Interfacial Nanostructure and Asymmetric Electrowetting of Ionic Liquids

Rewritable printing of ionic liquid nanofilm utilizing focused ion beam induced film wetting

Manipulating liquid flow over open solid substrate at nanoscale is important for printing, sensing, and energy devices. The predominant methods of liquid maneuvering usually involve complicated surface fabrications, while recent attempts employing external stimuli face difficulties in attaining nanoscale flow control. Here we report a largely unexplored ion beam induced film wetting (IBFW) technology for open surface nanofluidics. Local electrostatic forces, which are generated by the unique charging effect of Helium focused ion beam (HFIB), induce precursor film of ionic liquid and the disjoining pressure propels and stabilizes the nanofilm with desired patterns. The IBFW technique eliminates the complicated surface fabrication procedures to achieve nanoscale flow in a controllable and rewritable manner. By combining with electrochemical deposition, various solid materials with desired patterns can be produced.

Manipulation of Contact Angle Hysteresis at Electrified Ionic Liquid-Solid Interfaces

Room-temperature ionic liquids (RTILs) are intriguing fluids that have drawn much attention in applications ranging from tribology and catalysis to energy storage. With strong electrostatic interaction between ions, their interfacial behaviors can be modulated by controlling energetics of the electrified interface. In this work, we report atomic-force-microscope measurements of contact angle hysteresis (CAH) of a circular contact line formed on a micron-sized fiber, which is coated with a thin layer of conductive film and intersects an RTIL-air interface. The measured CAH shows a distinct change by increasing the voltage U applied on the fiber surface. Molecular dynamics simulations were performed to illustrate variations of the solidlike layer in the RTIL adsorbed at the electrified interface. The integrated experiments and computations demonstrate a new mechanism to manipulate the CAH by rearrangement of interfacial layers of RTILs induced by the surface energetics.

Effect of Surface Charge Characteristics of Ferroelectric LiNbO3 on Wettability of Ionic Liquids

Electrowetting is a widely used and effective method to tune the wettability of ionic liquids at solid-liquid interfaces, but it usually requires an external electric field. Here, we proposed a strategy for conveniently tuning ionic liquid wettability by adopting ferroelectric LiNbO₃ single crystals as functional substrates. A heating pretreatment process was applied to modulate the surface charge characteristics of LiNbO₃ substrates, leading to an improved wettability of [EMIM][BF₄] and [EMIM][NTf₂] on the LiNbO₃ substrates with both positively poled (+Z) and negatively poled (−Z) surfaces. This work may be of great interest in the field of ferroelectric-based microelectronics.

Initiating a high-temperature zinc ion battery through a triazolium-based ionic liquid

Triazolium-based ionic liquids (T1, T2 and T3) with or without terminal hydroxyl groups were prepared via Cu(i) catalysed azide–alkyne click chemistry and their properties were investigated using various technologies. The hydroxyl groups obviously affected their physicochemical properties, where with a decrease in the number of hydroxyl groups, their stability and conductivity were enhanced. T1, T2 and T3 showed relatively high thermal stability, and their electrochemical stability windows (ESWs) were 4.76, 4.11 and 3.52 V, respectively. T1S-20 was obtained via the addition of zinc trifluoromethanesulfonic acid (Zn(CF₃SO₃)₂) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to T1, displaying conductivity and ESW values of 1.55 × 10⁻³ S cm⁻¹ and 6.36 V at 30 °C, respectively. Subsequently, a Zn/Li₃V₂(PO₄)₃ battery was assembled using T1S-20 as the electrolyte and its performances at 30 °C and 80 °C were investigated. The battery showed a capacity of 81 mA h g⁻¹ at 30 °C, and its capacity retention rate was 89% after 50 cycles. After increasing the temperature to 80 °C, its initial capacity increased to 111 mA h g⁻¹ with a capacity retention rate of 93.6% after 100 cycles, which was much higher than that of the aqueous electrolyte (WS-20)-based zinc ion battery (71.8%). Simultaneously, the T1S-20 electrolyte-based battery exhibited a good charge/discharge efficiency, and its Coulomb efficiency was 99%. Consequently, the T1S-20 electrolyte displayed a better performance in the Zn/Li₃V₂(PO₄)₃ battery than that with the aqueous electrolyte, especially at high temperature.

ZIB with T1S-20 electrolyte displays good charge/discharge performances and dendrite-free structure at high temperature, which is better than that with aqueous electrolyte (WS-20).

Permeation by Electrowetting Actuation: Revealing the Prospect of a Micro-valve Based on Ionic Liquid

The electrowetting behavior of ionic liquid significantly promotes microfluidic technology due to the advantage of manipulation of ionic liquid without additional mechanical parts. Recently, a novel micro-valve that shows good prospects was proposed by MacArthur et al. based on the permeation of ionic liquid under electric field. Inspired by their work, the permeation process of ionic liquid (EMIM-Im) droplets actuated by electrowetting was investigated in this work using molecular dynamics simulation. The wettability of substrate, electric field strength and electric field polarity were varied to investigate their influences. On the substrate side, results showed that the hydrophilic substrates tend to stretch and adsorb the droplet and hence hinder the permeation process, whereas the hydrophobic substrates facilitate permeation due to their low attraction for liquid. Particularly, super hydrophilic substrates should be avoided in practice, because their strong adsorption effects will override the electric field effects and disable the permeation process. On the electric field side, results showed that increased electric field strength enhances the permeation, but varying electric field polarity will result in an asymmetric permeation behavior, which was found to be the result of the different evaporation rate of the ion species that ultimately caused a non-charge-neutral droplet. Our investigation then uncovered the two critical roles of the electric field: elongating the droplet and providing the driving force for the permeation.

Molecular insights into the electrowetting behavior of aqueous ionic liquids

Molecular dynamics (MD) simulations were applied to investigate the wettability of aqueous hydrophilic and hydrophobic imidazolium-based ionic liquid (IL) nano-droplets on a graphite surface under a perpendicular electric field. Imminent transformation in the droplet configuration was observed at E = 0.08 V Å^-1 both for hydrophobic ILs 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][NTF₂] and SPC/E water droplets. However, for the hydrophilic IL, 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF₄], the droplet was entirely elongated to column-shaped at E = 0.09 V Å^-1 for lower weight percentages of ILs and at E = 0.15 V Å^-1 for a higher weight percentage of ILs (i.e., 50 wt%). We explored the impact of the electric field through various parameters such as mass and charge density distribution across the droplet, contact angle of the droplet, orientation of water dipoles, and hydrogen bond analysis. The external electric field was found to influence the orientation of water dipoles and the accumulation of charge at various interfaces was observed with an increase in an electric field, which finally leads to shape deformation and depletion of ions from the liquid-vapor interface of the droplet. However, this behavior strongly depends on the hydrophilicity or hydrophobicity of the ILs and thus, is critically examined for both the ILs.

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 wetting behavior of aqueous imidazolium based ionic liquids: a molecular dynamics study

Aqueous ionic liquids are of particular interest due to their tunability of physical and chemical properties and a deeper understanding of their structure-property relationship is desired. Molecular dynamics (MD) simulations were conducted to study the wetting behavior of aqueous imidazolium-based ionic liquids (ILs), consisting of a 1-ethyl-3-methylimidazolium [EMIM]+ cation and either a hydrophilic boron tetrafluoride [BF4]- or a hydrophobic bis(trifluoromethylsulfonyl)imide [NTF2]- anion mixed in water. To understand the effect of anion and concentration of ILs at the graphite solid-liquid interface, wettability studies were performed with IL concentrations from 0-50 wt%. The contact angle of aqueous IL droplets decreases with increasing IL concentration. Droplet characteristics near the surface were investigated by profiling the density perpendicular (z-direction) and horizontal (r-direction) to the graphite sheet; this was further quantified by an orientation order parameter. Due to the preferred adsorption of ILs, water depletes near the surface as IL concentration increases. The hydrophobic [NTF2]- anion forces the IL toward the interface from the bulk, whereas the hydrophilic [BF4]- anion causes the IL to remain in the bulk of the droplet. Differences in water-anion hydrogen bonding, the nature of the anions, and their interfacial tensions are crucial factors in the wetting behavior of aqueous ionic liquids.

Molecular Dynamics Simulation on the Electrowetting Behaviors of the Ionic Liquid [BMIM][BF4] on a Solid Substrate

Compared with traditional aqueous solutions, ionic liquids have important application prospects in the field of wetting and electrowetting due to the advantages of high electric conductivity, long liquid range, and low volatility. In this paper, molecular dynamics method was employed to investigate the wetting and electrowetting behaviors of the nanodroplet of ionic liquid on a solid substrate, as well as the distribution of ionic groups. The ionic liquid is 1-butyl-3-methyl tetra-fluoroborate and coarse grained to simplify the molecular simulation model. The results show that the anion and cation groups are distributed in layers above the wall, and the peaks are different corresponding to different ionic groups. Due to the attraction of the solid substrate and the electrostatic force between anions and cations, the contact angle tends to increase slightly with the increase of ionic liquid pairs. To investigate the electrowetting behaviors of ionic liquid droplet, several electric fields of different strengths and directions have been applied to the system, respectively. The results show that the static contact angles decrease obviously with the increase of electric field, and the ionic liquid droplet wets the solid surface asymmetrically under electric fields in positive and negative directions due to different diffusion abilities of cationic and anionic coarse particles. However, for a hydrophilic surface (ε = 2.0 kcal/mol), the ionic liquid droplet wets symmetrically under the electric field E = ±0.18 V/Å because of the strong interaction from the solid surface. Thus, the wetting and electrowetting behaviors are determined by the combine effect of electric field, interaction among cationic/anionic coarse particles, and solid substrates.

Electrowetting of Ionic Liquid on Graphite: Probing via in Situ Electrochemical X-ray Photoelectron Spectroscopy

Thin films of ionic liquid 1-ethyl-3-methylimidazolium bis(fluoromethylsulfonyl)imide ([EMIm][FSI]) vapor-deposited on highly oriented pyrographite (HOPG) were studied by X-ray photoelectron spectroscopy and atomic force microscopy. The results revealed a reversible morphological transition from a "drop-on-layer" structure to a "flat-layer" structure at positive, and not at negative, polarization. The effect is rationalized in terms of electric-field-induced reduction of the liquid-solid transition temperature in the ionic liquid film, when its thickness is comparable to the charge screening length. The observed bias asymmetry of [EMIm][FSI] electrowetting on HOPG is tentatively explained by the bilayer structure at the interface driven by the affinity of the imidazolium ring to the HOPG surface.

Asymmetrical Electrowetting on Dielectrics Induced by Charge Transfer through an Oil/Water Interface

Electrowetting on dielectrics is a fascinating as well as a precise way in microfluid manipulation. As one of the controversial conclusions, charge trapping on the dielectric surface might be one of the causes which induces water contact angle saturation and forms one of the significant issues that bear on the applications of electrowetting on dielectrics. Recently, it was demonstrated that the contact angle saturation can be significantly reduced by employing an oil lubrication layer on the hydrophobic surface. In this work, we have investigated the influence of effects of an oil layer on the electrowetting behavior by dissolving a nonpolar oil-soluble dye in the oil phase. We monitored the contact angle of water drops with varying pH on an oil-lubricated hydrophobic insulator. Interestingly, we found asymmetry in the electrowetting curve. Several analysis methods were proceeded trying to explain this asymmetric electrowetting phenomenon. First and foremost, the electrochemical properties of dye were investigated by cyclic voltammetry which demonstrates that oxidation-reduction reactions of the dye can indeed happen on the electrode and one irreversible peak was found which indicated that the dye molecule might decompose at a higher voltage. Second, thin-layer cyclic voltammetry confirmed ions can transgress the oil/water interface. Also, the conductivity of the oil phase increases with the dissolved dye concentration, which indicates that charges can be transported in the oil phase. Finally, to further understand the transfer mechanism, the transient current of dye-doped oil was measured, which indicates that the formation of inverse micelles in the oil phase at high voltage could be one of the charge carriers. We demonstrated the oil-property-dependent asymmetry phenomenon of electrowetting and its association with charge transfer through the oil/water interface for the first time.

Modification of the Electrolyte/Electrode Interface for the Template-free Electrochemical Synthesis of Metal Nanowires from Ionic Liquids

In electrochemistry, the electrode/electrolyte interface (EEI) governs the charge/mass-transfer processes and controls the nucleation/growth phenomena. The EEI in ionic liquids (ILs) can be controlled by changing the cation/anion of the IL, salt concentration, electrode potential, and temperature. Here, we show that adding a dopant salt leads to the deposition of nanowires. To illustrate, zinc nanowires were electrodeposited from ZnCl₂/1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate in the presence of GaCl₃ as a dopant salt. The choice of Zn salt and its ratio to GaCl₃ were found to be crucial for Zn nanowires formation. AFM studies revealed that the solvation structure of Au(111)/IL changes significantly in the presence of GaCl₃ and ZnCl₂. Chronoamperometry showed changes in the nucleation/growth process, consequently leading to the formation of nanowires. A similar approach was adopted to synthesize Sn nanowires. Thus, modification of the EEI by adding a dopant to ILs can be a viable method to obtain nanowires.

Humidity-accelerated spreading of ionic liquids on a mica surface

The role of relative humidity (RH) on the wetting behavior of droplets of two [Rmim][NTf₂] ionic liquids (ILs) on a mica surface was investigated and water vapor adsorption was found to enhance the ILs precursor film formation and droplet spreading.