Julian
Mars
ab,
Binyang
Hou‡
ac,
Henning
Weiss
a,
Hailong
Li
a,
Oleg
Konovalov
c,
Sven
Festersen
d,
Bridget M.
Murphy
de,
Uta
Rütt
f,
Markus
Bier
gh and
Markus
Mezger
*ab
aMax Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany. E-mail: mezger@mpip-mainz.mpg.de
bInstitute of Physics and MAINZ Graduate School, Johannes Gutenberg University Mainz, 55128 Mainz, Germany
cESRF The European Synchrotron, 71 avenue des Martyrs, 38000 Grenoble, France
dInstitute of Experimental and Applied Physics, Kiel University, Leibnizstr. 19, 24098 Kiel, Germany
eRuprecht Haensel Laboratory, Kiel University, Leibnizstr. 19, 24098 Kiel, Germany
fDESY Photon Science, Notkestr. 85, 22607 Hamburg, Germany
gMax Planck Institute for Intelligent Systems, Heisenbergstr. 3, 70569 Stuttgart, Germany
hInstitute for Theoretical Physics IV, University of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany
First published on 26th September 2017
Surface induced smectic order was found for the ionic liquid 1-methyl-3-docosylimidazolium bis(trifluoromethlysulfonyl)imide by X-ray reflectivity and grazing incidence scattering experiments. Near the free liquid surface, an ordered structure of alternating layers composed of polar and non-polar moieties is observed. This leads to an oscillatory interfacial profile perpendicular to the liquid surface with a periodicity of 3.7 nm. Small angle X-ray scattering and polarized light microscopy measurements suggest that the observed surface structure is related to fluctuations into a metastable liquid crystalline SmA2 phase that was found by supercooling the bulk liquid. The observed surface ordering persists up to 157 °C, i.e. more than 88 K above the bulk melting temperature of 68.1 °C. Close to the bulk melting point, we find a thickness of the ordered layer of L = 30 nm. The dependency of L(τ) = Λln(τ/τ1) vs. reduced temperature τ follows a logarithmic growth law. In agreement with theory, the pre-factor Λ is governed by the correlation length of the isotropic bulk phase.
The bulk structure of ILs was extensively studied experimentally by X-ray scattering,9–19 neutron scattering10,11,13,20 coherent anti-Stokes Raman scattering21 and by computer simulations.16,22–29 One particular feature, that draws significant attention, is the appearance of a so-called pre-peak in scattering experiments.16 These pre-peaks were observed for a wide range of ILs with aliphatic side chains longer than a butyl group. Corresponding to a length scale of a few nanometers, this peak was attributed to a high degree of intermediate range mesoscopic order. It is caused by micro phase separation in the polar networks composed of the charged moieties and domains of the non-polar alkyl side chains.24 Beyond this intermediate range mesoscopic order, long range translational order is observed in Ionic Liquid Crystals (ILCs).9,30–33 Such ILCs with long aliphatic side chains can form smectic mesophases over an extended temperature range.34,35 From computer simulations, a detailed understanding of their phase behavior depending on electrostatic and vdW interactions has been obtained.36–40
These bulk heterogeneities affect their molecular scale arrangement at surfaces and interfaces. Simulations of imidazolium based IL surfaces indicate that the aliphatic part of the cation points towards the interface. Nevertheless, due to the low packing of the aliphatic alkyl-chains, the aromatic ring as well as the alkyl-chains were found to be present within the uppermost surface region.41–43 Experimentally, angle-resolved X-ray photoelectron spectroscopy shows an enrichment of the aliphatic chains at the interface of [CnC1im]+-based ILs with n > 2.44–49 Smaller anions tend to give a stronger enrichment. These observations are consistent with sum frequency generation spectroscopy showing that the imidazolium ring is parallel to the surface and the aliphatic chain is pointing toward the gas phase.50,51 Interfacial profiles with sub-nanometer resolution across free IL surfaces52–55 and buried interfaces56,57 were obtained by X-ray reflectivity (XRR) experiments.58,59 Jeon et al. observed a surface crystalline layer in [C4C1im]+[PF6]− using grazing incident X-ray diffraction (GIXD).60 Using resonant soft X-ray reflectivity, it was shown that the micro phase separation in ILs with long aliphatic side-chains drives an oscillatory near surface structure with a periodicity on the nanometer length scale.55 Towards the bulk, this structure is exponentially decaying with a correlation length matching the corresponding value found in the isotropic bulk liquid. Like in bulk, the oscillatory surface profiles originate from alternating regions enriched with ionic and aliphatic moieties.55 Recently, long ranged order has been found for ionic liquid films and in confinement.61,62
Here, we present a study of the 1-methyl-3-docosylimidazolium bis(trifluoromethlysulfonyl)imide ([C22C1im]+[NTf2]−, Fig. 1) surface. This imidazolium based IL with long C22 side chains exhibits pronounced microphase separation.63 Using X-ray reflectivity and grazing incidence scattering, we obtain information on the temperature dependent surface structure with molecular scale resolution. Complementary information from small angle X-ray scattering, polarized light microscopy and calorimetric measurements suggest that the observed surface structure is related to fluctuations into a metastable liquid crystalline phase that was found by supercooling the bulk liquid.
The temperature dependent molecular volume V = Vm[1 + α(T − Tm)] of the bulk liquid was determined by pycnometry. Interpolation by linear regression yield a molecular volume of Vm = 1.015 nm3 at the melting point Tm = 68.1 °C and a volumetric thermal expansion coefficient α = 0.745 × 10−3 K−1.
Phase transition temperatures and entropies ΔS = ΔH/T of [C22C1im]+[NTf2]− were determined by differential scanning calorimetry (Mettler Toledo DSC-822) using scan rates between 1 K min−1 and 10 K min−1 (ESI†). Equilibrium phase transition temperatures were obtained by extrapolating to zero heating and cooling rates.
Polarized light microscopy (POM) was carried out using a 10× objective and wave plate between crossed polarizer (Zeiss Photomicroscope III). The IL was contained between two glass cover slips with a sample thickness of approx. 100 μm. Temperature dependent measurements at a rate of approx. 20 K min−1 were performed using a heating microscope stage (Linkam THMS600).
The PTFE trough was cleaned in Piranha solution (3 parts H2SO4 98%, 1 part hydrogen peroxide 30%) for at least 20 minutes, thoroughly rinsed with ultrapure water, and dried in a nitrogen stream. [C22C1im]+[NTf2]− was filled into the heated trough at 90 °C until a convex meniscus was formed.
Molecular resolution is obtained by recording XRR patterns up to a sufficiently high momentum transfers qz perpendicular to the sample surface. Therefore, the specular reflected X-ray beam was recorded for incident angles αi ≤ 1.9°. This corresponds to momentum transfers qz = 4π/λsinαi ≤ 6 nm−1. Specular intensities were extracted from the 2D patterns by integration over an area of 40 × 40 detector pixels. The background signal predominantly originates from the diffuse scattering contributions of the bulk liquid. It exhibits an approximately concentric intensity distribution around the primary beam axis. Therefore, background intensities were measured at a horizontal offset of 20 pixels and a vertical offset, adjusted to keep the total scattering angle 2θ for the specular reflection and the background equal. XRR segments measured at different absorber settings were merged and interpolated on an equidistant qz-grid.
(1a) |
(1b) |
(2) |
Deviations from an ideal surface cause modulations in the Fresnel normalized XRR patterns. Within the Born approximation, the XRR pattern R(qz) is linked to its corresponding real space electron density profile ρe(z) across the liquid surface via Fourier transformation72,73
(3) |
(4) |
(5) |
The oscillatory function with periodicity d is expanded in a series
(6) |
The oscillatory function ψ(z) in eqn (6) corresponds to the microscopic structure of smectic layers of the IL moieties. The envelope ϕ(z) in eqn (4) plays the role of a locally varying smectic order parameter profile. Within a simple variant of the Landau-de Gennes formalism,77 one can determine that smectic order parameter profile ϕ(z) of a smectic surface layer of thickness L in between a vapor phase and the bulk liquid. Here, only the leading terms
(7) |
Reflectivity curves were numerically calculated using the Parratt formalism after dividing the profile into 0.05 nm slabs of constant density.72,78 Dispersion effects were included using X-ray form factors from Henke et al.69 Density profiles were extracted from the experimental XRR data by parameter refinement using a simulated annealing algorithm.79 Damping of the specular reflectivity due to capillary wave roughness of the liquid surface80,81 was taken into account by
(8) |
γ(L) = γlv + φ(L)Δγ | (9a) |
Δγ = γls + γsv − γlv | (9b) |
(9c) |
By minimization of the total free energy of the system, a logarithmic growth law of the ordered surface layer thickness L vs. reduced temperature is obtained.
(10a) |
(10b) |
Indeed, a focal conic texture, indicating a smectic A liquid crystalline mesophase, was observed in POM (Fig. 4c).63 Together with the SAXS data (Fig. 4) and the dimensions of the molecular moieties (Fig. 1) this mesophase is identified as a SmA2 liquid crystalline phase. While the absence of bâtonnets typically is linked to nematic to smectic phase transitions88 no signs for a nematic phase was found by DSC, POM and X-ray scattering. At 58.4 °C and 57.7 °C a double peak was observed in DSC. The higher temperature peak was attributed to the smectic to crystal transition. Its transition temperature strongly varied for different DSC measurements. The second peak is likely related to a solid–solid transition between two different crystal structures.
Above the melting point, the X-ray scattering patterns of [C22C1im]+[NTf2]− exhibit three diffuse peaks at q0 = 1.7 nm−1, 7.9 nm−1 and 12.9 nm−1 (Fig. 4a). The two peaks II and III at large momentum transfer q were associated with charge alternation and adjacency correlations.15,29 From the peak position, a periodicity 2π/q0 ≈ 3.6 nm is estimated at T = 96 °C. Having a periodicity similar to the molecular length scale (Fig. 1), the origin of the first sharp diffraction peak (FSDP) can be identified with the polarity alternation i.e. microphase separation of the ionic and aliphatic moieties.12,25,26,29
Quantitative analysis by fitting a generalized Teubner–Strey model to the scattering patterns yield a periodicity db(T) = 3.74 nm − 0.348 × 10−2 nm K−1 (T − Tm) and a correlation length ξb(T) = 4.79 nm − 0.0298 nm K−1 (T − Tm). A detailed X-ray scattering study of the [C22C1im]+[NTf2]− bulk structure in comparison to [C18C1im]+[FAP]−, [C18C1im]+[NTf2]−, [C18C1im]+[NNf2]− and [C22C1im]+[NNf2]− is found in ref. 67.
Upon rapid cooling below the SmA2 transition temperature TLC, the FSDP becomes much sharper. The periodicity dLC = 2π/q0 = 3.68 nm was found to be only 1.6% shorter then the value in the isotropic liquid. Albeit much weaker in intensity, a 2nd order reflection is observed at 2q0 = 3.4 nm−1. Analysis of high-resolution SAXS data by fitting a pseudo-Voigt function to the FSDP reveals a total FWHM of 0.0185 nm−1 (Fig. 4a). This value is close to the resolution limit of the instrument. From the Scherrer formula we estimate domain sizes ≥0.7 μm i.e. long ranged translational order. In contrast, the intensity, position and shape of the high-q peaks II and III remain almost unchanged. Together with the small change in the position of the FSDP, this indicates a strong structural similarity of the liquid crystalline phase and the mesoscopic structure found in the liquid phase.19
Fig. 5 (a) Temperature dependent XRR curves of the [C22C1im]+[NTf2]− surface normalized to the Fresnel reflectivity RF(qz) (symbols) and calculated patterns (solid lines). Vertically dashed lines at 2π/ds = 1.69 nm−1 and 3.37 nm−1 indicate the positions of the 1st and 2nd order quasi-Bragg peaks. (b) Electron density profile ρe(z) obtained by fitting eqn (4) to the experimental XRR data. Horizontal tics indicate the layer thickness L of the ordered structure near the IL surface. Curves measured at 115 °C (red), 87 °C (purple), 73 °C (yellow), 70 °C (green), and 68 °C (blue) are vertically shifted for clarity. |
Deviations from an ideal surface cause modulations in the Fresnel normalized XRR patterns (Fig. 5). For the highest temperature of 115 °C (red symbols), a step-like reflectivity curve is obtained. At lower temperatures, this step gradually transforms into a dip (purple curve, 87 °C). These modulations are directly linked to changes in the density profiles across the IL surface by Fourier transformation (eqn (3)). Therefore, the observed dip, resembling an inverted Lorentzian peak, correspond to an exponentially damped oscillatory density profile across the IL surface.57,89 Qualitatively similar XRR curves were obtained for [C18C1im]+[FAP]−.55 Using resonant scattering techniques, the oscillatory surface profile was attributed to alternating layers. They are composed of aliphatic side chains and ionic moieties i.e. the anions and positively charged imidazolium rings. The periodicity ds = 2π/q0 ≈ 3 nm of the oscillatory surface profiles estimated from the peak position q0 = 2 nm−1 is similar to the bulk value db.
Below 73 °C (yellow symbols) two Bragg-like peaks emerge in the XRR curves. They correspond to 1st and 2nd order reflections at q0 = 1.68 nm−1 and 2q0 = 3.35 nm−1, respectively. As temperature decreases towards the transition temperature TLC where long ranged translational order was observed in bulk, these peaks get sharper and their intensities increase (green and blue symbols). Like the dip, these peaks originate from an oscillatory surface profile. However, the appearance of two distinct Bragg-like 1st and 2nd order peaks indicate an increasing surface order. In analogy, in the SAXS bulk measurements the 2nd order FSDP was only observed in the liquid crystalline smectic phase. This suggests the formation of surface induced smectic order with a smectic layer of thickness L wetting the isotropic liquid bulk phase.
Fig. 6a shows the grazing incidence scattering patterns recorded from the [C22C1im]+[NTf2]− surface at 70.2 °C. At the grazing angle αi = 0.12°, corresponding to 80% of the critical angle of total reflection, the X-ray beam does not penetrate into the bulk liquid. Instead, an evanescent wave with a decay length of approx. 10 nm ≈ 3ds is formed. Therefore, in the grazing incidence experiments the bulk signal is strongly suppressed. This allows to detect the relatively weak scattering from the near surface region where oscillatory density profiles were observed. Long-range lateral order at the surface, e.g. a 2D crystalline structure, gives rise to sharp streaks along q⊥ at distinct q‖ values.90
However, in contrast to the experiments by Jeon et al. on [C4C1im]+[PF6]−60 at all temperatures no indications for long-ranged in-plane order have been found in our experiments. Instead, two broad diffuse peaks are observed (Fig. 6c). Their positions and widths are similar to the peaks II and III observed in bulk (Fig. 6d). However, unlike in bulk the relative peak intensity is strongly changing with temperature. Upon cooling, the intensity of peak III at 13 nm−1 is significantly increasing. In contrast, the corresponding bulk peak intensity remains almost unchanged.18 At low temperatures, the long axis of the smectic fluctuations adjacent to the surface are preferably aligned parallel to the surface normal. Thus, the in-plane scattering signals in the q‖-direction originating from charge alternation (peak II) and adjacency correlations (peak III) are enhanced. When the thickness of the smectic layer L reaches the decay length of the evanescent X-ray wave the peak intensities saturate. This texture effect affects only the peak intensities, leaving their position and shape unchanged with respect to the isotropic bulk liquid. This observation confirms our interpretation that the structure of the [C22C1im]+[NTf2]− surface above its melting point is governed by fluctuations of the metastable smectic mesophase rather than by formation of a 2D surface crystal.
T (°C) | L (nm) | ξ b (nm) | d (nm) | z 0 (nm) | σ s (nm) | S 0 | S − | B | a 2 | ξ 2 (nm) |
---|---|---|---|---|---|---|---|---|---|---|
a Parameters were fixed to the interpolated bulk values extracted from SAXS.67 b Parameters were determined by continuity and differential continuity conditions. ξs was fixed at 2000 nm. | ||||||||||
68 | 30.0 ± 2.0 | 4.78 | 3.73 | 2.28 | 0.18 | 1.71 | −98 | 126 | 0.15 | 26 |
70 | 16.8 ± 2.2 | 4.73 | 3.73 | 2.29 | 0.17 | 1.64 | −152 | 12.6 | 0.15 | 18 |
73 | 12.5 ± 1.2 | 4.64 | 3.73 | 2.24 | 0.10 | 1.62 | −189 | 6.50 | 0.12 | 19 |
87 | 6.5 ± 0.6 | 4.21 | 3.70 | 2.14 | 0.18 | 1.58 | −294 | 2.90 | 0.15 | 9 |
115 | 2.8 ± 2.5 | 3.38 | 3.56 | 1.89 | 0.25 | 1.55 | −503 | 0.19 | 0.05 | 20 |
Horizontal tics in Fig. 5b indicate the layer thickness L of the ordered structure near the IL surface. For z ≤ L, the envelope ϕ(z) of the density profile is given by eqn (7), whereas for z > L the envelope is exponentially decaying with the bulk correlation length ξb determined by X-ray scattering.67
Fig. 7 shows the layer thickness L/d of the normalized surface structure divided by its periodicity d vs. reduced temperature τ (red circles). The blue line is a fit to the theoretically predicted growth law assuming exponentially decaying short range interactions (eqn (10a)). Approaching the bulk transition temperature T0, the surface layer thickness L is diverging. From the fit we obtain T0 = 68.2 °C.
Fig. 7 Smectic wetting layer thickness L from fits (red circles) and fit to eqn (10a) (blue line) vs. reduced temperature τ = (T − T0)/T0. |
This value agrees well with the transition temperature TLC = 67.1 °C of the metastable smectic bulk phase determined from DSC measurements. On the other hand, surface induced ordering is found only below τ = 0.26. This corresponds to an onset temperature T1 = 157 °C. Above this temperature, the oscillatory structure near interfaces decays exponentially with a decay length given by the corresponding bulk correlation length ξb.91
For the structurally similar imidazolium based IL [C18C1im]+[FAP]− no such surface ordered structures were found in a previous XRR study.55 In contrast, over the entire accessible temperature range the decay length at the surface was found to be equal to the bulk correlation length. However, unlike for [C22C1im]+[NTf2]− no indications for metastable liquid crystalline mesophases have been found in this system.
In literature, at least three distinct types of interface induced smectic order are discussed: discrete, continuous, and continuous with a prewetting layer. Discrete interface induced smectic order has been found for several liquid crystals at the free surface,92–94 in pores,95 and by simulation.96 In these systems, surface freezing via a quantitized layer by layer growth of a smectic liquid crystal was found above the mesophase transition temperature. In contrast, continuous smectic order is characterized by a gradually decaying oscillating density profile. Notably, here the correlation length at the interface is larger than in the isotropic bulk. Several XRR studies,94,97–100 atomic-force microscopy101 and simulations102 show evidence for this phenomena. Surface induced continuous smectic order with prewetting is a mixture of the two cases discussed above. Here, a single smectic monolayer is wetting the surface. Below this first layer, similar to the continuous case an oscillating decaying profile is observed.94
However, these different varieties of interface induced smectic order phenomena can be described within microscopic wetting theory103,104 based on smectic order parameter profiles. There, the film growth of undersaturated phases upon approaching two phase coexistence is studied, and phenomena like layering transitions or prewetting can occur. Depending on the intermolecular interaction potentials, phase transition entropies, and temperature one or the other of structures discussed above is realized.
The first category are liquid crystals that exhibit continuous or quasi-continuous bulk phase transitions, e.g., the commonly used 8-CB and 8-OCB with nematic-smectic transition.100 These transitions are characterized by a vanishing or negligible transition entropy ΔS.87 Most of the previously studied LC systems fall in this category. In these systems, wetting theory predicts pinning of the smectic fluctuations at the surface of the nematic bulk. For continuous phase transitions, Ginzburg–Landau theory predicts that the correlation length ξs of the smectic fluctuations vs. reduced temperature τ scale with a power law
ξs(τ) = ξ0τ−ν | (11) |
By contrast, at the isotropic-smectic transition of [C22C1im]+[NTf2]− we find ΔSLC = 2.5 mol−1 K−1, which signals a first-order bulk phase transition. From the observed logarithmic growth law for the layer thickness L(τ) one infers complete wetting and effectively short-ranged interactions. Hence, in [C22C1im]+[NTf2]− the surface-induced smectic layer thickness L reaches values that are substantially larger than its bulk correlation length ξb.
Ultimately, the fundamental parameter connecting the system temperature with the surface structure is the pre-factor Λ in the growth law given by eqn (9c). According to theory, this parameter is linked to the bulk correlation length ξb in the isotropic phase.83,103,104 Indeed, we find a very good agreement of Λ/d = 1.2 with the bulk value ξb/d = 1.3 determined by SAXS.67
A similar behavior is expected for other ILs, exhibiting smectic mesophases. However, in equilibrium surface induced smectic order will only be found for systems where T1 > Tm. For example, by extrapolation a transition temperature TLC = −160 °C is predicted for the IL [C14C1im]+[NTf2]− with shorter C14 cation side chains.34 Assuming τ1 < 0.3, surface induced smectic order is only expected to be found below −130 °C. This temperature is about 170 Kelvin below its bulk melting point at 41 °C.
However, for smaller anions such as [BF4] and [PF6] ILC transition temperatures TLC strongly increase. For example, in [C14C1im]+[PF6]− a value of TLC − Tm = 5 K is found.34 Thus, surface induced smectic order might be also relevant for ILs composed of commonly used cations.
Fig. 8 shows a schematic representation of the surface structures at 68 °C and 115 °C. Notably, close to the melting point we find smectic layer thicknesses up to 30 nm. This corresponds to an ordered surface structure, extending 8 times the smectic periodicity d below the liquid surface. Therefore, unlike for other ILs such as [C18C1im]+[FAP]−, the oscillatory near surface structure persists far beyond the bulk correlation length of ξb/d = 1.3.
Fig. 8 Electron density profiles and schematic representation of the near surface structure at (a) 68 °C and (b) 115 °C. The size of anions (blue), imidazolium rings (red) and aliphatic side chains (yellow) are not to scale. Profiles are identical to Fig. 5b. The in-plane order (not shown) remains liquid-like for all temperatures. |
The ordered surface layer thickness L vs. temperature is quantitatively described by a logarithmic growth law. This dependency follows the prediction from a generic thermodynamic model for interface induced phase transitions. In agreement with theory, the pre-factor Λ in the logarithmic growth law agrees with the bulk correlation length ξb of the isotropic liquid phase. Upon cooling, the smectic layer thickness L/d in [C22C1im]+[NTf2]− diverges at a temperature T0. Interestingly, T0 agrees well with the transition temperature TLC of a metastable liquid crystalline smectic SmA2-mesophase formed from the supercooled isotropic liquid. This indicates that the observed layered surface structure in [C22C1im]+[NTf2]− is governed by surface induced smectic order, i.e. fluctuations into the metastable SmA2-phase. On the other hand, we find an upper transition temperature of T1 = 157 °C for the ordered surface layer. Note that this behavior is very different from the power law scaling found for LCs with quasi-continuous nematic-smectic phase transitions.97,100 In contrast to other soft matter systems such as liquid crystals or alkanes, surface induced smectic ordering in the ionic liquid [C22C1im]+[NTf2]− is relevant over a much larger temperatures range.
In bulk, smectic mesophases were found for a wide class of neat and mixed ILs.19 Based on our results, we therefore presume that similar surface structures are also formed in other ILs. From our previous bulk67 and surface studies,55 we anticipate that the tendency for surface induced smectic order is larger for smaller anions and longer aliphatic side chains. This tendency is supported by clear trends observed in a new volume based approach to predict the thermodynamical behavior of ILs and ILCs.34
The well ordered surface structures observed for [C22C1im]+[NTf2]− over a surprisingly large temperature range might significantly reduce the diffusion of guest molecules across an IL/gas interface. Indeed, for [C16C1im][NO3] MD simulations show 5-fold smaller diffusion constants perpendicular to the layers of the smectic phase.106 Formation of such diffusion barriers for reactants and products, might adversely affect the performance of ILs in SILP processes.8 Further experiments on different ILs will be required to shed light on the complex interplay between the molecular structure of the anions and cations and the tendency for surface induced smectic order.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7cp04852a |
‡ Present address: Department of Chemistry and Physical Science, School of Natural and Social Sciences, Mount Vernon Nazarene University, 800 Martinsburg Road, Mount Vernon, Ohio 43050, USA. |
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