Gold nanoparticle (Au NP) mirrors, which exhibit both high reflectance and electrical conductance, were self-assembled at a [heptane + 1,2-dichloroethane]/water liquid/liquid interface. The highest reflectance, as observed experimentally and confirmed by finite difference time domain calculations, occurred for Au NP films consisting of 60 nm diameter NPs and approximate monolayer surface coverage. Scanning electrochemical microscopy approach curves over the interfacial metallic NP films revealed a transition from an insulating to a conducting electrical material on reaching a surface coverage at least equivalent to the formation of a single monolayer. Reflectance and conductance transitions were interpreted as critical junctures corresponding to a surface coverage that exceeded the percolation threshold of the Au NP films at the [heptane + 1,2-dichloroethane]/water interface.
We describe a method of observing collisions of single femtoliter (fL) oil (i.e., toluene) droplets that are dispersed in water on an ultramicroelectrode (UME) to probe the ion transfer across the oil/water interface. The oil-in-water emulsion was stabilized by an ionic liquid, in which the oil droplet trapped a highly hydrophobic redox probe, rubrene. The ionic liquid also functions as the supporting electrolyte in toluene. When the potential of the UME was biased such that rubrene oxidation would be possible when a droplet collided with the electrode, no current spikes were observed. This implies that the rubrene radical cation is not hydrophilic enough to transfer into the aqueous phase. We show that current spikes are observed when tetrabutylammonium trifluoromethanesulfonate or tetrahexylammonium hexafluorophosphate are introduced into the toluene phase and when tetrabutylammonium perchlorate is introduced into the water phase, implying that the ion transfer facilitates electron transfer in the droplet collisions. The current (i)-time (t) behavior was evaluated quantitatively, which indicated the ion transfer is fast and reversible. Furthermore, the size of these emulsion droplets can also be calculated from the electrochemical collision. We further investigated the potential dependence on the electrochemical collision response in the presence of tetrabutylammonium trifluoromethanesulfonate in toluene to obtain the formal ion transfer potential of tetrabutylammonium across the toluene/water interface, which was determined to be 0.754 V in the inner potential scale. The results yield new physical insights into the charge balance mechanism in emulsion droplet collisions and indicate that the electrochemical collision technique can be used to probe formal ion transfer potentials between water and solvents with very low (ε < 5) dielectric constants.
We report the collisions of single emulsion oil droplets with extremely low dielectric constants (e.g., benzene, ε of 2.27, or cyclohexane, ε of 2.02) as studied via emulsion droplet reactor (EDR) on an ultramicroelectrode (UME). By applying appropriate potentials to the UME, we observed the electrochemical effects of single-collision signals from the bulk electrolysis of single emulsion droplets. Different hydrophobic redox species (ferrocene, decamethyl-ferrocene, or metalloporphyrin) were trapped in a mixed benzene (or cyclohexane) oil-in-water emulsion using an ionic liquid as the supporting electrolyte and emulsifier. The emulsions were prepared using ultrasonic processing. Spike-like responses were observed in each i-t response due to the complete electrolysis of all of the above-mentioned redox species within the droplet. On the basis of these single-particle collision results, the collision frequency, size distribution, i-t decay behavior of the emulsion droplets, and possible mechanisms are analyzed and discussed. This work demonstrated that bulk electrolysis can be achieved in a few seconds in these attoliter reactors, suggesting many applications, such as analysis and electrosynthesis in low dielectric constant solvents, which have a much broader potential window.
Chronoamperometry was used to study the dynamics of Pt nanoparticle (NP) collision with an inert ultramicroelectrode via electrocatalytic amplification (ECA) in the hydrogen evolution reaction. ECA and dynamic light scattering (DLS) results reveal that the NP colloid remains stable only at low proton concentrations (1.0 mm) under a helium (He) atmosphere, ensuring that the collision events occur at genuinely single NP level. Amperometry of single NP collisions under a He atmosphere shows that each discrete current profile of the collision event evolves from spike to staircase at more negative potentials, while a staircase response is observed at all of the applied potentials under hydrogen-containing atmospheres. The particle size distribution estimated from the diffusion-controlled current in He agrees well with electron microscopy and DLS observations. These results shed light on the interfacial dynamics of the single nanoparticle collision electrochemistry.
Emulsification of immiscible two-phase fluids, i.e., one condensed phase dispersed homogeneously as tiny droplets in an outer continuous medium, plays a key role in medicine, food, chemical separations, cosmetics, fabrication of micro- and nanoparticles and capsules, and dynamic optics. Herein, we demonstrate that water clusters/droplets can be formed in an organic phase via the spontaneous assembling of ionic bilayers. We term these clusters ionosomes, by analogy with liposomes where water clusters are encapsulated in a bilayer of lipid molecules. The driving force for the generation of ionosomes is a unique asymmetrical electrostatic attraction at the water/oil interface: small and more mobile hydrated ions reside in the inner aqueous side, which correlate tightly with the lipophilic bulky counterions in the adjacent outer oil side. These ionosomes can be formed through electrochemical (using an external power source) or chemical (by salt distribution) polarization at the liquid–liquid interface. The charge density of the cations, the organic solvent, and the synergistic effects between tetraethylammonium and lithium cations, all affecting the formation of ionosomes, were investigated. These results clearly prove that a new emulsification strategy is developed providing an alternative and generic platform, besides the canonical emulsification procedure with either ionic or nonionic surfactants as emulsifiers. Finally, we also demonstrate the detection of individual ionosomes via single-entity electrochemistry.
Single-emulsion toluene oil droplets (femtoliter) containing a hydrophobic redox probe that are dispersed in water stochastically collide with an ultramicroelectrode (UME). The fast-scan cyclic voltammetry (FSCV) or Fourier-transformed sinusoidal voltammetry (FTSV) is applied: the UME was scanned with a fast, repetitive triangular, or sinusoidal potential, and its current in time/frequency domains were monitored. The electron transfer at the UME/oil interface is coupled with ion transfer at the oil/water interface. Thus, the obtained transient voltammograms of a myriad of ions were used to estimate thermodynamics of ion transfer at the toluene/water interface. Additionally, the single-droplet voltammogram combined with finite element simulations reveal the droplet's size and shape distributions. Four collision mechanisms with new physical insights were also uncovered via comprehensive analysis of phase angle in the frequency domain, time domain FSCVs, and finite element simulations.
Graphene oxide (GO) in water was reduced heterogeneously by decamethylferrocene (DMFc) or ferrocene (Fc) in 1,2-dichloroethane (DCE), which could then act as a catalyst for an interfacial oxygen reduction reaction (ORR) and production of hydrogen peroxide (H 2 O 2 ). The reduced graphene oxide (RGO) produced at the liquid/liquid interface was characterized by using electron microscopy, spectroscopy (Raman, infrared, and electron energy loss), and electrochemical techniques.The oxygenated functional groups at the edge/defects of the RGO surface activate O 2 adsorption, forming superoxidelike adducts that can be protonated at the liquid/liquid interface and reduced by DMFc or Fc. This process is facilitated by the higher electrical conductivity of the RGO sheets. The key feature of this catalytic reaction is the in situ partial-reduction of GO at the liquid/liquid interface, forming an efficient and inexpensive catalyst for the production of H 2 O 2 .Electrochemistry at polarized interfaces between two immiscible electrolyte solutions (ITIES) has developed over the past 30 years, in which charge-transfer (electron-and ion-transfer) reactions have found applications in areas such as phase-transfer catalysis, solvent-extraction processes, chemical sensing, solar-energy-conversion systems, drug release and delivery, and in mimicking the function of biological membranes. [1] Liquid/liquid interfaces provide a unique platform at which to study ORRs, at which aqueous protons react with organic solubilized electron donors in the absence or presence of adsorbed catalysts, usually through a proton-coupled electron-transfer (PCET) reaction. [2] The molecular catalysts studied include cobalt, [3] free-base porphyrins, [4] and in situ-deposited platinum particles. [5] The ORR proceeds either by a 4 e À /4 H + pathway to produce water or a 2 e À /2 H + route to yield H 2 O 2 , which is considered a green oxidant.H 2 O 2 is widely used in many industrial areas, particularly in the chemical industry or for environmental protection, and is currently produced on an industrial scale through the biphasic anthrahydroquinone oxidation (AO) process (representing ca. 95 % of the world's H 2 O 2 production). [6] Generally, anthrahydroquinone is oxidized by O 2 to produce H 2 O 2 and anthraquinone and, subsequently, the formed anthraquinone is reduced back to the anthrahydroquinone by using H 2 in the presence of a metal catalyst. Both reactions occur in the organic phase, and H 2 O 2 is recovered by extraction to the aqueous phase. [6] The advantage of the AO process is the very high yield of H 2 O 2 generated per cycle. Conversely, side reactions generating organic byproducts need to be dealt with by regenerating the solution and by using separation techniques to eliminate such impurities. Conceptually, following the AO process, the reduction of O 2 was investigated at quinone-modified carbon surfaces. O 2 reduction to H 2 O 2 was mediated by surface-bound quinone groups via superoxide anion intermediates, [7] and such modified elec...
Initial oxygen concentration (ref. [2-4]) DMFc D 7.26 × 10 −6 cm•s −1 Diffusion coefficient of DMFc and DMFc + (ref. [5]) 2 O D 2.76 × 10 −5 cm•s −1 Diffusion coefficient of O2 (ref. [2]) ' OH w o o φ − ∆ ; however, to make the simulated CVs symmetric, and for the sake of convenience, −0.696 V was employed. 2 Component 1 (comp1) 2.1 Definitions 2.1.1 Variables Variables 3 Selection Geometric entity level Boundary Selection Boundary 2 Name Expression Description intcpl_source_Ibar pi*(rd^2)*F*(chds.ndflux_Maq-chds.ndflux_OHaq + chds.ndflux_Fcplusaq) 2.1.2 Component Couplings Integration 1 Coupling type Integration Operator name intop1 Source selection Geometric entity level Boundary Selection Boundary 2 2.2 Geometry 1 Geometry 1 Units Length unit m Angular unit deg Geometry statistics Property Value Space dimension 1 Number of domains 2 Number of boundaries 3 2.2.1 Interval 1 (i1) Selections of resulting entities Name Value Number of intervals One Left endpoint-0.01 Right endpoint 0 2.2.2 Interval 2 (i2) Selections of resulting entities Name Value Number of intervals One Left endpoint 0 Right endpoint 0.01 2.3 Transport of Diluted Species (chds) Transport of Diluted Species Selection Geometric entity level Domain Selection Domain 1 Equations Settings Description Value Concentration Linear Compute boundary fluxes On Apply smoothing to boundary fluxes On Value type when using splitting of complex variables Real Migration in electric field 0 2.5.1 Size (size) Settings Name Value Maximum element size 5e-6 Minimum element size 6.0E-6 Curvature factor 0.3 Maximum element growth rate 1.3 Custom element size Custom 2.5.2 Edge 1 (edg1) Selection Geometric entity level Remaining
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