Complementary Molecular Dynamics and X-ray Reflectivity Study of an Imidazolium-Based Ionic Liquid at a Neutral Sapphire Interface

Ordering of ionic liquids at a charged sapphire interface: Evolution with cationic chain length

HYPOTHESIS

Room Temperature Ionic Liquids (RTILs) bulk's molecular layering dominates their structure also at the RTIL/sapphire interface, increasing the layer spacing with the cationic alkyl chain length n. However, the negatively-charged sapphire surface compresses the layers, increases the layering range, and affects the intra-layer structure in yet unknown ways.

EXPERIMENTS

X-ray reflectivity (XR) off the RTIL/sapphire interface, for a broad homologous RTIL series 1-alkyl-3-methylimidazolium bis(trifluoromethansulfonyl)imide, hitherto unavailable for any RTIL.

FINDINGS

RTIL layers against the sapphire, exhibit two spacings: da and db. da is n-varying, follows the behavior of the bulk spacing but exhibits a downshift, thus showing significant layer compression, and over twofold polar slab thinning. The latter suggests exclusion of anions from the interfacial region due to the negative sapphire charging by x-ray-released electrons. The layering range is larger than the bulk's. db is short and near n-independent, suggesting polar moieties' layering, the coexistence mode of which with the da-spaced layering is unclear. Comparing the present layering with the bulk's and the RTIL/air interface's provides insight into the Coulomb and dispersion interaction balance dominating the RTIL's structure and the impact thereon of the presence of a charged solid interface.

Molecular Interactions between Ionic Liquid Lubricants and Silica Surfaces: An MD Simulation Study

The unique physicochemical properties of ionic liquids (ILs) attracted interest in their application as lubricants of micro/nano-electromechanical systems. This work evaluates the feasibility of using the protic ionic liquids [4-picH][HSO4], [4-picH][CH3SO3], [MIMH][HSO4], and [MIMH][CH3SO3] and the aprotic ILs [C6mim][HSO4] and [C6mim][CH3SO3] as additives to model lubricant poly(ethylene glycol) (PEG200) to lubricate silicon surfaces. Additives based on the cation [4-picH]+ exhibited the best tribological performance, with the optimal value for 2% [4-picH][HSO4] in PEG200 (w/w). Molecular dynamics (MD) simulations of the first stages of adsorption of the ILs at the glass surface were performed to portray the molecular behavior of the ILs added to PEG200 and their interaction with the silica substrate. For the pure ILs at the solid substrates, the MD results indicated that weak specific interactions of the cation with the glass interface are lost to accommodate the larger anion in the first contact layer. For the PEG200 + 2% [4-picH][HSO4] system, the formation of a more compact protective film adsorbed at the glass surface is revealed by a larger trans population of the dihedral angle -O(R)-C-C-O(R)- in PEG200, in comparison to the same distribution for the pure model lubricant. Our findings suggest that the enhanced lubrication performance of PEG200 with [4-picH][HSO4] arises from synergistic interactions between the protic IL and PEG200 at the adsorbed layer.

Anisotropic molecular diffusion in confinement II: A model for structurally complex particles applied to transport in thin ionic liquid films

HYPOTHESIS

Diffusion in confinement is an important fundamental problem with significant implications for applications of supported liquid phases. However, resolving the spatially dependent diffusion coefficient, parallel and perpendicular to interfaces, has been a standing issue and for objects of nanometric size, which structurally fluctuate on a similar time scale as they diffuse, no methodology has been established so far. We hypothesise that the complex, coupled dynamics can be captured and analysed by using a model built on the 2-dimensional Smoluchowski equation and systematic coarse-graining.

METHODS AND SIMULATIONS

For large, flexible species, a universal approach is offered that does not make any assumptions about the separation of time scales between translation and other degrees of freedom. The method is validated on Molecular Dynamics simulations of bulk systems of a family of ionic liquids with increasing cation sizes where internal degrees of freedom have little to major effects.

FINDINGS

After validation on bulk liquids, where we provide an interpretation of two diffusion constants for each species found experimentally, we clearly demonstrate the anisotropic nature of diffusion coefficients at interfaces. Spatial variations in the diffusivities relate to interface-induced structuring of the ionic liquids. Notably, the length scales in strongly confined ionic liquids vary consistently but differently at the solid-liquid and liquid-vapour interfaces.

Anisotropic molecular diffusion in confinement I: Transport of small particles in potential and density gradients

HYPOTHESIS

Diffusion in confinement is an important fundamental problem with significant implications for applications of supported liquid phases. However, resolving the spatially dependent diffusion coefficient, parallel and perpendicular to interfaces, has been a standing issue. In the vicinity of interfaces, density fluctuations as a consequence of layering locally impose statistical drift, which impedes the analysis of spatially dependent diffusion coefficients even further. We hypothesise, that we can derive a model to spatially resolve interface-perpendicular diffusion coefficients based on local lifetime statistics with an extension to explicitly account for the effect of local drift using the Smoluchowski equation, that allows us to resolve anisotropic and spatially dependent diffusivity landscapes at interfaces.

METHODS AND SIMULATIONS

An analytic relation between local crossing times in system slices and diffusivity as well as an explicit term for calculating drift-induced systematic errors is presented. The method is validated on Molecular Dynamics simulations of bulk water and applied to simulations of water in slit pores.

FINDINGS

After validation on bulk liquids, we clearly demonstrate the anisotropic nature of diffusion coefficients at interfaces. Significant spatial variations in the diffusivities correlate with interface-induced structuring but cannot be solely attributed to the drift induced by local density fluctuations.

Vacuum Interfacial Structure and X-ray Reflectivity of Imidazolium-Based Ionic Liquids with Perfluorinated Anions from a Theory and Simulations Perspective

We report studies of the vacuum interfacial structure of a series of 1-methyl-3-alkylimidazolium bis(perfluoroalkanesulfonyl)imide ionic liquids (ILs) and predict and explain their Fresnel-normalized X-ray reflectivity. To better interpret the results, we use a theory we recently developed dubbed "the peaks and antipeaks analysis of reflectivity" which splits the overall signal into that of different pair subcomponents. Whereas the overall reflectivity signal is not very informative, the peak and trough intensities for the pair subcomponents provide rich information for analysis. When species containing cationic alkyl or anionic fluoroalkyl tails are present at the interface, a tail layer is found next to a vacuum, and this tail layer can be composed of both alkyl and fluoroalkyl moieties. To maintain the positive-negative alternation of charged groups, alkyl and fluoroalkyl tails must necessarily be nearby and cannot segregate. Charged groups are found in the subsequent layer just below the interface and arranged to achieve lateral charge neutrality. In general, fluctuations at and away from the interface are based on polarity (i.e., heads and tails) and not on charge; when there are no significant alkyl or fluoroalkyl moieties in the IL, atomic density fluctuations away from the interface are small and appear to exist for the purpose of achieving lateral charge balance. For all the systems reported here, the persistence length of density fluctuations does not go beyond ∼7 nm.

From Water Solutions to Ionic Liquids with Solid State Nanopores as a Perspective to Study Transport and Translocation Phenomena

Solid state nanopores are single-molecular devices governed by nanoscale physics with a broad potential for technological applications. However, the control of translocation speed in these systems is still limited. Ionic liquids are molten salts which are commonly used as alternate solvents enabling the regulation of the chemical and physical interactions on solid-liquid interfaces. While their combination can be challenging to the understanding of nanoscopic processes, there has been limited attempts on bringing these two together. While summarizing the state of the art and open questions in these fields, several major advances are presented with a perspective on the next steps in the investigations of ionic-liquid filled nanopores, both from a theoretical and experimental standpoint. By analogy to aqueous solutions, it is argued that ionic liquids and nanopores can be combined to provide new nanofluidic functionalities, as well as to help resolve some of the pertinent problems in understanding transport phenomena in confined ionic liquids and providing better control of the speed of translocating analytes.

Interface between Water-Solvent Mixtures and a Hydrophobic Surface

The mechanism behind the stability of organic nanoparticles prepared by liquid antisolvent (LAS) precipitation without a specific stabilizing agent is poorly understood. In this work, we propose that the organic solvent used in the LAS process rapidly forms a molecular stabilizing layer at the interface of the nanoparticles with the aqueous dispersion medium. To confirm this hypothesis, n-octadecyltrichlorosilane (OTS)-functionalized silicon wafers in contact with water-solvent mixtures were used as a flat model system mimicking the solid-liquid interface of the organic nanoparticles. We studied the equilibrium structure of the interface by X-ray reflectometry (XRR) for water-solvent mixtures (methanol, ethanol, 1-propanol, 2-propanol, acetone, and tetrahydrofuran). The formation of an organic solvent-rich layer at the solid-liquid interface was observed. The layer thickness increases with the organic solvent concentration and correlates with the polar and hydrogen bond fraction of Hansen solubility parameters. We developed a self-consistent adsorption model via complementing adsorption isotherms obtained from XRR data with molecular dynamics simulations.

Recent Progresses in Nanometer Scale Analysis of Buried Layers and Interfaces in Thin Films by X-rays and Neutrons

In the early 1960s, scientists achieved the breakthroughs in the fields of solid surfaces and artificial layered structures. The advancement of surface science has been supported by the advent of ultra-high vacuum technologies, newly discovered and established scanning probe microscopy with atomic resolution, as well as some other advanced surface-sensitive spectroscopy and microscopy. On the other hand, it has been well recognized that a number of functions are related to the structures of the interfaces, which are the thin planes connecting different materials, most likely by layering thin films. Despite the scientific significance, so far, research on such buried layers and interfaces has been limited, because the probing depth of almost all existing sophisticated analytical methods is limited to the top surface. The present article describes the recent progress in the nanometer scale analysis of buried layers and interfaces, particularly by using X-rays and neutrons. The methods are essentially promising to non-destructively probe such buried structures in thin films. The latest scientific research has been reviewed, and includes applications to bio-chemical, organic, electronic, magnetic, spintronic, self-organizing and complicated systems as well as buried liquid-liquid and solid-liquid interfaces. Some emerging analytical techniques and instruments, which provide new attractive features such as imaging and real time analysis, are also discussed.

Molecular Dynamics Simulation of the Ionic Liquid 1-n-Butyl-3-Methylimidazolium Methylsulfate [Bmim][MeSO4 ]: Interfacial Properties at the Silica and Vacuum Interfaces

Molecular dynamics simulations are done to investigate the structure and dynamics of a thin [Bmim][MeO₄ ] film in contact with a hydroxylated silica surface on one side and with vacuum on the other. An examination of the microscopic structure of ionic liquid (IL) film shows that strong layered anionic/cationic structures are formed at both interfaces. At the silica interface, the imidazolium rings are closer to the silica surface (compared to anions) and are coplanar with it. At the vacuum interface, the charged imidazolium ring more concentrates in the interior of the film, but the butyl side chain stretches out toward the vacuum interface. While there exists an excess concentration of the cations at the silica interface, at the vacuum interface an excess concentration of anions (dissolved in the butyl chain) is found. The influence of the interface on the dynamical properties is shown to depend on their time scales. A short-time dynamical property, such as hydrogen bond formation is not noticeably perturbed at the interface. In contrary, long-time properties such as ion-pair formation/rupture and translation of ions across the film are largely decelerated at the silica interface but are accelerate at the vacuum interface. Our findings indicate that the structural relaxation time of ion-pairs, is comparable to diffusion time scale in the IL film. Therefore, ion-pairs are not stable species; the IL is composed of short-lived ion-pairs and freely diffusing ions. However, the structural relaxation times of ion-pairs is still long enough (comparable to the time scale of diffusion) to conclude that correlated motions of counterions influence the macroscopic properties of IL, such as diffusion and ionic conductivity. In this respect, we have shown that correcting the Nernst-Einstein equation for the joint translation of ion-pairs considerably improves the accuracy of calculated ionic conductivities.

Structure and Dynamics of Confined Liquids: Challenges and Perspectives for the X-ray Surface Forces Apparatus

The molecular-scale structure and dynamics of confined liquids has increasingly gained relevance for applications in nanotechnology. Thus, a detailed knowledge of the structure of confined liquids on molecular length scales is of great interest for fundamental and applied sciences. To study confined structures under dynamic conditions, we constructed an in situ X-ray surface forces apparatus (X-SFA). This novel device can create a precisely controlled slit-pore confinement down to dimensions on the 10 nm scale by using a cylinder-on-flat geometry for the first time. Complementary structural information can be obtained by simultaneous force measurements and X-ray scattering experiments. The in-plane structure of liquids parallel to the slit pore and density profiles perpendicular to the confining interfaces are studied by X-ray scattering and reflectivity. The normal load between the opposing interfaces can be modulated to study the structural dynamics of confined liquids. The confinement gap distance is tracked simultaneously with nanometer precision by analyzing optical interference fringes of equal chromatic order. Relaxation processes can be studied by driving the system out of equilibrium by shear stress or compression/decompression cycles of the slit pore. The capability of the new device is demonstrated on the liquid crystal 4'-octyl-4-cyano-biphenyl (8CB) in its smectic A (SmA) mesophase. Its molecular-scale structure and orientation confined in 100 nm to 1.7 μm slit pores was studied under static and dynamic nonequilibrium conditions.

Ionic liquid/ZnO(0001[combining macron]) single crystal and epitaxial film interfaces studied through a combination of electrochemical measurements and a pulsed laser deposition process under vacuum

O-Polar ZnO(0001[combining macron]) single crystals and ZnO and Mg-doped ZnO (MgZnO) films which were subsequently deposited on the ZnO crystals by a pulsed laser deposition (PLD) method were electrochemically investigated through the interfaces with ionic liquid (IL) in a vacuum. The sample surfaces were confirmed to be atomically clean and flat by reflection high energy electron diffraction (RHEED) observation, prior to their electrochemical measurements. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were then performed, and the donor density, flat band potential of these ZnO samples, and the electric double layer capacitance at the IL/ZnO interfaces were successfully evaluated. The flat band potentials of ZnO and MgZnO films were found to shift to more negative potentials relative to those of the single crystal ZnO, with different values for thicker films, respectively. Some possible origins of the different flat band potentials between ZnO and MgZnO films, and their film thickness dependence of the flat band potential will be discussed in this paper.

Reaction mechanisms at the homogeneous–heterogeneous frontier: insights from first-principles studies on ligand-decorated metal nanoparticles

The frontiers between homogeneous and heterogeneous catalysis are progressively disappearing.

Ionic Conductivity in Ionic Liquid Nano Thin Films

Thin film approaches are powerful methods for gaining a nanoscale understanding of interfacial ionic liquids (ILs) in the vicinity of solids. These approaches are used to directly elucidate the interfacial contributions to the physical properties of ILs as nanoscale thin films have significant proportions of the surface or interface region with respect to their total volume. Here, we report the growth of a uniform [emim][TFSA] thin film ionic liquid on a chemically modified, well-wettable sapphire, thereby allowing the in situ measurement of its ionic conductivity on the nanoscale. We observed the thickness-dependent behavior of the ionic conductivity, which gradually decreased especially when the thickness was less than 10 nm, and found it to be quantitatively analyzed well by using an empirical two-layer model. The molecular dynamics (MD) simulations show that the thickness-dependent ionic conductivity originates from the solid-like structuring of the IL near the substrate, reproducing a thickness-dependent ionic conductivity. The MD simulation results suggest that the thickness of the low conductivity region determined in the two-layer model should roughly correspond to the thickness of the solid-like structuring of the IL near the substrate.

Evidence of Tailoring the Interfacial Chemical Composition in Normal Structure Hybrid Organohalide Perovskites by a Self-Assembled Monolayer

Current-voltage hysteresis is a major issue for normal architecture organo-halide perovskite solar cells. In this manuscript we reveal a several-angstrom thick methylammonium iodide-rich interface between the perovskite and the metal oxide. Surface functionalization via self-assembled monolayers allowed us to control the composition of the interface monolayer from Pb poor to Pb rich, which, in parallel, suppresses hysteresis in perovskite solar cells. The bulk of the perovskite films is not affected by the interface engineering and remains highly crystalline in the surface-normal direction over the whole film thickness. The subnanometer structural modifications of the buried interface were revealed by X-ray reflectivity, which is most sensitive to monitor changes in the mass density of only several-angstrom thin interfacial layers as a function of substrate functionalization. From Kelvin probe force microscopy study on a solar cell cross section, we further demonstrate local variations of the potential on different electron-transporting layers within a solar cell. On the basis of these findings, we present a unifying model explaining hysteresis in perovskite solar cells, giving an insight into one crucial aspect of hysteresis for the first time and paving way for new strategies in the field of perovskite-based opto-electronic devices.

Surface induced smectic order in ionic liquids - an X-ray reflectivity study of [C22C1im]+[NTf2]. Phys Chem Chem Phys 2018;19:26651-26661. [PMID: 28960006 DOI: 10.1039/c7cp04852a]

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) = Λ ln(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.

Molecular scale structure and dynamics at an ionic liquid/electrode interface

The structural arrangement and dynamics of ions near the IL/electrode interface during charging and discharging was studied by a combination of time resolved X-ray reflectivity and impedance spectroscopy.

In Situ Probing of Ion Ordering at an Electrified Ionic Liquid/Au Interface

Charge transport at the interface of electrodes and ionic liquids is critical for the use of the latter as electrolytes. A room-temperature ionic liquid, 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide (EMMIM TFSI), is investigated in situ under applied bias voltage with a novel method using low-energy electron and photoemission electron microscopy. Changes in photoelectron yield as a function of bias applied to electrodes provide a direct measure of the dynamics of ion reconfiguration and electrostatic responses of the EMMIM TFSI. Long-range and correlated ionic reconfigurations that occur near the electrodes are found to be a function of temperature and thickness, which, in turn, relate to ionic mobility and different configurations for out-of-plane ordering near the electrode interfaces, with a critical transition in ion mobility for films thicker than three monolayers.

Interfacial Structures of Trihexyltetradecylphosphonium-bis(mandelato)borate Ionic Liquid Confined between Gold Electrodes

Atomistic molecular dynamics simulations have been performed to study microscopic the interfacial ionic structures, molecular arrangements, and orientational preferences of trihexyltetradecylphosphonium-bis(mandelato)borate ([P_6,6,6,14][BMB]) ionic liquid confined between neutral and charged gold electrodes. It was found that both [P_6,6,6,14] cations and [BMB] anions are coabsorbed onto neutral electrodes at different temperatures. The hexyl and tetradecyl chains in [P_6,6,6,14] cations lie preferentially flat on neutral electrodes. The oxalato and phenyl rings in [BMB] anions are characterized by alternative parallel-perpendicular orientations in the mixed innermost ionic layer adjacent to neutral electrodes. An increase in temperature has a marginal effect on the interfacial ionic structures and molecular orientations of [P_6,6,6,14][BMB] ionic species in a confined environment. Electrifying gold electrodes leads to peculiar changes in the interfacial ionic structures and molecular orientational arrangements of [P_6,6,6,14] cations and [BMB] anions in negatively and positively charged gold electrodes, respectively. As surface charge density increases (but lower than 20 μC/cm²), the layer thickness of the mixed innermost interfacial layer gradually increases due to a consecutive accumulation of [P_6,6,6,14] cations and [BMB] anions at negatively and positively charged electrodes, respectively, before the formation of distinct cationic and anionic innermost layers. Meanwhile, the molecular orientations of two oxalato rings in the same [BMB] anions change gradually from a parallel-perpendicular feature to being partially characterized by a tilted arrangement at an angle of 45° from the electrodes and finally to a dominant parallel coordination pattern along positively charged electrodes. Distinctive interfacial distribution patterns are also observed accordingly for phenyl rings that are directly connected to neighboring oxalato rings in [BMB] anions.

π+–π+ stacking of imidazolium cations enhances molecular layering of room temperature ionic liquids at their interfaces

The interfacial structure of room temperature ionic liquids (RTILs) is governed by the competing effects of the randomization due to the molecular polarizability and the ordered structure stabilized by π⁺–π⁺ interactions between the cationic molecules of RTILs.

Observing structural reorientations at solvent–nanoparticle interfaces by X-ray diffraction – putting water in the spotlight

Nanoparticles are attractive in a wide range of research genres due to their size-dependent properties, which can be in contrast to those of micrometre-sized colloids or bulk materials. This may be attributed, in part, to their large surface-to-volume ratio and quantum confinement effects. There is a growing awareness that stress and strain at the particle surface contribute to their behaviour and this has been included in the structural models of nanoparticles for some time. One significant oversight in this field, however, has been the fact that the particle surface affects its surroundings in an equally important manner. It should be emphasized here that the surface areas involved are huge and, therefore, a significant proportion of solvent molecules are affected. Experimental evidence of this is emerging, where suitable techniques to probe the structural correlations of liquids at nanoparticle surfaces have only recently been developed. The recent validation of solvation shells around nanoparticles has been a significant milestone in advancing this concept. Restructured ordering of solvent molecules at the surfaces of nanoparticles has an influence on the entire panoply of solvent–particle interactions during, for example, particle formation and growth, adhesion forces in industrial filtration, and activities of nanoparticle–enzyme complexes. This article gives an overview of the advances made in solvent–nanoparticle interface research in recent years: from description of the structure of bulk solids and liquidsviamacroscopic planar surfaces, to the detection of nanoscopic restructuring effects. Water–nanoparticle interfaces are given specific attention to illustrate and highlight their similarity to biological systems.

Crowding and Anomalous Capacitance at an Electrode-Ionic Liquid Interface Observed Using Operando X-ray Scattering

Room temperature ionic liquids are widely recognized as novel electrolytes with properties very different from those of aqueous solutions, and thus with many potential applications, but observing how they actually behave at electrolytic interfaces has proved to be challenging. We have studied the voltage-dependent structure of [TDTHP](+)[NTF2](-) near its interface with an electrode, using in situ synchrotron X-ray reflectivity. An anion-rich layer develops at the interface above a threshold voltage of +1.75 V, and the layer thickness increases rapidly with voltage, reaching ∼6 nm (much larger that the anion dimensions) at +2.64 V. These results provide direct confirmation of the theoretical prediction of "crowding" of ions near the interface. The interfacial layer is not purely anionic but a mixture of up to ∼80% anions and the rest cations. The static differential capacitance calculated from X-ray measurements shows an increase at higher voltages, consistent with a recent zero-frequency capacitance measurement but inconsistent with ac capacitance measurements.

Ordered ionic liquid structure observed at terraced graphite interfaces

Reflection high-energy electron diffraction is presented as a contactless, surface-specific method to probe the ion organization and layering at the ionic liquid–solid interfaces.

Structure and Dynamics of Ionic Liquids Confined in Amorphous Porous Chalcogenides

Besides the abundant literature on ionic liquids in porous silica and carbon, the confinement of such intriguing liquids in porous chalcogenides has received very little attention. Here, molecular simulation is employed to study the structural and dynamical properties of a typical ionic liquid confined in a realistic molecular model of amorphous chalcogenide with various pore sizes and surface chemistries. Using molecular dynamics in the isobaric-isothermal (NPT) ensemble, we consider confinement conditions relevant to real samples. Both the structure and self-dynamics of the confined phase are found to depend on the surface-to-volume ratio of the host confining material. Consequently, most properties of the confined ionic liquid can be written as a linear combination of surface and bulk-like contributions, arising from the ions in contact with the surface and the ions in the pore center, respectively. On the other hand, collective dynamical properties such as the ionic conductivity remain close to their bulk counterpart and almost insensitive to pore size and surface chemistry. These results, which are in fair agreement with available experimental data, provide a basis for the development of novel applications using hybrid organic-inorganic solids consisting of ionic liquids confined in porous chalcogenides.