Interfacial pH Research Articles

Switchable Biocatalytic Reactions Controlled by Interfacial pH Changes Produced by Orthogonal Biocatalytic Processes.

Enzymes immobilized on a nano-structured surface were used to switch the activity of one enzyme by a local pH change produced by another enzyme. Immobilized amyloglucosidase (AMG) and trypsin were studied as examples of the pH-dependent switchable "target enzymes." The reactions catalyzed by co-immobilized urease or esterase were increasing or decreasing the local pH, respectively, thus operating as "actuator enzymes." Both kinds of the enzymes, producing local pH changes and changing biocatalytic activity with the pH variation, were orthogonal in terms of the biocatalytic reactions; however, their operation was coupled with the local pH produced near the surface with the immobilized enzymes. The "target enzymes" (AMG and trypsin) were changed reversibly between the active and inactive states by applying input signals (urea or ester, substrates for the urease or esterase operating as the "actuator enzymes") and washing them out with a new portion of the background solution. The developed approach can potentially lead to switchable operation of several enzymes, while some of them are inhibited when the others are activated upon receiving external signals processed by the "actuator enzymes." More complex systems with branched biocatalytic cascades can be controlled by orthogonal biocatalytic reactions activating selected pathways and changing the final output.

PH Changes in the Micelle-Water Interface of Surface-Active Ionic Liquids Dictate the Stability of Encapsulated Curcumin: An Insight Through a Unique Interfacial Reaction between Arenediazonium Ions and t-Butyl Hydroquinone.

The chemical kinetic (CK) method, which involves the reduction of 4-hexadecylbenzenediazonium ions (16-ArN2+) by antioxidants (in the present case, TBHQ) occurring exclusively at the interface of the association colloids, was employed to establish the changes in the chemical reactivity of anionic surface-active ionic liquids (SAILs) as a function of the concentration and the composition in their mixed states. We used sodium dodecyl sulfate and different SAILs based on the dodecylsulfate surfactant containing 1-alkyl-3-methylimidazolium cations as counterions having a varying alkyl chain length of 4 (bmim), 8 (omim), and 12 (ddmim) carbon atoms. The structural transitions of aggregates of the SAILs from the micellar to vesicular form were observed as a function of concentration in single surfactant systems and as a function of composition in mixed surfactant systems. Results of the reduction kinetics of 16-ArN2+ at the interface of such aggregates, which depends on the acid/base equilibria at the interface, gave an insight into the changes in the interfacial H+ ions with the change in the hydrophobicity of the counterions of SAILs and the morphological changes from micelles to vesicles as a function of concentration or composition. These changes in the interfacial pH correlate very well with the stability of curcumin within these self-assemblies, which exclusively depends on the pH of the medium and highlights the importance of the results obtained from the CK method in selecting the appropriate medium/conditions for the stabilization of the bioactive molecules.

Mineral cathodic electro-precipitation and its kinetic modelling in thin-film microfluidic reactor during advanced electro-oxidation process

It is presented for the first time the occurrence and modelling of cathodic mineral electro-precipitation in microfluidic thin-film configuration of electrochemical advanced oxidation process (EAOP), in order to better understand and predict the cathode's scaling when treating water containing Mg2+ and Ca2+. Upon a systematic study, the main results show that when an inter-electrode distance of 500 µm is used at 4 mA cm−2, local alkalization on cathode surface occurs despite the absence of dissolved O2 suggesting that it takes place due predominantly to water reduction (current efficiency > 99%) and not dissolved O2 reduction. Moreover, electromigration phenomenon could be neglected when varying ionic strength from 0.02 to 0.1 mol L−1, while diffusion was the kinetic rate-limiting step. In addition, Ca(OH)2 and MgCO3 precipitates were not formed under all investigated conditions. Mg(OH)2 electro-precipitation was found to be highly dependent on current density, but independent of other ionic species jointly present in electrolyte. Mg(OH)2 precipitated once interfacial pH of 10.2 was reached. More CaCO3 was electro-precipitated at lower current density (ca. 7.2%) owing to more vigorous gas evolution when higher current density was applied. 12% less CaCO3 electro-precipitation was found in the presence of Mg2+ confirming its inhibiting effect towards CaCO3 scaling. The mathematical model proposed could fit well the experimental curves (Root mean square error (RMSE) < 0.1163) and permit to predict the evolution of concentration of depositing species (i.e. Mg2+, Ca2+ and CO32−). Furthermore, the role of hydroxyl radicals (•OH) produced at boron-doped diamond (BDD) anode could be neglected upon the applied conditions, meaning that the outcome of this study is reliable across all types of water containing Mg2+, Ca2+ and/or CO32− in compliance to their ubiquity when it is to be treated by electrolysis in general.

Efficient Electrocatalytic N2 Reduction on Three-Phase Interface Coupled in a Three-Compartment Flow Reactor for the Ambient NH3 Synthesis.

The electrochemical N2 reduction reaction (eNRR) represents a carbon-free alternative to the Haber-Bosch process for a sustainable NH3 synthesis powered by renewable energy under ambient conditions. Despite significant efforts to develop catalyst activity and selectivity toward eNRR, an appropriate electrochemical system to obstruct the drawback of low N2 solubility remains broadly unexplored. Here, we demonstrate an electrocatalytic system combining a ruthenium/carbon black gas diffusion electrode (Ru/CB GDE) with a three-compartment flow cell, enabling solid-liquid-gas catalytic interfaces for the highly efficient Ru-catalyzed eNRR. The electrolyte optimization and the Ru/CB GDE development through the hydrophobicity, the Ru/CB loading, and the post-treatment have revealed the crucial contribution of interfacial N2 transportation and local pH environment. The optimized hydrophobic Ru/CB GDE generated excellent eNRR performance, achieving a high NH3 yield rate of 9.9 × 10-10 mol/cm2 s at -0.1 V vs RHE, corresponding to the highest faradaic efficiency of 64.8% and a specific energy efficiency of 40.7%, exceeding the most reported system. This work highlights the critical role of design and optimization of the GDE-flow cell combination and provides a valuable practicable solution to enhance the electrochemical reaction involving gas-phase reactants with low solubility.

Evaluation of Interfacial pH Using Surface Forces Apparatus Fluorescence Spectroscopy.

The fluorescence spectrum measurement of a fluorescence pH probe, C. SNARF-4F, was performed for monitoring the interfacial pH of aqueous electrolytes between mica or silica surfaces while varying the surface separation (D) using surface force apparatus (SFA) fluorescence spectroscopy. The pH of the aqueous CsCl between mica exponentially decreased with decreasing D. The order of the decay lengths of the interfacial pH obtained from the exponential fitting (L) at various electrolyte concentrations was L1mM > L0.1mM ≈ L0.4mM > L10mM. For studying the mechanisms of these changes, we performed the electric double layer (EDL) model calculation of the interfacial pH based on the surface potentials, which were evaluated from the EDL forces between the substrates in aqueous electrolytes using the same SFA. The calculated pH value for the 0.1 mM aqueous electrolyte corresponded to the values obtained from fluorescence spectroscopy, indicating that the interfacial pH was attributed to only the general EDL effect. On the other hand, the measured pH value for the higher concentrations of aqueous electrolytes (0.4-10 mM) decreased in the longer D ranges than the values calculated from the model, indicating that there was an additional factor affecting the interfacial pH for those concentrations. We also studied the effects of the cationic species of the electrolytes (Cs+, Na+, and Li+) and of the silica substrate on the interfacial pH. The systematic studies of the interfacial pH revealed that it depended on all three factors studied here, that is, the electrolyte concentration, electrolyte species, and the substrates. The results also suggested that the interfacial pH was not only due to the simple EDL theory but could also be affected by an additional factor due to the ion adsorption at the interface and chemical states of the substrates.

PH Overpotential for Unveiling the pH Gradient Effect of H + /OH − Transport in Electrode Reaction Kinetics

pH Overpotential for Unveiling the pH Gradient Effect of H ⁺ /OH ⁻ Transport in Electrode Reaction Kinetics

Evaluation of interfacial pH change during water splitting at pulsed regime using finite element method

Electrochemical water splitting is a well-known reaction, investigated since 1869 due to its wide applicability for different purposes. Despite occurring at the electrode/solution interface, this set of reactions can be governed not only by interfacial properties but also by reactant transport from the solution toward the electrode surface. For water splitting specifically, the reaction mechanism itself leads to a pH change at the interface although the bulk value remains constant. In this work, we simulated such pH variations using the finite element method. The results show an interfacial pH change of up to 9 pH units. This modification in the reaction condition affects many related parameters such as electric power consumption, which increases. Thence, we investigated the effect of using an AC perturbation. In the conditions we have studied, it is observed an important delay in the pH change, although it always occurs.

On the assessment of electrocatalysts for nitrate reduction

The electrochemical reduction of nitrate is attracting attention in the context of producing ammonia, besides the traditional removal to harmless N2. To make progress in this complex reaction and facilitate the search for active and selective catalysts, we need to establish generalized testing protocols which will enable to compare and complement data from different laboratories. The purpose of this article is to raise awareness on the importance of (i) solution processes that involve products of the electrode reaction, (ii) determination of products with appropriate, product-specific quantitative methods, (iii) the strong sensitivity of the reaction on experimental parameters, (iv) the cell design for the separation of anode from cathode processes, and (v) the increase in the interfacial and solution pH that occurs during the electrolysis at high current densities.

Template‐free electrochemically polymerized polypyrrole nanowires and their application in flexible solid‐state supercapacitors

AbstractTreated carbon cloths combined with polypyrrole (PPy) nanowires (TCC@PPyW) were fabricated by template‐free electrochemical deposition of pyrrole in the presence of sodium citrate dihydrate and sodium perchlorate. The morphologies and electrochemical performances of the samples were studied using scanning electron microscopy, cyclic voltammetry, galvanostatic charge–discharge measurements and electrochemical impedance spectroscopy. The morphologies of PPy nanostructure were controlled by regulating interfacial pH, deposition time and concentration of sodium perchlorate. PPy nanorods were generated with the aid of water oxidation when the pH of pyrrole solution was 7.9, and PPy nanowires could be obtained by increasing the deposition time or the concentration of sodium perchlorate. The resulting PPy nanowires were about 100 nm in diameter and up to 600 nm in length. TCC@PPyW electrodes provided a large contact area for electrolyte, which facilitated the rapid delivery of electric charge. As a result, the TCC@PPyW electrodes exhibited a high areal specific capacitance of 841.2 mF cm−2 at 0.4 mA cm−2 and a high energy density of 420.6 μWh cm−2 at a power density of 0.2 mW cm−2. An all‐solid‐state flexible symmetric supercapacitor without any binder, incorporated with TCC@PPyW as electrodes and poly(vinyl alcohol)–LiCl gel as gel electrolyte, is also reported. The TCC@PPyW supercapacitor displayed excellent flexibility and rate capability. © 2021 Society of Chemical Industry

Galvanic corrosion behavior of friction stir welded AZ31B magnesium alloy and 6N01 aluminum alloy dissimilar joints

The corrosion behavior and corrosion rate of friction stir welded AZ31B magnesium alloy and 6N01 aluminum alloy dissimilar joints were studied using real-time imaging, potentiodynamic polarization, EIS, and hydrogen evolution tests. The corrosion rate was found to increase, in order, Al base metal (BM-6N01) < Mg base metal (BM-AZ31B) < stirred zone. The galvanic coupling of the Mg phase to the other phases enhanced the corrosion of both BM-AZ31B and BM-6N01. Accompanying the intense corrosion at BM-AZ31B, water reduction on the cathodic particles of BM-6N01 elevated the interfacial pH to a level where the Al matrix corroded to AlO2−.

Effect of ionic strength in low salinity water injection processes

Low salinity water injection has been frequently studied as an enhanced oil recovery process (EOR), mainly due to promising experimental results and because operational needs are not very different from those of the conventional water injection. However, there is no agreement on the mechanisms involved in increasing the displacement of crude oil, except for the effects of wettability changes. Water injection is the oil recovery method mostly used, and considering the characteristics of Colombian oil fields, this study analyses the effect of modifying the ionic composition of the waters involved in the process, starting from the concept of ionic strength (IS) in sandstone type rocks.&#x0D; The experimental plan for this research includes the evaluation of spontaneous imbibition (SI), contact angles, and displacement efficiencies in Berea core plugs. Interfacial tension and pH measurements were also carried out. The initial scenario consists in formation water (FW), with a total concentration of 9,800 ppm (TDS) (IS ~ 0.17) and a 27 °API crude oil. Magnesium and Calcium brine were also used in a first approach to assess the effect of the divalent ions.&#x0D; Displacement efficiency tests are performed using IS of 0.17, 0.08, and 0.05, as secondary and tertiary oil recovery and the recovery of oil increases in both scenarios. Spontaneous imbibition curves and contact angle measurements show variations as a function of the ionic strength, validating the displacement efficiencies.&#x0D; Interfacial tension and pH collected data evidence that fluid/fluid interactions occur due to ionic strength modifications. However, as per the conditions of this research, fluid/fluid mechanisms are not as determining as fluid/rock.

Cation Effect on Interfacial CO2 Concentration in the Electrochemical CO2 Reduction Reaction

The capacity of larger alkali metal cations to maintain the interfacial CO2 concentration by buffering the interfacial pH has been proposed to enhance the electrochemical CO2 reduction reaction (CO...

Investigation of the calcareous deposits formation controlled by interfacial pH and its effect on the hydrogen entry into AISI 4135 steel in seawater

The influence of interfacial pH between AISI 4135 steel and seawater under different polarization potentials on the formation of calcareous deposits has been studied. An interfacial pH of 9.61 at −0.9 V vs. SCE using state of the art iridium oxide microelectrode was found to be the critical pH for the precipitation of magnesium hydroxide. Calcareous deposits with a double-layer structure comprising an inner-brucite layer and an outer-aragonite layer were found to form at potentials between −1.0 V and −1.2 V vs. SCE. Furthermore, the facilitation of hydrogen permeation into steel induced by the formation of calcareous deposits was verified using the Devanathan-Stachurski electrochemical test. The mechanism of calcareous deposits facilitates hydrogen permeation into steel is related to its inhibition on hydrogen recombination and escape processes.

Fluorescent pH-Nanosensors for the in-Operando Visualization of Fast Dynamics, Interfacial pH in Electrochemical Reactions

The in-operando visualization of interfacial pH distribution, at electrode-electrolyte interface, provides crucial insights into the mechanism and performance of many electrochemical processes. However, due to the fast dynamics and wide pH range, the spatiotemporal visualization of such interfacial pH changes requires the development of appropriate probes capable of capturing evolution of pH changes in the harsh chemical environments that normally occur in electrochemical processes. In this study, ratiometric, fluorescent, pH-sensitive nanosensors were fabricated with a wide pH measurement range (pH 3.5 to 11), for the in-operando visualisation of fast-dynamic, interfacial pH changes during an electrocoagulation process. The developed nanosensors were synthesised from the crosslinked polyacrylamide matrix, covalently linked to three pH-sensitive dyes and a reference fluorophore, to provide ratiometric nanosensors with protective but proton permeable matrix and comparable sensitivity and responsiveness to the free dyes. Using the developed nanosensors and an electrochemically coupled laser confocal fluorescence microscope, the in-operando visualization of the dynamic interfacial pH distribution during electrocoagulation of synthetic and real oil-sands produced water was achieved for the first time. The results (Figure 1) revealed the differences between the real and synthetic produced water and reflected the crucial roles of factors such as the rate of water electrolysis and chemical composition of the system on the evolution of the pH boundary layer and the interfacial pH. Overall, this study provides powerful probes (nanosensors) that can be utilized to study interfacial pH changes in a wide range of electrochemical reactions such as batteries and electrolyzers. Figure 1: Selected time series, processed images shows the differences in the interfacial pH boundary layers evolved during the electrocoagulation treatment of synthetic and real produced water. In-operando imaging was conducted using the developed nanosensors and an electrochemically coupled laser scanning confocal microscopy. Figure 1

Electrochemically Stimulated Molecule Releaseassociated with Interfacial pH Changes

Stimuli responsive materials are extremely important in many applications in biomedical and biotechnological field.1 Over past several decades, a large number of materials/systems which respond to the various activating/deactivating signals have been proposed and developed. External stimuli controlled release of therapeutic agents is one among such systems. In this field a large variety of stimuli such as pH, temperature, light, magnetic field and many more have been exploited to achieve signal stimulated release of drugs/biomolecules. Electrochemically triggered release of (bio)molecules is gaining attention due to the simplicity and easy integration of those systems with sophisticated electronic devices. These methods take advantage of some common phenomenon such as formation/cleavage of redox bonds (for example; disulfide linkages), electrostatic association/dissociation2,3, and entrapping/releasing the (bio)molecules from the (bio)polymer gels for achieving immobilization and electrochemically stimulated release. While these methods are powerful, they have their own limitations (for example, reduction of disulfide bonds is significantly complicated in presence of other redox species). Hence to design an universal approach for electrochemically triggered release of (bio)molecules, we investigated the development of a hydrolysable linker at mild basic (pH 8-10) conditions. Herein we report a new linker with a hydrolysable phenolic ester bond at basic pH (~8-10) for electrode modification. Basic pH locally produced at the electrode surface upon electrochemical reduction of oxygen resulted in the hydrolytic cleavage of the phenolic ester bond and release of the immobilized fluorescent dye used as a model compound4.

Effects of Solid-Solute V on the Phosphatability of Hot-Rolled Steel Sheets

The phosphatability of hot-rolled steel sheets has become increasingly important with application of higher strength and thinner steel sheets to automotive parts. Although vanadium (V) is an alloying element which is often added to high strength hot-rolled steel sheets, the effect of V on phosphatability was still unclear. This study investigated the phosphatability of V-added hot-rolled steel sheets by using V-free, 0.20% V, and 0.47% V steel sheets as test specimens. After phosphate treatment, phosphate crystals covered the whole surface of the V-free and 0.20% V steel sheets, but no phosphate crystals were observed on the surface of the 0.47% V steel sheets. In order to clarify this difference, potentiostatic polarization measurement was carried out in the phosphate treatment solution. Phosphate crystals were found on the surface of the V-free steel sheet after both cathodic and anodic polarization. In contrast, no phosphate crystals were found on the surface of the 0.47% V steel sheet after anodic polarization, but similarly to the V-free steel sheet, phosphate crystals had formed after cathodic polarization. A surface analysis by XPS revealed that V oxides had precipitated on the surface of the 0.47% V steel sheet after anodic polarization. V oxidation reaction involves the generation of hydrogen ions and prevent the steel/solution interfacial pH from rising. This reaction identified as the deterioration mechanism of the phosphatability of V-added hot-rolled steel sheets.

In vitro osteogenic performance of two novel strontium and zinc-containing glass polyalkenoate cements.

Glass polyalkenoate cements (GPCs) are under investigation as potential bone adhesives, as they may provide an alternative to polymethylmethacrylate-based cements. GPCs containing strontium (Sr) and zinc (Zn) in place of aluminium (Al) are of particular interest because these ions are known stimulators of osteoprogenitor differentiation. GPCs have been manufactured from a novel bioactive glass (SiO2 :0.48, ZnO:0.36, CaO:0.12, SrO:0.04) in the past, but, while such materials have been assessed for their influence on viability, their influence on osteogenic function has not been investigated until now. For this study, two GPCs were formulated from the same glass precursor evaluated in previous studies. These GPCs were named GPC A and GPC B, and they differed in glass particle size, polyacrylic acid molecular weight, and their powder: liquid ratios. The effect of these two GPCs on osteogenic differentiation of primary rat osteoblasts were evaluated using three culture systems: culture with dissolution extracts, indirect contact with transwell-inserts and direct contact. Additionally, the degradation characteristics of GPCs were assessed, including their interfacial pH and surrounding pH. The experimental outcomes revealed that collagen deposition, alkaline phosphatase expression, and mineralization were largely dependent on GPC composition as well as the mode of interaction with cells. These markers were found to be significantly elevated in response to GPC A's dissolution products. However, osteogenic differentiation was inhibited when osteoblasts were cultured indirectly and directly with GPCs, with, overall, GPC B significantly outperforming GPC A. These results suggest that GPC degradation products effect osteogenic differentiation in a dose-dependent manner.

Bell shape vs volcano shape pH dependent kinetics of the electrochemical oxidation of formic acid and formate, intrinsic kinetics or local pH shift?

The interfacial pH at the electrode/electrolyte interface (pHs) and reaction kinetics of electrochemical oxidation of formic acid in the presence or absence of buffers are carried out. The transform from bell shape to volcano shape pH dependence of formic acid oxidation reaction (FOR) is clarified by considering both pHs drift and buffer effect. The Diagram of pHs as a function of FOR current density (j) in solutions with bulk pH (pHb) from 0 to 14 with/without buffers are constructed by numeral calculations, which are compared with experimental results of FOR on Pt and Au. Our study reveals that i) In solutions with 5< pHb <10 free of buffer or with limit buffer capability, there is significant pHs drop and drift of the thermodynamic equilibrium potential (Eeq); ii) FOR on both Pt and Au has a bell shape intrinsic pH dependence with maximum activity close to the pKa of HCOOH; iii) The observed volcano shape pH dependence of FOR activity with a plateau at 5<pHb<10 on top of the volcano is a coincidence related to pHs drift.

New insight to experimental study of ionic solutions with a non-ionic surfactant on wettability, interfacial tension and micro-model flooding

Wettability alteration (Δθ), changes in interfacial tension (ΔIFT) and fluid pH are investigated for Asmari formation carbonate and sandstone reservoirs of the large Ahwaz oil field (Iran) with a range of low salinity water-based fluids accompanied by a non-ionic surfactant (LSWF-surfactant) and the acidic (total acidic number = 0.58 mg KOH g−1) Asmari crude oil. On initial contact with the LSWF-surfactant the carefully prepared reservoir rock slices, soaked and aged in the crude oil all displayed oil-wet states. Metal-salt (potassium chloride, sodium sulfate, magnesium sulfate heptahydrate, calcium chloride dihydrate and lithium chloride) enriched aqueous solutions of various concentration (1000–4000 ppm) mixed with 400 ppm Triton™ X-100 (surfactant) were used in the Δθ, ΔIFT and pH tests. The test results for these variables suggest that the calcium chloride dihydrate (4000 ppm) solution with surfactant has the most beneficial impacts on both the carbonate and sandstone reservoir samples evaluated in terms of improving oil mobilization. Glass micro-model fluid-flow experiments to evaluate and compare oil recovered by secondary recovery (LSWF only injected) and by tertiary recovery (LSWF-surfactant injection) confirm the rock slice tests, with calcium chloride dihydrate achieving substantially higher ultimate oil recovery than the other LSWF solutions tested.

Tuning Redox State and Ionic Transfers of Mg/Fe-Layered Double Hydroxide Nanosheets by Electrochemical and Electrogravimetric Methods.

Studying the electrogravimetric behavior of Mg/Fe-layered double hydroxide (LDH) nanoparticles with an electrochemical quartz crystal microbalance demonstrates its pseudocapacitance properties of mix cation and anion exchanger. The electrochemical control of the oxidation state of iron constituting the layered sheets allowed anion intercalation/deintercalation into the LDH interlayer space. Concomitantly, in agreement with the pH of zero point of net charge of the Mg/Fe-LDH, the interfacial pH increase via catalyzed hydrogen evolution reaction allows cation electroadsorption onto the external surfaces of the nanoplatelets.