Crystallization and temperature-dependent structure deflection of C₆mimBr ionic liquid intercalated in LAPONITE®

Abstract

The physicochemical properties of large molecules confined in nanopores are expected to be different from those of the bulk. This study investigates the cation–anion relative position and the molecular orientation of the 1-hexyl-3-methylimidazolium bromide (C₆mimBr) ionic liquid intercalated in LAPONITE® by a temperature-dependent X-ray absorption fine structure (XAFS) and some traditional methods (TEM, DSC, XRD, and IR). DSC and TEM analyses revealed the formation of C₆mimBr crystals intercalated in LAPONITE®. The XAFS and XRD analyses at ambient temperature revealed that an ordered monolayer structure, with C₆mim⁺ cations intercalated in the interlayer nanospace and Br located on the edge of the LAPONITE®, was formed when ILs were intercalated in LAPONITE®. The results also demonstrated that the enhanced interactions, the formed hydrogen bonds, as well as the ordered monolayer arrangement induced the formation of C₆mimBr crystals when intercalated in LAPONITE®. In situ XAFS analysis with a combination of XRD patterns at varied temperatures revealed that the structure orientation of the intercalated C₆mim⁺ cations tends to deflect and maintain the ordered monolayer arrangement with the elevation of temperature. The ordered crystal structure still exists at 120 °C and disappears at a higher temperature.

---

Introduction

Ionic liquids (ILs) have received much attention due to their importance in a broad range of applications.^1,2 These applications stem from their special physical and chemical properties including negligible vapor pressure, high thermal stability, and wide electrochemical window.³ In recent years, ILs immobilized on solid supports have gained increasing attention because these compounds are applied in material expansion.^4,5 In order to approach a smart-design for this kind of IL based hybrids, many theoretical and experimental efforts have been made to study the microstructures and phase behavior of ILs immobilized on a solid surface or incorporated into a porous solid. For instance, Sha et al.⁶ reported the bilayer structure formation of ILs loaded on SiO₂ solid surface by atom molecular dynamics simulations. Singh et al.⁷ found that the T_m (melting point) of imidazolium-based ILs immobilized on a nanoporous silica matrix was depressed significantly compared to bulk ILs. Wang et al.⁸ investigated the microscopic ionic structures and orientational preferences of imidazolium-based ILs absorbed on chemically different quartz surfaces by atomistic molecular dynamics simulations, concluding that the imidazolium rings lie preferentially perpendicular to Si(OH)₂ surface and the anions are mainly absorbed on positively charged SiH₂ surface. Zhou et al.⁹ evaluated the nanoscale interactions of ILs at uncharged graphene and charged mica solid surfaces by using high resolution X-ray interface scattering and fully atomistic molecular dynamics simulations, suggesting that the structure of ILs exhibits a mixed cation/anion layering at uncharged graphene surfaces and an alternating cation/anion layering at the charged mica surface. Gong et al.¹⁰ performed atomic force microscopy (AFM) to reveal the extended layering structure of ILs confined to an amorphous carbon surface. Hence, the phase behavior and microstructure of ILs in various supports are relatively different, which will further determine the physicochemical properties of the ILs-based hybrids.

LAPONITE® (LAPONITE®) is composed of two silica tetrahedral sheets with a central magnesium octahedral sheet. The special structure of LAPONITE® caused a wide range of applications in paints, cosmetics, and inks.¹¹ The reported LAPONITE® empirical formula is Na_0.7⁺[(Si₈Mg_5.5Li_0.3)O₂₀(OH)₄]_0.7⁻. The occurrence of silanol groups at the edges of the sheets is ascribed to the finite dimensions (diameter, 25 nm; height, 0.92 nm) of the clay platelets.^12,13 The isomorphic substitution of some magnesium cations with lithium in the central sheet induces a partially negative charge. This charge is balanced by the presence of sodium ions in the interlayer space.¹² Various ionic organic molecules can be easily intercalated in the interlayer of LAPONITE® through ion exchange. Recently, the increasing attentions have been focused on the IL-modified LAPONITE® because of the combination of the advantages of them, leading to the applications in catalysis, antibacterial, luminescence.^14–16 For instance, Joshi et al.¹⁷ reported the coexistence of unique LAPONITE® colloidal gel–glass when LAPONITE® were dispersed in the ILs solution. Fraile et al.¹⁸ immobilized [Bmim][PF₆] on the LAPONITE® surface and found that a solid phase was obtained at low coverage by solid NMR analysis; they speculated that the solid layer of [bmim][PF₆] interacts with the surface through the protons in α position to the imidazolium ring. Nevertheless, the reported investigations only focused on the phase behavior of the LAPONITE® and the loaded ILs at room temperature. No systematic analyses have been performed on the cation–anion relative position and the molecular orientation of ILs incorporated into the interlayers of LAPONITE®, especially with the synergy between the temperature-dependent effects and the nanoconfinement.

In the present study, we mainly performed an in situ synchrotron radiation XAFS analysis with aid of TEM, DSC, IR and XRD methods to probe the cation–anion relative position and the molecular orientation of C₆mimBr IL intercalated in LAPONITE®. We firstly investigated the crystallization and temperature-dependent structure deflection of C₆mimBr ILs intercalated in LAPONITE® surface. Our results will provide a detailed theoretical basis for understanding the structural changes of ILs intercalated in LAPONITE® with the synergy between the temperature-dependent effects and the nanoconfinement.

Materials and methods

Materials

LAPONITE® (LAP, RD grade) was obtained from Southern Clay Products, Inc. The 1-butyl-3-methylimidazolium bromide (C₆mimBr) used in our investigation was purchased from Sigma (purity 99%). The ILs were purified carefully in our laboratory by washing with water/CH₂Cl₂ to remove halogen impurities, followed by recrystallization, and the purity was confirmed by ¹H NMR and HPLC. The properties of ILs are known to change with purity, particularly the water content. Therefore, the ILs were preheated at 70 °C in vacuum for 48 h before using.

Methods

Up to 100 mg of C₆mimBr was added to a suspension containing 600 mg of LAPONITE® RD and 100 mL of C₂H₅OH. This mixture was stirred at room temperature for 3 days. Afterward, the final product (C₆mimBr/LAPONITE®) was generated by drying at 100 °C for 24 h under high vacuum to remove C₂H₅OH completely.

Transmission electron microscope (TEM). TEM observations were conducted using a FEI Tecnai G2 S-TWIN (accelerating at 200 kV) microscope. The microscope was equipped with a super-atmospheric thin window X-ray detector.

Differential scanning calorimetry (DSC). DSC measurements were performed by using a PerkinElmer DSC-822^e analyzer; 5–10 mg samples were sealed in aluminum pans for measurements. The measurements were performed from −100 to 200 °C with a heating rate of 10 °C min⁻¹. All the performances were carried out in a nitrogen atmosphere with a gas flow of 20 mL min⁻¹.

X-ray absorption fine structure (XAFS). X-ray absorption data of the Se K-edge (12 658 eV) of the samples were recorded at room temperature in transmission mode by using ion chambers at the beam line BL14W1 of Shanghai Synchrotron Radiation Facility. The station was operated using a Si(III) double-crystal monochromator. During measurement, the synchrotron was operated at an energy of 3.5 GeV and a current of 230 mA. The photon energy was calibrated using the first inflection point of the Se K-edge in selenium metal foil. Data was analyzed using IFEFFIT.¹⁹ The k-space range was set from 1.2 Å⁻¹ to 4.2 Å⁻¹. The theoretical backscattering phase was calculated using the FEFF 8.2 code.²⁰

Fourier transform infrared spectroscopy (FT-IR). FT-IR measurement was conducted on a Bruker Tensor 27 FT-IR spectrometer at a transmission mode by pressing potassium bromide troche (KBr) from 4000 to 400 cm⁻¹ with a spectral resolution of 4 cm⁻¹.

X-ray diffraction (XRD). The XRD patterns were recorded using a Bruker D8 Advance diffractometer with a Bragg–Brentano geometry with a Cu Kα radiation (λ = 1.5418 Å) ranging from 5 to 90 2θ at an operating voltage of 40 kV and an electric current of 40 mA.

Ion chromatography (IC). Dionex ICS-2100 (USA) ion chromatographic instruments with amperometric detection were used for all analyses. The analytical chromatographic system consisted of a separation column IonPac AS19 (4 × 250 mm i.d.) and a guard column AG19 (4 × 250 mm i.d.). The IC conditions were as follows: the ion chromatographic separation of all samples were performed at 30 °C, using KOH solution as mobile phase at a flow rate of 1 mL min⁻¹. All samples (200 μL) were automatically injected into the column.

Results and discussion

Fig. 1 shows TEM images, high resolution TEM (HRTEM) images and the corresponding energy-dispersive X-ray (EDX) spectra of the LAPONITE® and LAPONITE® intercalated with C₆mimBr (C₆mimBr/LAPONITE®). The EDX spectra of C₆mimBr/LAPONITE® (Fig. 1F) confirmed the existence of Br atom, indicating the successful adsorption of C₆mimBr ILs on the LAPONITE®. On the basis of the ion chromatography analysis, the amount of bromide bounded to LAPONITE® is approximately 0.45 wt%. Compared with HRTEM image of the LAPONITE® (Fig. 1C), some crystal planes can obviously be observed in HRTEM image of C₆mimBr/LAPONITE® (Fig. 1D), demonstrating the formation of crystal for intercalated C₆mimBr ILs. Moreover, selected area electron diffraction pattern (inset in Fig. 1D) clearly indicates the formation of C₆mimBr crystals.

Fig. 1 TEM images, HRTEM images and the corresponding EDX of LAPONITE® (A, C and E) and C₆mimBr/LAPONITE® (B, D and F). Inset: electron diffraction of the selected area of LAPONITE® and C₆mimBr/LAPONITE®.

To investigate the phase behavior change of C₆mimBr ILs, differential scanning calorimetry (DSC) experiment was carried out. Fig. 2 displays the DSC heating curves for C₆mimBr, C₆mimBr/LAPONITE® and LAPONITE®. No fusion peaks for LAPONITE® can be observed at temperature below 200 °C, implying that the melting of LAPONITE® is above 200 °C. The melting point of pure C₆mimBr was −66 °C, while one sharp fusion peak at 137 °C was clearly observed in the DSC curve of C₆mimBr/LAPONITE®. The melting point of the C₆mimBr IL on the LAPONITE® increased by ca. 200 °C compared with that of the neat IL, indicating the transition of C₆mimBr from liquid to crystal. Obtaining stable IL crystals is very difficult or nearly impossible by simply cooling because solidification of IL by cooling often results in the glass formation.²¹ However, the TEM and DSC analyses indicated the formation of C₆mimBr crystal by intercalating the C₆mimBr ILs on the layer of LAPONITE®. In order to more clearly clarify the formation of C₆mimBr crystal on LAPONITE®, XAFS analysis was further carried out.

Fig. 2 DSC heating curves for C₆mimBr, C₆mimBr/LAPONITE® and LAPONITE®.

Fig. 3 shows the X-ray absorption near edge structure (XANES) spectra and the corresponding Fourier transform (FT) R-space spectra for the C₆mimBr and C₆mimBr/LAPONITE® at ambient temperature. At the Br K-edge, the intensity and position changes of the white line peak at 13 476 eV can be interpreted by variations in the electron density of the Br center atom. As shown in Fig. 3A, compared with pristine C₆mimBr ILs, the intensity increase and the shift to higher energy side of the white line peak for the C₆mimBr/LAPONITE® were attributed to the electron loss of the Br⁻. The electron loss of the Br⁻ possibly resulted from the charge transfer from anion to cation in intercalated C₆mimBr and from anion to LAPONITE®, implying the existence of IL–IL and IL–LAPONITE® interactions.

Fig. 3 The XANES spectra in the Br K-edge of pure C₆mimBr and C₆mimBr/LAPONITE® (A) and the corresponding Fourier transform R-space spectra of pure C₆mimBr and C₆mimBr/LAPONITE® at ambient temperature (B).

To obtain a clear local structure around the Br atoms, a series of FT R-space spectra for the C₆mimBr and C₆mimBr/LAPONITE® at ambient temperature are given in Fig. 3B. These spectra provide the definite distance between the anion and cation of C₆mimBr. As shown in Fig. 3B, the distances between the Br⁻ anions and [C₆mim]⁺ cations in the C₆mimBr and C₆mimBr/LAPONITE® are 2.61 Å and 2.42 Å, respectively, implying that the distance between Br⁻ and [C₆mim]⁺ was significantly decreased for the IL after intercalating in the LAPONITE®. The shorter distance between anions and cations will lead to the enhancement of weak interactions (such as electrostatic forces and H-bonding in C₆mimBr ILs). Notably, the new peak at 3.51 Å, ascribing to the O–H⋯Br bond, appeared in the spectrum for C₆mimBr/LAPONITE®. This result indicates the formation of hydrogen bond between Br anion of the C₆mimBr and O–H group of LAPONITE®, which is consistent with XANES analysis to confirm the existence of ILs–LAPONITE® interactions. This finding was further certified by FT-IR spectra of the LAPONITE®, C₆mimBr, and C₆mimBr/LAPONITE® and the results are shown in Fig. 4. As shown in Fig. 4, the FT-IR spectrum of the LAPONITE® shows a broad peak at 3445 cm⁻¹ attributed to the Si–OH groups present on the edge of the clay.²² By contrast, vibration bands at 3705 and 3666 cm⁻¹ were observed for the C₆mimBr/LAPONITE®, indicating that the anion Br⁻ strongly interacted with the hydroxyl groups of the outer surface of LAPONITE®. Similar result was investigated by Yang's group,²³ they observed that the IR vibration bands at 3648 and 3572 cm⁻¹ for BmimBF₄ IL–SiO₂ was indicative of the existence of the stronger interaction between the anion and the hydroxyl group of silanols. From the perspective of the formed hydrogen bonds, bromide ions locate on the outer edge of LAPONITE® and are not incorporated within the interlayer space when C₆mimBr IL is intercalated in LAPONITE®. Similar results were reported by Bourgeat-Lami groups,²⁴ concluded that monoalkoxy silanes form a flat layer on the edge of the clay by a condensation reaction with the clays edge silanol groups. Additionally, LAPONITE® presents a layered structure and exchangeable sodium cations in the interlayer space to compensate the excess layer charge. As a result, C₆mim⁺ cations can intercalate in the interlayer nanospace by ion-exchange reaction. The major driving force for ion-exchange reaction should account for a coulombic stabilization by the neutralization of the charges on the LAPONITE®. This finding was further indicated by XRD pattern of the LAPONITE® and C₆mimBr/LAPONITE® at ambient temperature and the results are shown in Fig. 5. As shown in Fig. 5, the diffraction peak at 2θ = 6.86° was attributed to the characteristic diffraction plane (001) of LAPONITE®. It is worth noting that the 2θ at 6.86° (d₀₀₁ = 1.28 nm) in the LAPONITE® shifts to 6.32° (d₀₀₁ = 1.39 nm) in the C₆mimBr/LAPONITE®, indicating the intercalation of C₆mim⁺ cations into the LAPONITE® interlayer. Moreover, we obtained an interlamellar space of 0.47 nm by subtracting the thickness of the clay layer (the thickness of the clay layer is approximately 0.92 nm (ref. 25)) in LAPONITE®/C₆mimBr (d₀₀₁ = 1.39 nm). The structure of C₆mim⁺ was calculated by Gaussian-09 C1 package, as shown in Fig. S1 (ESI†), with the molecular dimensions of C₆mim⁺ (10.8 × 4.2 × 3.6 in Å). This result suggests an inclination disposition of the long molecular axes of C₆mim⁺ along the clay layers. The tilt angle (θ) of C₆mim⁺ in the LAPONITE® interlayer can be calculated as follows:²⁶

Δd = d₀₀₁ − 0.92 = (ionic length)sin θ (1)

where the ionic length is the diameter of C₆mim⁺ ion. The corresponding tilt angle of C₆mim⁺ was calculated as 25.80° based on formula (1). We speculated that one monolayer of C₆mim⁺ inclined to the silicate layer at angle 25.80°. The intercalated C₆mim⁺ are parallel to each other to form an ordered monolayer structure on the interlayer space of LAPONITE®, and a strong π–π stacking interaction is formed, which leads to a reduction in the potential energy and entropy cost. For instance, Ogawa et al. indicated that the intercalated molecules are aligned themselves in an ordered structure within the interlayer space.²⁷ Here, an ordered monolayer structure, with C₆mim⁺ cation intercalated in the interlayer nanospace and Br located on the edge of the LAPONITE®, will be formed when C₆mimBr ILs are intercalated in the LAPONITE®.

Fig. 4 FT-IR spectra of C₆mimBr, LAPONITE®, and C₆mimBr/LAPONITE®.

Fig. 5 XRD patterns of LAPONITE® and C₆mimBr/LAPONITE® at room temperature.

Based on the above analysis, we draw a conclusion that the enhanced interactions in intercalated ILs, the hydrogen bonds formed between the Br anion of ILs and the OH group of LAPONITE®, as well as the ordered monolayer arrangement between intercalated cations and anions induced the formation of C₆mimBr crystals intercalated in the LAPONITE®, which is in accordance with the appearance of new peak at 2θ = 8.84°, 21.18°, 25.80°, 26.78°, 28.90°, 39.37°, 42.76°, 50.82°, and 53.08°, belongs to the different diffraction planes of C₆mimBr ILs crystal, in XRD pattern at ambient temperature (Fig. 5).

As the practical applications of ILs have considerable relevance with the temperature effect, the temperature-dependent microstructure changes of the intercalated ILs should be investigated to expand their applications. In our work, an in situ temperature XAFS experiment was performed to reveal the structural characteristics of the intercalated ILs at varied temperatures. Fig. 6 shows the XANES spectra and FT R-space spectra of C₆mimBr/LAPONITE® at varied temperature, 25, 90, 120 and 160 °C, respectively. As shown in Fig. 6A, the slightly decreased intensity and the shift of white line peak to lower energy can be investigated with raising temperature from 25 °C to 120 °C, indicating the electron reception of the Br⁻. These results maybe contribute to the slight cleavage of hydrogen bond between Br anion of C₆mimBr ILs and O–H groups of LAPONITE®. The electronegativity of Br atoms was stronger than that of the hydrogen atom in O–H group of the LAPONITE®, leading to a charge transfer from LAPONITE® to Br anion. However, the slight cleavage of hydrogen bond is not enough to change the structure of the intercalated ionic liquid. Therefore, with increasing temperature from 25 °C to 120 °C, the structure of intercalated ILs has no significantly change and maintains an ordered monolayer structure. However, a sudden decrease in intensity of the white line peak for the C₆mimBr/LAPONITE® at 160 °C may be attributed to the complete cleavage of hydrogen bond.

Fig. 6 The XANES spectra in the Br K-edge of C₆mimBr/LAPONITE® (A) and the corresponding Fourier transforms R-space spectra of C₆mimBr/LAPONITE® (B) at varied temperature, 25 °C, 90 °C, 120 °C and 160 °C, respectively.

To obtain the local structure around the Br atoms, a series of FT R-space spectra for the C₆mimBr/LAPONITE® at varied temperatures are shown in Fig. 6B. As shown in Fig. 6B, the distance between the Br anion and [C₆mim]⁺ cation in C₆mimBr/LAPONITE® slightly increased with increasing the temperature from 25 °C to 120 °C. Singh et al.²⁸ concluded that the mobility of ion confined in a slit-like graphitic nanopore increases with increasing temperature. Similarly, the increasing temperature promoted the movement of intercalated C₆mim⁺ cations. Therefore, the slightly increase in the distance between cations and anions with increasing the temperature from 25 °C to 120 °C can be interpreted by the slight structure deflection of intercalated C₆mim⁺ cations, in which C₆mim⁺ cations maintain the ordered monolayer arrangement. This finding was further indicated by XRD patterns of the C₆mimBr/LAPONITE® at varied temperatures and the results are shown in Fig. 7. It is revealed that some C₆mimBr crystal diffraction peaks can be still observed at 120 °C, indicating the existence of ordered crystal structure. Moreover, the intensities of diffraction peaks at 26.78°, 42.76° slightly decreased and no new diffraction peak appeared with increasing the temperature from 25 °C to 120 °C, indicating that the structure of ILs crystal has no obvious change in the temperature range 25–120 °C. It is worth noting that the 2θ (d₀₀₁) of LAPONITE® shifts to the lower value with increasing temperature, implying the expanding basal spacing (d₀₀₁) of LAPONITE®. The increase in basal spacing of LAPONITE® indicated the structure deflection of the intercalated monolayer of C₆mim⁺ cation. The XRD patterns of C₆mimBr/LAPONITE® at 120 °C display a d₀₀₁ basal spacing (2θ = 6.11°) of 1.44 nm. The corresponding tilt angle of C₆mim⁺ was calculated to be 28.80° based on formula (1). In terms of the statistical average value, we speculated that the monolayer of C₆mim⁺ inclined to the silicate layer with angle change from 25.80° to 28.80°, corresponding to a temperature change from 25 to 120 °C. This result is consistent with XAFS spectra with temperature from 25 °C to 120 °C. Additionally, the distance (2.68 Å) between cation and anion in intercalated C₆mimBr ILs at 160 °C have clearly changed when compared with that of intercalated C₆mimBr IL at 120 °C (2.47 Å), it is very close to that of pristine ILs (2.61 Å). These findings indicate that the structure of intercalated IL altered from ordered crystal structure to disordered amorphous structure with increasing the temperature from 120 to 160 °C and the intercalated IL was completely melted at 160 °C. These results are in well agreement with the disappearance of characteristic diffraction peak of C₆mimBr ILs at 160 °C (Fig. 7). Therefore, at temperature lower than the melting point of the intercalated ILs, the intercalated C₆mim⁺ cations tend to deflect and maintain the ordered monolayer structure with the rising temperature. The proposed structure change is depicted schematically in Scheme 1.

Fig. 7 XRD patterns of C₆mimBr/LAPONITE® at varied temperature, 25 °C, 90 °C, 120 °C and 160 °C, respectively.

Scheme 1 Schematic demonstration on the structure deflection of C₆mimBr ionic liquid intercalated in LAPONITE® with the rising temperature.

Conclusion

Crystallization and temperature-dependent structure deflection of C₆mimBr ILs intercalated in LAPONITE® were investigated by in situ XAFS, with the aid of TEM, DSC, IR and XRD measurements. The TEM and DSC analyses verified the formation C₆mimBr crystal. The XAFS spectra, with a combination of IR spectra and XRD pattern, indicated the formation an ordered monolayer structure with C₆mim⁺ cation intercalated in the interlayer nanospace and Br located on the edge of the LAPONITE®. Moreover, the formation of C₆mimBr crystals on the LAPONITE® may result from the following three factors: (1) the enhanced interactions in intercalated ILs, (2) the formation hydrogen bonds between the Br anion of the ILs and the OH group of the LAPONITE®, (3) the ordered monolayer arrangement between intercalated cations and anions. Furthermore, the XAFS spectra, with aid of XRD pattern at varied temperatures, demonstrated that the structure of intercalated C₆mim⁺ cations tend to deflect and maintain the ordered monolayer arrangement with the rising temperature. The ordered crystal structure still exists at 120 °C and disappears only at a higher temperature. Our results will provide a detailed theoretical basis for understanding the temperature-dependent structural changes of the ILs intercalated in LAPONITE®, and should be helpful in expanding many ILs applications in which the confinement effects or solid–liquid interfaces are related.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (11079007, 21306220, 21276257, 91534109). XAFS experiment was carried out at beamline BL14W1 SSRF (Shanghai Synchrotron Radiation Facility).

References

1. Y. Zhao and T. Bostrom, Application of Ionic Liquids in Solar Cells and Batteries: A Review, Curr. Org. Chem., 2015, 19(6), 556–566.
2. H. Lin, P. Bai and X. Guo, Ionic Liquids for SO₂ Capture: Development and Progress, Asian J. Chem., 2014, 26(9), 2501–2506.
3. J. D. Holbrey and K. R. Seddon, The phase behaviour of 1-alkyl-3-methylimidazolium tetrafluoroborates; ionic liquids and ionic liquid crystals, J. Chem. Soc., Dalton Trans., 1999, 13, 2133–2139.
4. B. Xin, C. Jia and X. Li, Supported Ionic Liquids: Efficient and Reusable Green Media in Organic Catalytic Chemistry, Curr. Org. Chem., 2016, 20(5), 616–628.
5. M. Shirani, A. Semnani, S. Habibollahi, H. Haddadi and M. Narimani, Synthesis and application of magnetic NaY zeolite composite immobilized with ionic liquid for adsorption desulfurization of fuel using response surface methodology, J. Porous Mater., 2016, 23(3), 701–712.
6. M. Sha, G. Wu, Q. Dou, Z. Tang and H. Fang, Double-Layer Formation of BmimPF₆ Ionic Liquid Triggered by Surface Negative Charge, Langmuir, 2010, 26(15), 12667–12672.
7. M. P. Singh, R. K. Singh and S. Chandra, Studies on Imidazolium-Based Ionic Liquids Having a Large Anion Confined in a Nanoporous Silica Gel Matrix, J. Phys. Chem. B, 2011, 115(23), 7505–7514.
8. Y.-L. Wang and A. Laaksonen, Interfacial structure and orientation of confined ionic liquids on charged quartz surfaces, Phys. Chem. Chem. Phys., 2014, 16(42), 23329–23339.
9. H. Zhou, M. Rouha, G. Feng, S. S. Lee, H. Docherty, P. Fenter, P. T. Cummings, P. F. Fulvio, S. Dai, J. McDonough, V. Presser and Y. Gogotsi, Nanoscale Perturbations of Room Temperature Ionic Liquid Structure at Charged and Uncharged Interfaces, ACS Nano, 2012, 6(11), 9818–9827.
10. X. Gong, A. Kozbial, F. Rose and L. Li, Effect of π–π Plus Stacking on the Layering of Ionic Liquids Confined to an Amorphous Carbon Surface, ACS Appl. Mater. Interfaces, 2015, 7(13), 7078–7081.
11. H. Z. Cummins, Liquid, glass, gel: the phases of colloidal LAPONITE®, J. Non-Cryst. Solids, 2007, 353(41–43), 3891–3905.
12. R. Sasai, T. Itoh, W. Ohmori, H. Itoh and M. Kusunoki, Preparation and Characterization of Rhodamine 6G/Alkyltrimethylammonium/LAPONITE® Hybrid Solid Materials with Higher Emission Quantum Yield, J. Phys. Chem. C, 2009, 113(1), 415–421.
13. M. Jaber and J.-F. Lambert, A New Nanocomposite: L-DOPA/LAPONITE®, J. Phys. Chem. Lett., 2010, 1(1), 85–88.
14. A. V. Martinez, J. A. Mayoral and J. I. Garcia, Pd nanoparticles immobilized in bmimPF₆ supported on LAPONITE® clay as highly recyclable catalysts for the Mizoroki–Heck reaction, Appl. Catal., A, 2014, 472, 21–28.
15. A. Sharma, P. Prakash, K. Rawat, P. R. Solanki and H. B. Bohidar, Antibacterial and Antifungal Activity of Biopolymers Modified with Ionic Liquid and LAPONITE®, Appl. Biochem. Biotechnol., 2015, 177(1), 267–277.
16. Y. Yao, Z. Li and H. Li, Modification of Eu³⁺-beta-diketonate complex-intercalated LAPONITE® with a terpyridine-functionalized ionic liquid, RSC Adv., 2015, 5(87), 70868–70873.
17. N. Joshi, K. Rawat and H. B. Bohidar, Coexistence of Iso-Nonergodic LAPONITE® Gel and Glass in 1-Methyl-3-Octylimidazolium Chloride Ionic Liquid Solutions, J. Phys. Chem. B, 2014, 118(23), 6329–6338.
18. M. Rosa Castillo, J. M. Fraile and J. A. Mayoral, Structure and Dynamics of 1-Butyl-3-methylimidazolium Hexafluorophosphate Phases on Silica and LAPONITE® Clay: From Liquid to Solid Behavior, Langmuir, 2012, 28(31), 11364–11375.
19. J. J. Rehr and R. C. Albers, Theoretical approaches to X-ray absorption fine structure, Rev. Mod. Phys., 2000, 72(3), 621–654.
20. T. Burchardt, V. Hansen and T. Valand, Microstructure and catalytic activity towards the hydrogen evolution reaction of electrodeposited NiP_x alloys, Electrochim. Acta, 2001, 46(18), 2761–2766.
21. W. Xu, L. M. Wang, R. A. Nieman and C. A. Angell, Ionic liquids of chelated orthoborates as model ionic glassformers, J. Phys. Chem. B, 2003, 107(42), 11749–11756.
22. S. Salleres, F. L. Arbeloa, V. Martinez, T. Arbeloa and I. L. Arbeloa, Photophysics of Rhodamine 6G Laser Dye in Ordered Surfactant (C12TMA)/Clay (LAPONITE®) Hybrid Films, J. Phys. Chem. C, 2009, 113(3), 965–970.
23. H. Yang, C. Yu, Q. Song, Y. Xia, F. Li, Z. Chen, X. Li, T. Yi and C. Huang, High-temperature and long-term stable solid-state electrolyte for dye-sensitized solar cells by self-assembly, Chem. Mater., 2006, 18(22), 5173–5177.
24. E. Bourgeat-Lami, N. N. Herrera, J. L. Putaux, S. Reculusa, A. Perro, S. Ravaine, C. Mingotaud and E. Duguet, Surface assisted nucleation and growth of polymer latexes on organically-modified inorganic particles, Macromol. Symp., 2005, 229, 32–46.
25. M.-A. Neouze, J. Le Bideau, P. Gaveau, S. Bellayer and A. Vioux, Ionogels, new materials arising from the confinement of ionic liquids within silica-derived networks, Chem. Mater., 2006, 18(17), 3931–3936.
26. T. Wan, H. Xu, Y. Yuan and W. He, Preparation and photochemical behavior of a cationic azobenzene dye-montmorillonite intercalation compound, J. Wuhan Univ. Technol., Mater. Sci. Ed., 2007, 22(3), 466–469.
27. M. Ogawa and K. Kuroda, Photofunctions of intercalation compounds, Chem. Rev., 1995, 95(2), 399–438.
28. R. Singh, J. Monk and F. R. Hung, Heterogeneity in the Dynamics of the Ionic Liquid BmimPF₆ Confined in a Slit Nanopore, J. Phys. Chem. C, 2011, 115(33), 16544–16554.

---

Footnotes

† Electronic supplementary information (ESI) available: The structure of C₆mim⁺ simulated by Gaussian-09 C1 package. See DOI: 10.1039/c6ra18618a

‡ These authors contributed equally to this work.

---