Influence of the electrode potential and in situ STM scanning conditions on the phase boundary structure of the single crystal Bi(111)|1-butyl-4-methylpyridinium tetrafluoroborate interface

Molecular dynamics simulation studies on the ionic liquid N-butylpyridinium tetrafluoroborate on the gold surface

The study of solid/liquid interface is of great significance for understanding various phenomena such as the nanostructure of the interface, liquid wetting, crystal growth and nucleation. In this work, the nanostructure of the pyridinium ionic liquid [BPy]BF4 on different gold surfaces was studied by molecular dynamics simulation. The results indicate that the density of the ionic liquids near the gold surface is significantly higher than that in the bulk phase. Cation's tail (the alkyl chain) orients parallel to the surface under all studied conditions. Cation's head (the pyridine ring) orientation varies from parallel to perpendicular, which depends on the temperature and corrugation of the Au(hkl) surface. Interestingly, analysis of simulated mass and number densities revealed that surface corrugation randomizes the cations packing. On smooth Au(111) and Au(100) surfaces, parallel and perpendicular orientations are well distinguished for densely packed cations. While on corrugated Au(110), cations' packing density and order are decreased. Overall, this study explores the adsorption effect of the gold surface on ionic liquids, providing some valuable insights into their behavior on the solid/liquid interface.

Another Piece of the Ionic Liquid's Puzzle: Adsorption of Cl- Ions

Classical electrochemical and microscopy methods were used to characterize the interfacial processes of the adsorption of chloride ions from ionic liquids at the Bi(111) single crystal electrode. The mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium chloride was electrochemically characterized by using cyclic voltammetry and electrochemical impedance spectroscopy. In situ scanning tunneling microscopy images showed the formation of superstructures at the electrode's surface over an extended period of time. The specific adsorption of chloride ions reaches an equilibrium state in a more viscous ionic liquid medium slower than in aqueous and organic solvents. Capacitance values increase considerably (also depending on alternative current frequency) at the potential region, where the specific adsorption of chloride ions with partial charge transfer occurs.

The Influence of Ionic Liquids Adsorption on the Electronic and Optical Properties of Phosphorene and Arsenene with Different Phases: A Computational Study

Density functional theory (DFT) calculations have been performed to investigate the interfacial interactions of ionic liquids (ILs) on the α- and β-phases of phosphorene (P) and arsenene (As). Nine representative ILs based on the combinations of 1-ethyl-3-methylimidazolium ([EMIM]⁺), N-methylpyridinium ([MPI]⁺), and trimethylamine ([TMA]⁺) cations paired to tetrafluoroborate ([BF₄]⁻), trifluoromethanesulfonate ([TFO]⁻), and chloridion (Cl⁻) anions were used as adsorbates on the 2D P and As nanosheets with different phases to explore the effect of IL adsorption on the electronic and optical properties of 2D materials. The calculated structure, adsorption energy, and charge transfer suggest that the interaction between ILs and P and As nanosheets is dominated by noncovalent forces, and the most stable adsorption structures are characterized by the simultaneous interaction of the cation and anion with the surface, irrespective of the types of ILs and surfaces. Furthermore, the IL adsorption leads to the larger change in the electronic properties of β-phase P and As than those of their α-phase counterparts, which demonstrates that the adsorption properties are not only related to the chemical elements, but also closely related to the phase structures. The present results provide insight into the further applications of ILs and phosphorene (arsenene) hybrid materials.

Microstructural and Dynamical Heterogeneities in Ionic Liquids

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

The metal–ionic liquid interface as characterized by impedance spectroscopy and in situ scanning tunneling microscopy

Electrochemical measurements including impedance spectroscopy and in situ scanning tunneling microscopy were performed to study the interface between solid electrodes and ionic liquids. We could reveal that the double layer rearrangement processes are not instantaneous, but that the ions can form ordered clusters at the interface.

Electrochemical characteristics of ideal polarizable interfaces with limited number of charge carriers

Recent progress in material chemistry and surface engineering has led to emergence of new electrode materials with unique physical and electrochemical properties. Here, we introduce a physical model describing charging of ideal polarizable electrode-electrolyte interface where the electrode is characterized by a limited capacity to store charge. The analytical model treats the electrode and electrolyte phases as independent nonlinear capacitors that are eventually coupled through the condition of equality of the total stored electrical charge opposite in sign. Gouy-Chapman and condensed layer theories applied to a general 1:n valent electrolyte are used to predict dependencies of differential capacitance of the electrolyte phase and surface concentration of the electrical charge on the applied potential. The model of the nonlinear capacitor for the electrode phase is described by a theory of electron donors and acceptors present in conductive solids as a result of thermal fluctuations. Both the differential capacitance and the surface concentration of the electrical charge in the electrode are evaluated as functions of the applied potential and related to the capacity of the electrode phase to accumulate charge and its ability to form electron donors and acceptors. The knowledge of capacitive properties of both phases allows to predict electrochemical characteristics of ideal polarizable interfaces, e.g., current responses in linear sweep voltammetry. The coupled model also shows significant potential drops in the electrode comparable to those in the electrolyte phase for materials with low charge carrier concentrations.