Alkyl Tail Aggregations Break Long Range Ordering of Ionic Liquids

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Alkyl Tail Aggregations Break Long Range Ordering of Ionic Liquids Confined in Sub-Nanometer Pores Jiale Ma, Qiangqiang Meng, Chun Chan, Zhen Li, Yonghui Zhang, and Jun Fan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06263 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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The Journal of Physical Chemistry

Alkyl Tail Aggregations Break Long Range Ordering of Ionic Liquids Confined in Sub-nanometer Pores

Jiale Ma,a Qiangqiang Meng,a,b Chun Chan,a Zhen Li a, Yonghui Zhang a, Jun Fan*a,b aDepartment bCenter

of Materials Science and Engineering, City University of Hong Kong, Hong Kong

for Advanced Nuclear Safety and Sustainable Development, City University of Hong Kong, Hong Kong, China

*Email: [email protected] *Phone: +852-3442-9978

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ABSTRACT Manipulation of the lateral structure of electric double layers emerges to become an important method to improve the differential capacitance of supercapacitor, due to the correlation between the differential capacitance and lateral structural evolution. Space confinement is one of methods that can significantly influence the lateral structures of EDLs. In this paper, all-atom molecular dynamics simulations were employed to study in-plane structure of room temperature ionic liquids [Cnmim][PF6] (n = 1, 2, 4, 6) confined in sub-nanometer slit pores. Lateral ordering of ions and orientation of imidazole rings are systematically characterized under the influences of size of slit pore and length of alkyl tail linking to imidazole rings. In our simulations, crystalline, partially ordered and disordered phases of ions are observed. The ordering of ions can be manipulated by the formation of tail aggregations through the influence of both tail length and pore size. With increasing of alkyl tail and pore size, number and size of tail aggregation increase, and the ordering of ions in pores decreases. The formation of the alky tail aggregation can decrease the columbic ordering of ILs in the slit pore. Besides the lateral ordering, orientation of imidazole ring is found to strongly correlated with pore size but is independent with the alkyl tail length. These results highlight the importance of geometric structure of ions on ordering and provide new insights in the manipulation of lateral ordering to increase the energy density of supercapacitors.

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1. INTRODUCTION Room temperature ionic liquids (RTILs), usually composed of a class of organic and inorganic ions, are at liquid state at or near room temperature.1 RTILs have attracted much attention due to a lot of advanced properties, such as wide electrochemical windows, high charge density, high thermal stability, low volatility, etc. These properties can be modified by utilizing different combination of cation and anion. In the electrochemistry field, the wider electrochemical window significantly increases the energy density of electric double layer (EDL) capacitors. In this energy storage device, the electrode−electrolyte interface, i.e. the EDL structure, plays a key role in the energy storage process. Lateral structure of EDLs of ionic liquids (ILs) has been studied extensively in the past 10 years from both experimental and computational point of views. Unlike the liquid state in the bulk, ordered interfacial structure of ILs on charged walls have been observed in various previous experiments. Ions of both roundish2,3 and irregular shapes3-5 can form ordered phases. Driven by electric voltage, transitions between ordered and disordered phases of ILs were observed.2-5 Moreover, in computer simulation studies, the transition from disordered multilayer to ordered monolayer of ions at electrode surface was systematically studied by Kirchner et al..6-9 The formation of the ordered monolayer structure is closely related with the elimination of overscreening phenomenon of first interfacial ion layer. In 2014, Merlet et al. found the maxima of differential capacitance correlates with the lateral ordering transition of interfacial ions in EDL structure.10 This work comprises an important step into the understanding of three-dimensional EDL structure and triggered the interests of theorists in this field.11,12 In our previous work, the mechanism of the correlation between the differential capacitance and the lateral ordering transition of ions was investigated. It was found that this correlation origins from the structural 2 ACS Paragon Plus Environment


correlation of ions between lateral and normal directions.13 Due to such a correlation, manipulation of the lateral structure can be an important method to control the differential capacitance. Space confinement is one of the methods that can significantly influence the lateral structure of ILs. Since it is difficult to experimentally characterize the structure of ILs under space confinement, MD simulations have been utilized to explore the lateral structure of ILs in slit pore. It was found that space confinement has a dramatic influence on the lateral structure and electrical behavior of ILs. For example, in the all-atom MD simulation of [DMIM]Cl, the crystalline monolayer of ions was observed in slit pore with pore size of 7 Å.14 When the pore size increases to 11 Å, a crystalline bilayer structure was observed.15 Those crystalline monolayer and bilayer structures are quite different from the disordered structure in ILs bulk. Similar ordered monolayer was also reported in simulations of [EMIM][Br] ILs in slit pore.16 Besides these all-atom MD simulations, crystalline structures in slit pore were also observed in coarse-grained simulations17 and hard sphere simulations18,19. Generally, ordering of molecules is sensitive to their geometrical structure. For instance, in our previous all-atom MD simulation of [BMIM][PF6], crystalline monolayer of roundish PF6-ions can be observed near anode at high surface charge density, while long-range ordering was not observed for the cation near the cathode. The structural origin of this difference is the structure of imidazole ring in cation is much more complicated than the roundish anions.13 Besides, alkyl chains of ions also play a key role in the self-assembling nanostructures in both experiment20-22 and simulation23-27 works. In previous coarse-grained MD simulations of ILs in slit pore, ordering of ILs with side chain is different from that without side chain. 28 So far, structure of RTILs with both imidazole ring and alkyl tails under nano-confinement has not been systematically investigated.


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Space confinement of ions can occur in porous electrode materials. In electric double layer capacitors, electrode with subnano pores (pore size < 2 nm) has been intensively investigated recently. In 2006 and 2008, Simon and Gogotsi’s group observed anomalous increase of capacitance in sub-nanometer pores, when average pore size was decreased to the size of ions.29,30 The profile of capacitance versus pore size was reproduced by three MD simulation works in 2011.31-33 The origin of the anomalous increase of the capacitance was explained by the “superionic states” of ions in sub-nanopores.34 The term “superionic states” is used to define the state of ions screened by free electron of pores. As a result, packing of ions of the same sign becomes easier and coulombic ordering of ions break down. Recently, this superionic phenomenon was characterized by the combination of X ray scattering and hybrid Reverse Monte Carlo (RMC) simulation.35 Recently, breaking of coulombic ordering was studied by molecular dynamics simulation with roundish ions.36 The knowledge of in-plane structure of complex RTIL ions in subnano slit pore is also important for the understanding of superionic phenomenon. In this study, in order to reveal the lateral structure of ions under nano-confinement, all-atom MD simulations were applied to study different types of ILs confined in sub-nano slit pore with different pore sizes. To check the influence of ion geometric structure on the in-plane structure, 1alkyl-3-methylimidazolium hexafluorophosphate ILs [Cnmim][PF6] (n = 1, 2, 4, 6) with tails in different length were chosen. Then, roundish ion, 2D ring plane and alkyl tail in different length are involved in current simulations. The in-plane ordering of ILs was quantitatively characterized using structure factor S(k), radial distribution function gαβ(r), and coordination number (CN). The influence of tail length of imidazolium ions and the pore size on the ordering of ILs were investigated. We found that the ordering of the in-plan structure can be manipulated by both the



pore size and alkyl tail length through the formation of alkyl tail aggregation. Besides, the orientation of imidazole ring of ions in slit pore were investigated.

2. METHODS The simulation model of slit pore immersed in ILs bath are illustrated in Figure 1. The slit pore consists of two parts of walls. Each wall is modeled by three graphene layers and the distance between two neighboring graphene layers is 3.4 Å. As shown in Figure 1b, the pore size h is defined by the distance between two opposing walls. The pore size h in this work is 7, 8, 9, 10, 11, and 12 Å. Four ionic liquids with different lengths of side chains [Cnmim][PF6] (n = 1, 2, 4, 6), i.e., [DMIM][PF6] (n=1), [EMIM][PF6] (n=2), [BMIM][PF6] (n=4) and [HMIM][PF6] (n=6), were simulated to investigate their lateral structures inside the pore. Since the two carbon atoms next to the imidazole ring are constrained in the ring plane, the length of flexible alkyl tails of these ILs are 0, 1, 3, and 5, respectively. All MD simulations in this work were performed in NVT ensemble using LAMMPS package.37 The simulation system was maintained at 400 K by Nose-Hoover thermostat with an integration time-step of 1 fs.38 3D periodic boundary condition was applied. Carbon atoms in slit pore were constrained at its initial positions by strong springs with spring constant of 5000 eV/Å. A set of non-polarizable force field from Lopes and Pádua’s work was applied to bulk ionic liquid molecules39.39,40 The van der Waals parameters from OPLS force field were applied to C atoms on the electrode.41 Geometric mixing rule was applied to generate the LJ parameters between graphite electrode and IL molecules. The cutoff of van der Waals and the real space electrostatic interaction are 10 Å. The long range electrostatic interaction was solved by particle-particle particle-mesh (PPPM) algorithm.42 The C-H bond is maintained by the SHAKE algorithm.43 5 ACS Paragon Plus Environment

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In this work, before the equilibrium and production run, a pure IL system without graphite has been performed to obtain the density of IL. Based on this mass density, ILs were randomly placed in the simulation box to generate the initial configuration by PACKMOL package.44 In the equilibrium part, the initial configuration was first heated from 1 K to 1000 K in 1 ns and equilibrated for 4 ns at 1000 K. If the ILs were only heated to 400 K, ions can also entry pores and the high equilibrating temperature did not artificially make the ions get inside pores. Then the system was gradually quenched to 400 K in 6 ns. The equilibration run at 400 K is 10 ns. A 20-ns production run was performed for the investigation of the lateral structure in the slit pore. In the analysis of the in-plane structures, ions near the pore entrance (10 Å in x direction) are not included.

3. RESULTS AND DISCUSSION 3.1. The ordering of ILs is correlated with pore size and alkyl tail length of cation We first observed that ILs form different number of layers as the pore size h varies. The layering structures along the z direction of four different types of ILs in the pore are similar, for demonstration purpose, Figure 2 plots the IL [EMIM][PF6] distribution along the z direction in slit pore with different pore size h. More ion distribution profiles in z direction have been shown in Figure S1. Ions start to enter the pore at h = 7 Å. When h >=7 Å and h

Alkyl Tail Aggregations Break Long Range Ordering of Ionic Liquids

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