the strongest interaction between PVDF and [B(CN)4]-. Bhargava et al.21 and Huang et al.19 reported the same phenomenon in CO2/[bmim][PF6] systems and concluded that it is the stronger interaction between CO2 and anions that gives rise to the displacement of RDFs peak position of cations-cations and anions-anions. The largest displacement of [bmim][B(CN)4] means that it is able to well disperse in the PVDF matrix compared with another two ILs containing different anions, which is consistent with the results presented in Figure 3 and 4. 3.1.3. SDFs analysis Spatial distribution functions (SDFs) can be more clearly to depict the density distribution of other particles around a given particle within a three-dimensional space, and the higher density distribution means the stronger interaction. The threedimensional SDFs for F atoms and H atoms in PVDF and cations around anions of three different ILs/PVDF systems with ILs concentration of 50% are shown in Figure 7. It is clear that the anions are mainly surrounded by cations and H atoms in PVDF, which is most likely due to the more positive atomic charges of cations and H atoms in PVDF. On the other hand, the high-density areas of cations and H atoms in PVDF around anions are competitive because they have certain overlapped regions. Besides, some un-lapped density areas of cations and H atoms in PVDF around [Tf2N]- anions are also observed, which can be ascribed to its more dispersed atomic charges and less symmetrical structure compared with [PF6]- and [B(CN)4]-. Interestingly, the highdensity areas of F atoms in PVDF around [PF6]- are smaller than those around [Tf2N]and B(CN)4]-. Results shown in Figure 3 indicate that the degree of ILs aggregation in [bmim][PF6]/PVDF is more remarkable, thus more [PF6]- anions are coordinated by cations and the high-density areas of F atoms in PVDF around [PF6]- anions are so small. 3.1.4. HB interaction analysis It is well known that there is strong hydrogen bond (HB) interaction among polymer chains. Due to the strong HB interaction, polymer chains are combined to form high-molecular polymer complexes with superior mechanical properties. In order to elucidate the effect of ILs on the HB interaction among PVDF chains, the number of HB among PVDF chains of all simulated systems was calculated. An HB can be



identified if the angle of donor-hydrogen-acceptor is less than 30o and the distance between donor and acceptor is less than or equal to 0.35 nm.56 Here, the possible donors are hydrogen atoms in PVDF chains and the potential acceptors are all fluorine atoms in PVDF chains. Therefore, the number of HB between F atoms and H atoms was calculated and depicted in Figure 8. As shown in Figure 8, it is obvious to see that the number of HB among PVDF chains decreases with increasing ILs concentration for three different ILs/PVDF systems, which mainly originates from the shielding effect of ILs. This effect hinders the contact of PVDF chains by destroying the HB network among PVDF chains. On the other hand, the ability of destroying HB networks among PVDF chains of three [bmim]-based ILs with different anions are also found to be different. For [bmim][PF6]/PVDF, the number of HB among PVDF chains has a little slow rate of descent from 251 to 116 with the increased ILs content to 50%. For [bmim][Tf2N]/PVDF, the number of HB among PVDF chains with 0% and 50% ILs are 251 and 87, respectively. However, for [bmim][B(CN)4]/PVDF, the number of HB among PVDF chains significantly decreases from 251 to 79 with increasing ILs content from 0% to 50%. The order of decreased HB number among PVDF chains for three different ILs/PVDF composite is [bmim][B(CN)4]/PVDF > [bmim][Tf2N]/PVDF > [bmim][PF6]/PVDF, which obviously demonstrates that ILs possess the capacity to penetrate into PVDF matrix to form composite membranes and further testifies the best dispersibility of [bmim][B(CN)4] in the PVDF matrix. The results are in accordance with the results displayed in Table 2. Wang et al.67 fabricated a polymer complex with superior mechanical properties which results from the strong HB interaction among polymer chains. The superior mechanical properties mean the high rigidity of polymer chains. In other words, destroying the HB among polymer chains can facilitate the mobility of polymer chains. While the mobility of polymer chains is related to the diffusion of gases and the higher mobility of polymer chains leads to the faster diffusion of gases.30, 33, 68 Therefore, the destruction of HB among PVDF chains is helpful for the diffusion of CO2. 3.1.5. Self-diffusion coefficients analysis The mobility of polymer has a great effect on the gas diffusion, while the existence


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of ILs in polymer matrix plays a vital role in the mobility of polymer. Many researchers have reported that pure polymer membrane has extremely low CO2 permeability, while the incorporation of ILs leads to the enhanced mobility of polymer chains by experiment, thus dramatically increasing CO2 permeability. Therefore, studying the diffusion behavior of polymer in ILs/polymer systems has significant meaning on the design of ILs/polymer composite membranes for CO2 separation. In this work, mean square displacements (MSDs) were calculated to study translational motion. Selfdiffusion coefficients of PVDF, cations and anions of ILs in whole simulated systems were calculated by using the slop of MSDs, fitting the Einstein relationship,69

〈∑

1 𝑑 D= lim 6𝑁𝑡→∞ 𝑑𝑡

𝑁

〉

[𝑟𝑖(𝑡) ― 𝑟𝑖(0)]2

𝑖=1

where the quantity in angle bracket is the ensemble-averaged of the MSDs of centerof-mass of molecules, N is the total number, ri(0) and ri(t) is the initial and final position. In the process of calculating self-diffusion coefficients of all simulated systems, it is very important to define the proper time range for the diffusive Einstein regime. When the MSD curve is linear with simulation time, the Einstein’s equation is valid. Borodin70 reported that in comparison with the correctly determined values extracted from the linear diffusive regime, the ion self-diffusion coefficient will be overestimated by more than 20% if extracting MSDs in a too short time. Other researchers53, 69, 71 also pointed out that for obtaining a reliable self-diffusion coefficient with Einstein’s equation, the long-time simulations should be conducted, making the MSD curve in a linear regime. In the present work, the long-time simulations of 50 ns under NpT were carried out for thoroughly equilibrating all simulated ILs/PVDF systems. Subsequently, a further 50 ns simulation runs were implemented to compute the self-diffusion coefficients. As shown in Figure S5-7 in the Supporting Information, the all MSDs are linear from 50 ns to 100 ns, which means that the obtained self-diffusion coefficients of cations, anions and PVDF are reliable. The results of self-diffusion coefficients of PVDF, cations and anions in all simulated systems calculated through Gromacs package are shown in table 3. The selfdiffusion coefficients of cations and anions are larger than those of PVDF, and the



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cations always diffuse faster than anions in spite of ILs species. For example, the diffusion coefficients for [bmim]+ cation, [Tf2N]- anion and PVDF in the mixture of 50% [bmim][Tf2N] in PVDF are 9.74×10-11, 7.86 ×10-11 and 5.84×10-11 m2/s, respectively. The same phenomenon was also found by other researchers.19, 21, 46 Compared with [bmim][PF6] and [bmim][Tf2N], [bmim][B(CN)4] has the largest self-diffusion coefficients regardless of cations or anions. For instance, when the ILs content is 30%, the

self-diffusion

coefficients

of

[bmim]+

cation,

[B(CN)4]-

anion

for

[bmim][B(CN)4]/PVDF system are 8.98×10-11 and 6.95×10-11 m2/s, respectively, while those for [bmim]+ cation and [Tf2N]- anion for [bmim][Tf2N]/PVDF system are 5.27×10-11 and 4.81×10-11 m2/s, respectively. The fastest diffusion of [bmim]+ cation and [B(CN)4]- anion in [bmim][B(CN)4]/PVDF mainly originates from the lowest viscosity of ILs based on [B(CN)4]- anions as reported in literature.24 As for three different ILs/PVDF systems, the effect of ILs on the self-diffusion coefficients of PVDF is similar. With the gradual increase of ILs content, the selfdiffusion coefficients of PVDF are gradually increased. However, the rates of increase are different. For [bmim][PF6]/PVDF, the self-diffusion coefficients for PVDF with 0% and 50% ILs are 0.15×10-11 and 2.07×10-11 m2/s, respectively. For [bmim][Tf2N]/PVDF, the self-diffusion coefficients for PVDF with 0% and 50% ILs are 0.15×10-11 and 5.84×10-11 m2/s. However, for [bmim][B(CN)4]/PVDF, the values of self-diffusion coefficients sharply increase from 0.15×10-11 to 11.71×10-11 m2/s with increased ILs content to 50%, which means that PVDF in [bmim][B(CN)4]/PVDF system has the fastest translational motion. The increased self-diffusion coefficients of PVDF can be explained in two aspects. One is that the larger self-diffusion coefficients of ILs relative to PVDF and the strong interactions between ILs and PVDF force the motion of PVDF. The other is that the HB network among PVDF chains is broken up with the addition of ILs, thus increasing the PVDF motion. At the same time, the enhancive PVDF motion facilitates the diffusion of CO2, especially in [bmim][B(CN)4]/PVDF. These results are consistent with the experimental studies conducted by Chen et al.33 The addition of ILs can reduce the crystallinity of PVDF and increase the free volume of membranes, thus leading to the higher mobility of PVDF chains. As a result, CO2


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molecules move faster among PVDF chains, which increases the self-diffusion coefficients of CO2. The same phenomenon was also reported by using different ILs/polymer membranes for CO2 separation through experimental studies.29, 32, 33, 68 3.2. ILs-PVDF-CO2 systems 3.2.1. Thermodynamic and dynamic properties of CO2 It is well known that gas molecules passing through the membrane follow the dissolution-diffusion mechanism. Thus, it is very important to calculate CO2 solubility and self-diffusion coefficient to estimate the CO2 separation performance of the membrane. The CO2 solubility was evaluated through Henry’s constant obtained by calculating the free energy of solvation for CO2 in three different ILs/PVDF systems, K𝐻 =

𝑅𝑇𝜌 𝑀

∆𝐺𝑠𝑜𝑙

( )

𝑒𝑥𝑝

𝑅𝑇

where R is the universal gas constant, T is the temperature, ρ is the density of systems in the limit of infinite dilution, M is the molecular weight and ΔGsol is the free energy of solvation. Free energy of solvation (ΔGsol) and Henry’s law constant (KH) for CO2 in three different ILs/PVDF systems with ILs content of 50% were calculated and shown in Table 4. It is obvious that the values of ΔGsol and KH show the same order [bmim][PF6]/PVDF > [bmim][Tf2N]/PVDF > [bmim][B(CN)4]/PVDF. Smaller ΔGsol and KH mean higher gas solubility.24 Therefore, the results indicate that the solubility of CO2 in [bmim][B(CN)4]/PVDF is the highest. Siqueira et al.49 performed MD simulations on the mixtures of [N4111][Tf2N] with PEO for SO2 absorption at 350 K and 1 bar and found that the SO2 absorption capacity of the mixture is related to the ΔGsol. With the increase of PEO content (from P(EO)2.7-ILs to P(EO)15.6-ILs), the free energy of solvation of SO2 in [N4111][Tf2N]/PEO decreases from -8.61 kJ/mol to -9.78 kJ/mol, which means the higher PEO content results in a more negative solvation free energy, thus significantly enhancing SO2 solubility and absorption capacity. The results also reflect that the smaller ΔGsol facilitate the more dissolution of gas in ILs/polymer composite. In the latest research, the same group48 simulated the CO2 absorption in the same ILs/polymer system. The CO2 solubility is inversely proportional to the



temperature. With the increase of temperature from 300 K to 400 K, the value of ΔGsol is increased from -1.39 kJ/mol to 1.36 kJ/mol, and KH is increased from 33.06 bar to 108.29 bar for [N4111][Tf2N]/PEO system containing 0.22 PEO. Therefore, they concluded that the CO2 solvation in [N4111][Tf2N]/PEO is an exothermic process and the increased temperature always leads to the increase of ΔGsol. As seen in Table 4, the values of ΔGsol and KH for CO2 in [bmim][PF6]/PVDF (8.67 kJ/mol, 486.79 bar), [bmim][Tf2N]/PVDF (7.00 kJ/mol, 299.78 bar) and [bmim][B(CN)4]/PVDF (3.16 kJ/mol, 110.35 bar) are larger than those of CO2 in [N4111][Tf2N]/PEO, which may originate from the higher temperature (493 K) and different ILs/polymer system. As far as we know, there is almost no report about the ΔGsol and KH for CO2 in other ILs/polymer composite membranes. Therefore, the comparison of the ΔGsol and KH for CO2 in other ILs/polymer composite membranes is not given. Moreover, the effect of ILs content on the Henry’s constant of CO2 in [bmim][B(CN)4]/PVDF was also studied. The values of KH for CO2 in [bmim][B(CN)4]/PVDF systems with ILs content of 10%, 30% and 50% are 716.64, 339.71 and 110.35 bar, respectively. The results illustrate that the ILs/PVDF membrane containing higher ILs content (lower PVDF concentration) is beneficial to the more dissolution of CO2. Self-diffusion coefficients of CO2 in three different ILs/PVDF systems with ILs content of 50% were calculated by using the slop of MSDs, fitting the Einstein relationship. The calculated self-diffusion coefficients are also shown in Table 4. The self-diffusion coefficient of CO2 in [bmim][B(CN)4]/PVDF is the highest compared with that in [bmim][PF6]/PVDF and [bmim][Tf2N]/PVDF, which means that the translational motion of CO2 in [bmim][B(CN)4]/PVDF is the fastest. The permeability, which is directly determined by solubility and self-diffusion coefficient, is a vital factor in evaluating the gas separation performance of the membrane. The results of free energy of solvation, Henry’s constant and self-diffusion coefficients of CO2 in three different ILs/PVDF systems reveal that the order of CO2 permeability is [bmim][B(CN)4]/PVDF > [bmim][Tf2N]/PVDF > [bmim][PF6]/PVDF. Higher permeability indicates better separation performance. Hence, [bmim][B(CN)4]/PVDF has better CO2 separation performance, which is in accordance with the results obtained


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from experimental studies31, 33 as well as the results gained from Table 3. 3.2.2. Interactions of CO2 with ILs/PVDF In order to investigate the interaction of CO2 with ILs/PVDF, the site-site RDFs of CO2-cations, CO2-anions and CO2-PVDF of three different ILs/PVDF systems with ILs content of 50% were calculated. The C, Panion, Nanion, Banion, CRcation, FPVDF and HPVDF were chosen to represent the sites on the CO2, [PF6]-, [Tf2N]-, [B(CN)4]-, [bmim]+ and PVDF, respectively. Figure 9 shows that the shape of RDFs peaks for three different ILs/PVDF systems is similar, while the height and position of RDFs peaks are different. In addition, it also can be found that the RDFs peaks of C-antions and C-CRcation are higher than those of C-FPVDF and C-HPVDF, which indicates that CO2 mainly interact with ILs, while the interaction of CO2-PVDF is relatively weak. Therefore, the formation of continuous ionic channels is beneficial to the transportation of CO2 through the ILs/PVDF membrane, which further demonstrates the results obtained from Figure 2. 4. CONCLUSIONS In this work, all-atom MD simulations were performed to study the microstructure, interactions and dynamic properties of several ILs/PVDF composite systems, which are based on [PF6]-, [Tf2N]- and [B(CN)4]- anions coupling with the same [bmim]+ cation. The content of ILs was set from 0% to 100%. The RDFs of cations-anions and the snapshots of [bmim][PF6]/PVDF, [bmim][Tf2N]/PVDF and [bmim][B(CN)4]/PVDF were investigated. The aggregation of all ILs in PVDF is found, and the continuous ionic channels are gradually formed with increasing ILs content. Thus, the ILs/PVDF membrane containing high ILs content is beneficial to the diffusion of CO2. However, when content of ILs in PVDF is the same, the degree of ILs aggregation follows the order of [bmim][PF6]/PVDF > [bmim][Tf2N]/PVDF > [bmim][B(CN)4]/PVDF, indicating that it is easiest to form continuous ionic channels in [bmim][B(CN)4]/PVDF for CO2 diffusion. In addition, the RDFs of PVDF-anions and the corresponding coordination numbers show that the interaction of PVDF-[B(CN)4] is stronger than that of PVDF-[Tf2N] and PVDF-[PF6]. The largest shift of the first peak of cations-cations and anions-anions is observed in [bmim][B(CN)4]/PVDF system, indicating that



[bmim][B(CN)4] is able to well disperse in the PVDF matrix. Furthermore, the hydrogen bond (HB) numbers among PVDF chains gradually decrease with increasing ILs concentration for three different ILs/PVDF systems, and the least HB number for PVDF in [bmim][B(CN)4] indicates that [bmim][B(CN)4] has the strongest ability to destroy the inter-chain HB among PVDF chains. Meanwhile, the self-diffusion coefficients of PVDF increase with the addition of ILs, which could be explained as that ILs compel the motion of PVDF and the HB among PVDF chains are broken up with the addition of ILs. The increased PVDF motion can facilitate the diffusion of CO2 among PVDF chains. Moreover, the systems of ILs/PVDF with CO2 were also studied. It is found that CO2 is strongly interacted with ILs, while the interaction of CO2 with PVDF is weaker. The free energy of solvation and Henry’s law constant for CO2 in three different ILs/PVDF systems indicate that the solubility of CO2 in [bmim][B(CN)4]/PVDF is the highest. Meanwhile, the self-diffusion coefficient of CO2 in [bmim][B(CN)4]/PVDF is the largest compared with that in [bmim][PF6]/PVDF and [bmim][Tf2N]/PVDF. The results imply that [bmim][B(CN)4]/PVDF can well separate CO2 from gas mixtures in relative to [bmim][Tf2N]/PVDF and [bmim][PF6]/PVDF. These discoveries provide a molecular insight into the interaction between ILs and polymer as well as useful information for designing ILs/polymer composite membranes with an excellent separation performance of CO2. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org Figures showing the radial distribution functions of cations-anions, F-H in PVDF for C20 and C70 systems; radial distribution functions of cations-cations and anionsanions for C20 and C70 systems with ILs content of 50% and 100%; the changing process of solvation free energy for CO2 in three different ILs-PVDF systems with ILs content of 50%; the mean square displacement curves of cations, anions and PVDF in three different ILs-PVDF systems.


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AUTHOR INFORMATION Corresponding Author. *Email:

[email protected]

*Email:

[email protected]

*Email:

[email protected]

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (No. 51674234, U1704251, 21606233), and “Transformational Technologies for Clean Energy and Demonstration”, Strategic Priority Research Program of the Chinese Academy of Sciences, No. XDA 21030500. REFERENCES (1) Hammond, G. P.; Akwe, S. S. O.; Williams, S. Techno-economic appraisal of fossilfuelled power generation systems with carbon dioxide capture and storage. Energy 2011, 36 (2), 975-984. (2) Zeng, S.; Zhang, X.; Bai, L.; Zhang, X.; Wang, H.; Wang, J.; Bao, D.; Li, M.; Liu, X.; Zhang, S. Ionic-Liquid-Based CO2 Capture Systems: Structure, Interaction and Process. Chem. Rev. 2017, 117 (14), 9625-9673. (3) Tome, L. C.; Marrucho, I. M. Ionic liquid-based materials: a platform to design engineered CO2 separation membranes. Chem. Soc. Rev. 2016, 45 (10), 2785-2824. (4) Kenarsari, S. D.; Yang, D.; Jiang, G.; Zhang, S.; Wang, J.; Russell, A. G.; Wei, Q.; Fan, M. Review of recent advances in carbon dioxide separation and capture. RSC Adv. 2013, 3 (45), 22739-22773. (5) Service, R. F. Choosing a CO2 Separation Technology. Science 2004, 305 (5686), 963-963. (6) Zhang, X.; Zhang, X.; Dong, H.; Zhao, Z.; Zhang, S.; Huang, Y. Carbon capture with ionic liquids: overview and progress. Energy Environ. Sci. 2012, 5 (5), 6668-6681. (7) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Green processing using ionic liquids and CO2. Nature 1999, 399 (6731), 28-28. (8) Rogers, R. D.; Seddon, K. R. Ionic Liquids-Solvents of the Future? Science 2003, 302 (5646), 792-793. (9) Smiglak, M.; Reichert, W. M.; Holbrey, J. D.; Wilkes, J. S.; Sun, L.; Thrasher, J. S.; Kirichenko, K.; Singh, S.; Katritzky, A. R.; Rogers, R. D. Combustible ionic liquids by design: is laboratory safety another ionic liquid myth? Chem. Commun. 2006, 24 (24), 2554-2556. (10) Anderson, J. L.; Ding, R.; Ellern, A.; Armstrong, D. W. Structure and Properties of High Stability Geminal Dicationic Ionic Liquids. J. Am. Chem. Soc. 2005, 127 (2), 593-604. (11) Earle, M. J.; Esperança, J. M. S. S.; Gilea, M. A.; Canongia Lopes, J. N.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. The distillation and volatility of



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(55) Spoel, D. V. D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.; Berendsen, H. J. C. GROMACS: Fast, flexible, and free. J. Comput. Chem. 2005, 26 (16), 1701-1718. (56) Hess, B.; Kutzner, C.; van der Spoel, D.; Lindahl, E. GROMACS 4: Algorithms for Highly Efficient, Load-Balanced, and Scalable Molecular Simulation. J. Chem. Theory Comput. 2008, 4 (3), 435-447. (57) Martínez, L.; Andrade, R.; Birgin, E. G.; Martínez, J. M. PACKMOL: A package for building initial configurations for molecular dynamics simulations. J. Comput. Chem. 2009, 30 (13), 2157-2164. (58) Darden, T.; York, D.; Pedersen, L. Particle mesh Ewald: An N⋅log(N) method for Ewald sums in large systems. J. Chem. Phys. 1993, 98 (12), 10089-10092. (59) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G. A smooth particle mesh Ewald method. J. Chem. Phys. 1995, 103 (19), 8577-8593. (60) Braga, C.; Travis, K. P. A configurational temperature Nosé-Hoover thermostat. J. Chem. Phys. 2005, 123 (13), 134101. (61) Parrinello, M.; Rahman, A. Polymorphic transitions in single crystals: A new molecular dynamics method. J. Appl. Phys. 1981, 52 (12), 7182-7190. (62) Hess, B.; Bekker, H.; Berendsen, H. J. C.; Fraaije, J. G. E. M. LINCS: A linear constraint solver for molecular simulations. J. Comput. Chem. 1997, 18 (12), 14631472. (63) Bennett, C. H. Efficient estimation of free energy differences from Monte Carlo data. J. Comput. Phys. 1976, 22 (2), 245-268. (64) Mistry, M. K.; Subianto, S.; Choudhury, N. R.; Dutta, N. K. Interfacial Interactions in Aprotic Ionic Liquid Based Protonic Membrane and Its Correlation with High Temperature Conductivity and Thermal Properties. Langmuir 2009, 25 (16), 9240-9251. (65) Xing, C.; Zhao, M.; Zhao, L.; You, J.; Cao, X.; Li, Y. Ionic liquid modified poly(vinylidene fluoride): crystalline structures, miscibility, and physical properties. Polym. Chem. 2013, 4 (24), 5726-5734. (66) Mejri, R.; Dias, J. C.; Lopes, A. C.; Bebes Hentati, S.; Silva, M. M.; Botelho, G.; Mão de Ferro, A.; Esperança, J. M. S. S.; Maceiras, A.; Laza, J. M.; Vilas, J. L.; León, L. M.; Lanceros-Mendez, S. Effect of ionic liquid anion and cation on the physicochemical properties of poly(vinylidene fluoride)/ionic liquid blends. Eur. Polym. J. 2015, 71, 304-313. (67) Wang, Y.; Liu, X.; Li, S.; Li, T.; Song, Y.; Li, Z.; Zhang, W.; Sun, J. Transparent, Healable Elastomers with High Mechanical Strength and Elasticity Derived from Hydrogen-Bonded Polymer Complexes. ACS Appl. Mater. Interfaces 2017, 9 (34), 29120-29129. (68) Rabiee, H.; Ghadimi, A.; Mohammadi, T. Gas transport properties of reverseselective poly(ether-b-amide6)/[Emim][BF4] gel membranes for CO2/light gases separation. J. Membr. Sci. 2015, 476, 286-302. (69) Morrow, T. I.; Maginn, E. J. Molecular Dynamics Study of the Ionic Liquid 1-nButyl-3-methylimidazolium Hexafluorophosphate. J. Phys. Chem. B 2002, 106 (49), 12807-12813. (70) Borodin, O. Polarizable Force Field Development and Molecular Dynamics Simulations of Ionic Liquids. J. Phys. Chem. B 2009, 113 (33), 11463-11478.



(71) Koller, T. M.; Ramos, J.; Schulz, P. S.; Economou, I. G.; Rausch, M. H.; Fröba, A. P. Thermophysical Properties of Homologous Tetracyanoborate-Based Ionic Liquids Using Experiments and Molecular Dynamics Simulations. J. Phys. Chem. B 2017, 121 (16), 4145-4157.

Table and Figure captions ACS Paragon Plus Environment

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Table 1. Simulation systems of ILs-PVDF. Table 2. Coordination numbers of F-H in all ILs/PVDF systems at 493 K and 1 bar. Table 3. Self-diffusion coefficients (1 × 10-11 m2/s) of all simulated ILs/PVDF systems at 493 K and 1 bar. Table 4. Free energy of solvation, ΔGsol (kJ/mol), Henry’s law constant, KH (bar), and self-diffusion coefficients, D (1 × 10-9 m2/s), of CO2 in three different ILs/PVDF systems at 493 K and 1 bar with ILs content of 50%. Figure 1. Structure of cation, anions and PVDF (a) [bmim]+, (b) [PF6]-, (c) [Tf2N]-, (d) [B(CN)4]- and (e) PVDF. Figure 2. Snapshots of three kinds of ILs/PVDF systems (a) [bmim][PF6]/PVDF, (b) [bmim][Tf2N]/PVDF and (c) [bmim][B(CN)4]/PVDF with 10%, 20%, 30%, 40%, 50% ILs from left to right at 493 K and 1 bar. Magenta and white represent ILs and PVDF chains, respectively. Figure 3. Center-of-mass RDFs for cations-anions in (a) [bmim][PF6]/PVDF, (b) [bmim][Tf2N]/PVDF and (c) [bmim][B(CN)4]/PVDF at 493 K and 1 bar. Figure 4. Site-site RDFs between F atoms and H atoms in PVDF in (a) [bmim][PF6]/PVDF, (b) [bmim][Tf2N]/PVDF and (c) [bmim][B(CN)4]/PVDF at 493 K and 1 bar. Figure 5. Site-site RDFs of PVDF-anions for three different ILs/PVDF systems at 493 K and 1 bar with ILs content of 50%. Figure 6. RDFs of cations-cations for pure ILs and with 50% ILs in (a) [bmim][PF6]/PVDF, (c) [bmim]Tf2N]/PVDF and (e) [bmim][B(CN)4]/PVDF and anions-anions

in

(b)

[bmim][PF6]/PVDF,

(d)

[bmim]Tf2N]/PVDF

and

(f)

[bmim][B(CN)4]/PVDF at 493 K and 1 bar. Figure 7. Three-dimensional probable distribution of F atoms (red) and H atoms (green) in PVDF and cations(purple) around anions in ILs/PVDF composite systems (a) [bmim][PF6]/PVDF with the multiple of average density 1.00, 1.15, 5.00, (b) [bmim][Tf2N]/PVDF with the multiple of average density 1.13, 1.15, 5.00 and (c) [bmim][B(CN)4]/PVDF with the multiple of average density 1.16, 1.15, 2.70 at 493 K and 1 bar with ILs content of 50%.



Figure 8. The number of HB between PVDF-PVDF as a function of ILs concentration at 493 K and 1 bar. Figure 9. Site-site RDFs of CO2-cations, CO2-anions and CO2-PVDF (a) [bmim][PF6]/PVDF, (b) [bmim][Tf2N]/PVDF and (c) [bmim][B(CN)4]/PVDF at 493 K and 1 bar with ILs content of 50%.

Table 1. Simulation systems of ILs-PVDF Number of ILs


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PVDF

ILs

0%

10%

20%

30%

40%

50%

Pure ILs

150

[bmim][PF6]

0

39

88

150

232

349

349

150

[bmim][Tf2N]

0

39

88

150

232

349

349

150

[bmim][B(CN)4]

0

39

88

150

232

349

349

aThe

number of 150 denotes the number of PVDF chains and others are the number of ionic pairs.

Table 2. Coordination numbers of F-H in all ILs/PVDF systems at 493 K and 1 bar 0%

10%

20%


30%

40%

50%


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[bmim]PF6]/PVDF

4.60

4.44

4.35

4.24

4.19

4.10

[bmim][Tf2N]/PVDF

4.60

4.39

4.29

4.14

4.05

3.96

[bmim]B(CN)4]/PVDF

4.60

4.37

4.27

4.13

4.01

3.94

Table 3. Self-diffusion coefficients (1 × 10-11 m2/s) of all simulated ILs/PVDF systems at 493 K and 1 bar


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[bmim][PF6]/PVDF

[bmim][Tf2N]/PVDF

[bmim][B(CN)4]/PVDF

cation

anion

PVDF

cation

anion

PVDF

cation

anion

PVDF

0%

-

-

0.15±0.01

-

-

0.15±0.01

-

-

0.15±0.01

10%

1.25±0.40

0.61±0.17

0.42±0.02

1.70±0.15

1.20±0.18

0.52±0.06

2.65±0.01

1.66±0.07

0.81±0.14

20%

1.88±0.21

1.18±0.14

0.63±0.08

3.48±0.27

2.81±0.88

1.59±0.15

4.65±0.33

3.73±0.04

1.90±0.04

30%

2.77±0.17

1.60±0.19

1.02±0.00

5.27±0.64

4.81±0.89

2.91±0.06

8.98±0.11

6.95±0.98

5.46±0.10

40%

4.37±0.33

2.64±0.26

1.40±0.13

7.70±0.69

6.08±0.22

3.40±0.08

12.95±0.83

9.95±0.21

7.61±0.60

50%

6.47±0.03

3.95±0.21

2.07±0.24

9.74±0.06

7.86±0.00

5.84±0.38

21.43±1.30

17.34±1.62

11.71±0.27

pure

15.97±0.51

12.04±0.69

-

21.02±1.77

17.58±2.53

-

49.67±2.11

43.38±0.05

-

aThe

digits after operational symbol are the standard deviation calculated from block averages.

Table 4. Free energy of solvation, ΔGsol (kJ/mol), Henry’s law constant, KH (bar), and self-diffusion coefficients, D (1 × 10-9 m2/s), of CO2 in three different ILs/PVDF



Page 32 of 42

systems at 493 K and 1 bar with ILs content of 50% [bmim][PF6]/PVDF [bmim][Tf2N]/PVDF [bmim][B(CN)4]/PVDF

aThe

ΔGsol

8.67±0.30

7.00±0.32

3.16±0.18

KH

486.79±34.36

299.78±22.51

110.35±4.74

D

1.49±0.03

1.82±0.32

2.11±0.17

digits after operational symbol are the standard deviation obtained from block averages.


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Figure 1. Structure of cation, anions and PVDF (a) [bmim]+, (b) [PF6]-, (c) [Tf2N]-, (d) [B(CN)4]- and (e) PVDF.



Figure 2. Snapshots of three kinds of ILs/PVDF systems (a) [bmim][PF6]/PVDF, (b) [bmim][Tf2N]/PVDF and (c) [bmim][B(CN)4]/PVDF with 10%, 20%, 30%, 40%, 50% ILs from left to right at 493 K and 1 bar. Magenta and white represent ILs and PVDF chains, respectively.


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Figure 3. Center-of-mass RDFs for cations-anions in (a) [bmim][PF6]/PVDF, (b) [bmim][Tf2N]/PVDF and (c) [bmim][B(CN)4]/PVDF at 493 K and 1 bar.



Figure 4. Site-site RDFs between F atoms and H atoms in PVDF in (a) [bmim][PF6]/PVDF, (b) [bmim][Tf2N]/PVDF and (c) [bmim][B(CN)4]/PVDF at 493 K and 1 bar.


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Figure 5. Site-site RDFs of PVDF-anions for three different ILs/PVDF systems at 493 K and 1 bar with ILs content of 50%.



Page 38 of 42

Figure 6. RDFs of cations-cations for pure ILs and with 50% ILs in (a) [bmim][PF6]/PVDF, (c) [bmim]Tf2N]/PVDF and (e) [bmim][B(CN)4]/PVDF and anions-anions

in

(b)

[bmim][PF6]/PVDF,

(d)

[bmim]Tf2N]/PVDF

[bmim][B(CN)4]/PVDF at 493 K and 1 bar.


and

(f)

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Figure 7. Three-dimensional probable distribution of F atoms (red) and H atoms (green) in PVDF and cations(purple) around anions in ILs/PVDF composite systems (a) [bmim][PF6]/PVDF with the multiple of average density 1.00, 1.15, 5.00, (b) [bmim][Tf2N]/PVDF with the multiple of average density 1.13, 1.15, 5.00 and (c) [bmim][B(CN)4]/PVDF with the multiple of average density 1.16, 1.15, 2.70 at 493 K and 1 bar with ILs content of 50%.



Figure 8. The number of HB between PVDF-PVDF as a function of ILs concentration at 493 K and 1 bar.


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Figure 9. Site-site RDFs of CO2-cations, CO2-anions and CO2-PVDF (a) [bmim][PF6]/PVDF, (b) [bmim][Tf2N]/PVDF and (c) [bmim][B(CN)4]/PVDF at 493 K and 1 bar with ILs content of 50%.



TOC/Abstract graphic


Page 42 of 42

Poly(vinylidene fluoride

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