Interfacial pH Research Articles

Combined effect of cathodic potential and sulfur species on calcareous deposition, hydrogen permeation, and hydrogen embrittlement of a low carbon bainite steel in artificial seawater

Calcareous deposition, hydrogen permeation, and hydrogen embrittlement of E690 steel in thiosulfate-containing artificial seawater (ASW) under cathodic polarization are investigated. Sulfur species in acidic ASW eliminate the aragonite precipitation, attributed to the variation of cathodic reaction, favorable adsorption of HS− and enhanced hydrogen evolution. The maximum permeation current was achieved at −850 mVSCE due to the change of interfacial pH and promotion ability of sulfur species. Hydrogen embrittlement susceptibility increases linearly with logarithm increase of hydrogen concentration, while the preferential cracking pathway evolves from lath boundary separation at low hydrogen concentration to prior austenite grain boundary failure at high concentrations.

Extraction Effect of Ni-Zn By Electrodeposition from Aqueous Acid Solutions

The electrodeposition of Zn-Ni is classified by Brenner (1) as anomalous codeposition, because zinc, the less noble metal, is preferentially deposited to nickel the more noble metal. The inhibition of H+ reduction occurs with increasing Zn ions concentration in solution (2). Anomalous codeposition is favored in chloride medium and inhibited in boric acid (3). The reaction kinetic of Zn-Ni codéposition was investigated in acid solutions .The effects of solution composition and pH were analyzed. The inhibition of H+ reduction occurs with increasing Zn ions concentration in solution. Increasing pH values causes Zn deposition during Ni-Zn codeposition. Anomalous codeposition is favored in chloride medium and inhibited in boric acid. When alloy deposition becomes the main process, the interfacial pH is governed by the individual metal deposition that controls the kinetic behavior. The interfacial pH increases during separate Ni deposition, meaning that it occurs with simultaneous consumption of H+. Anomalous codeposition process is not due to a saturation of species at the electrode surface. Results show that it is possible to obtain alloys rich in either nickel or zinc , from an acidic solution,depending on the Ni/Zn concentration ration. For Ni/Zn concentration ratios in the range of 1-5, zinc rich alloys are obtained despite Zn is the less noble metal and even when it is present lower concentrations than nickel. For the highest Ni/Zn concentrations ratio ≥100, the formation of the alloy occurs by a normal deposition, resulting in Ni-rich alloys. Boric acid extends the proton discharge potential to more cathodic values. Thus it can reduce anomalous codeposition since the presence of 0,8M of acid reduces the intensity of the peak corresponding to δ phase that is rich in zinc and enhances the intensity of γ phase containing more nickel than the δ phase.

The formation and properties of polypyrrole doped with an immobile antibiotic

Oxacillin, a semisynthetic penicillinase-resistant penicillin, was incorporated as a dopant within polypyrrole. The oxacillin-doped polymer was deposited at gold, platinum and the biocompatible substrate, titanium. A doping level of 0.12 was obtained during the early stages of polymer formation. Smooth polymer films were deposited, but more structure was observed in the cross sections of the polymer. This was attributed to a reduction in the concentration of the anionic oxacillin as the interfacial pH decreased due to the electropolymerisation reaction, shifting the equilibrium in favour of the undissociated oxacillin, HOx. Although HOx has poor solubility and may precipitate at the polymer solution interface during formation of the polymer, there was no evidence of any reduction in the conducting properties of the polymer. High charging capacitance values of 12 to 15 mF cm−2 were estimated for the oxidised polymer. These conducting properties were attributed to a slow rate of electropolymerisation that prevented significant acidification at the electrode interface, preventing the build-up of HOx. The oxacillin-doped polypyrrole showed cation exchange properties. Oxacillin was maintained within the polymer, as the polymer was cycled between its oxidised and reduced states to give an immobilised antibiotic with very little oxacillin detected in the solution phase. The application of a layer of chitosan gave rise to a slight reduction in the charging capacitance of PPyOx. However, improved adhesion was observed for the reduced PPyOx with the application of the chitosan layer.

Extraction Effect of Ni-Zn By Electrodeposition from Aqueous Acid Solutions

The electrodeposition of Zn-Ni is classified by Brenner (1) as anomalous codeposition, because zinc, the less noble metal, is preferentially deposited to nickel the more noble metal. The inhibition of H+ reduction occurs with increasing Zn ions concentration in solution (2). Anomalous codeposition is favored in chloride medium and inhibited in boric acid (3). The reaction kinetic of Zn-Ni codéposition was investigated in acid solutions .The effects of solution composition and pH were analyzed. The inhibition of H+ reduction occurs with increasing Zn ions concentration in solution. Increasing pH values causes Zn deposition during Ni-Zn codeposition. Anomalous codeposition is favored in chloride medium and inhibited in boric acid. When alloy deposition becomes the main process, the interfacial pH is governed by the individual metal deposition that controls the kinetic behavior. The interfacial pH increases during separate Ni deposition, meaning that it occurs with simultaneous consumption of H+. Anomalous codeposition process is not due to a saturation of species at the electrode surface. Results show that it is possible to obtain alloys rich in either nickel or zinc , from an acidic solution,depending on the Ni/Zn concentration ration. For Ni/Zn concentration ratios in the range of 1-5, zinc rich alloys are obtained despite Zn is the less noble metal and even when it is present lower concentrations than nickel. For the highest Ni/Zn concentrations ratio ≥100, the formation of the alloy occurs by a normal deposition, resulting in Ni-rich alloys. Boric acid extends the proton discharge potential to more cathodic values. Thus it can reduce anomalous codeposition since the presence of 0,8M of acid reduces the intensity of the peak corresponding to δ phase that is rich in zinc and enhances the intensity of γ phase containing more nickel than the δ phase.

Threats to adhesive/dentin interfacial integrity and next generation bio-enabled multifunctional adhesives.

Nearly 100 million of the 170 million composite and amalgam restorations placed annually in the United States are replacements for failed restorations. The primary reason both composite and amalgam restorations fail is recurrent decay, for which composite restorations experience a 2.0-3.5-fold increase compared to amalgam. Recurrent decay is a pernicious problem-the standard treatment is replacement of defective composites with larger restorations that will also fail, initiating a cycle of ever-larger restorations that can lead to root canals, and eventually, to tooth loss. Unlike amalgam, composite lacks the inherent capability to seal discrepancies at the restorative material/tooth interface. The low-viscosity adhesive that bonds the composite to the tooth is intended to seal the interface, but the adhesive degrades, which can breach the composite/tooth margin. Bacteria and bacterial by-products such as acids and enzymes infiltrate the marginal gaps and the composite's inability to increase the interfacial pH facilitates cariogenic and aciduric bacterial outgrowth. Together, these characteristics encourage recurrent decay, pulpal damage, and composite failure. This review article examines key biological and physicochemical interactions involved in the failure of composite restorations and discusses innovative strategies to mitigate the negative effects of pathogens at the adhesive/dentin interface. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B:2466-2475, 2019.

Spatial Distribution of Biomaterial Microenvironment pH and Its Modulatory Effect on Osteoclasts at the Early Stage of Bone Defect Regeneration.

It is generally accepted that biodegradable materials greatly influence the nearby microenvironment where cells reside; however, the range of interfacial properties has seldom been discussed due to technical bottlenecks. This study aims to depict biomaterial microenvironment boundaries by correlating interfacial H+ distribution with surrounding cell behaviors. Using a disuse-related osteoporotic mouse model, we confirmed that the abnormal activated osteoclasts could be suppressed under relatively alkaline conditions. The differentiation and apatite-resorption capability of osteoclasts were "switched off" when cultured in titrated material extracts with pH values higher than 7.8. To generate a localized alkaline microenvironment, a series of borosilicates were fabricated and their interfacial H+ distributions were monitored spatiotemporally by employing noninvasive microtest technology. By correlating interfacial H+ distribution with osteoclast "switch on/off" behavior, the microenvironment boundary of the tested material was found to be 400 ± 50 μm, which is broader than the generally accepted value, 300 μm. Furthermore, osteoporotic mice implanted with materials with higher interfacial pH values and boarder effective ranges had lower osteoclast activities and a thicker new bone. To conclude, effective proton microenvironment boundaries of degradable biomaterials were depicted and a weak alkaline microenvironment was shown to promote regeneration of osteoporotic bones possibly by suppressing abnormal activated osteoclasts.

Localized corrosion of carbon steel in seawater: Processes occurring in cathodic zones

The composition of the corrosion product layer forming on carbon steel in seawater depends on the cathodic/anodic character of the metal surface underneath. This variability is mainly linked to that of interfacial pH, that decreases in anodic zones and increases in cathodic zones. To study the role of pH on these phenomena, two types of experiments were achieved. First, carbon steel coupons were immersed 10 days in artificial seawater in conditions favoring differential aeration cells and the resulting corrosion product layer was characterized by X‐ray diffraction. Lepidocrocite (γ‐FeOOH) and aragonite (CaCO3) proved to be the major components of this layer in the anodic and cathodic zones, respectively, while the sulfate and carbonate green rusts, GR(SO42‐) and GR(CO32‐), were observed as minor components. Secondly, aqueous suspensions of GR(SO42‐) and GR(CO32‐) were prepared in the laboratory to study the equilibrium conditions where both compounds can co‐exist. A pK value of 7.85 ± 0.35 could be estimated, leading to an equilibrium pH of 8.24 ± 0.35 in seawater, consistent with the observed predominance of GR(SO42‐) in the anodic zones and GR(CO32‐) in the cathodic zones.

Electrochemically stimulated molecule release associated with interfacial pH changes.

A new linker with a hydrolyzable aryl ester bond was used for electrode modification. Basic pH locally produced at the electrode surface upon electrochemical reduction of O2 resulted in the hydrolytic cleavage of the aryl ester bond and release of the immobilized fluorescent dye used as a model compound.

Dimethylaniline functionalised pyrene fluorophores; dual colour pH switching in solution and self-assembled monolayers.

A pyrene charge transfer fluorophore with three ionizable N,N-dimethylaniline moieities was explored as an interfacial pH switch. The parent carboxylate compound and the thiolated derivative were shown by spectroscopy combined with DFT calculation to be successively and reversibly protonated. Protonation leads to progressive decrease of intensity of the 550 nm centered N,N-dimethylaniline to pyrene charge transfer emission which on protonation of the third site, leads to extinction of this transition and evolution of an intense blue (450 nm) pyrene-centered emission. Concomitant loss of the charge transfer absorbance was observed and the changes are reversed on neutralization of pH. A self-assembled monolayer of the thiolated derivative was prepared on gold and found from voltammetry of ferricyanide/ferrocyanide probe to form close packed monolayers. The probe voltammetry, label-free electrochemical impedance spectroscopy of the film was monitored as a function of pH and progressive, but reversible protonation steps were reflected in decreasing film resistance. The Stokes shift of the probe prevents self-quenching so a broad, charge transfer fluorescence centered around 540 nm was recorded for the self-assembled monolayer where as per solution, progressive and reversible reduction in intensity was observed. The facile assembly, impedance and optical switching make these materials potentially interesting as on-off or two colour on-off-on fluorescence switches with potential applications in logic gates or in responsive surface applications.

Studies on the formation and properties of polypyrrole doped with ionised β-cyclodextrins: influence of the anionic pendants

Polypyrrole films doped with sulfonated β-cyclodextrin (PPy/sβ-CD) and carboxymethyl β-cyclodextrin (PPy/CMβ-CD) were formed on electropolymerisation of pyrrole in a solution of the cyclodextrin sodium salts. Lower rates of electropolymerisation were evident with the carboxymethyl β-CD and this was attributed to protonation of the carboxylate groups due to an increase in the interfacial pH at the electrode–solution interface. The PPy/sβ-CD and PPy/CMβ-CD films showed cation exchange behaviour with the ingress of Na+ and its associated water molecules on reduction. The PPy/CMβ-CD film showed less electroactive behaviour which may be related to the lower number of anionic pendants on the CMβ-CD or to the possible precipitation of the less soluble neutral CD which arises due to the protonation of the carboxylate groups. The PPy/sβ-CD showed a high capacitance of 6.0 mF cm−2 corresponding to a specific capacitance of 71 F g−1 over a wide potential range extending from − 0.60 to 0.60 V vs SCE. Over-oxidation was observed at relatively low potentials, at about 0.60 V vs SCE. A diffusion coefficient of 6.47 × 10−6 cm2 s−1 and a rate constant of 3.73 × 10−2 cm s−1 were obtained for the ferricyanide redox couple at the PPy/sβ-CD, indicating little or no inhibition of the ferricyanide redox reaction.

Nano-sized bubbles in solution of hydrophobic dyes and the properties of the water/air interface

This article is devoted to colloidal aggregates formed in air-saturated water in the presence of 3–10 vol% ethanol, either without or with long-tailed hydrophobic dyes. In 5 × 10−6 M solution of n‑decylfluorescein or n‑decyleosin, particles with size of about 100–300 nm and PDI around 0.1–0.3 were observed by the dynamic light scattering method. Varying of pH and the dyes concentration is accompanied by measurements of visible absorption spectra, size distribution, and ζ-potential. The last parameter is strongly pH-dependent, with isoelectric point within the pH range of 3 to 4. The character of variation of ζ-potential in buffer solutions is similar to the data reported by other authors for gas bubbles of bigger size. However, the presence of ethyl alcohol makes the ζ values at higher pH less negative. Though the particles appear also without the dyes the introduction of the later makes the parameters of colloidal species somewhat different. The fluorescence and absorption spectra give no evidence for dye dimerization or other kinds of aggregation, including formation of homomicelles, and therefore it is reasonable to infer that the interfacial region of the air bubbles, partly occupied by ethanol molecules, is a most probable locus of dyes.The apparent ionization constants of the dyes are determined via the spectrophotometric method either in buffer systems at ionic strength 0.005 to 0.10 M, or in HCl solutions. The pKaapp values at higher pH resemble those typical for these indicators in micelles of anionic surfactants, whereas those in the acidic region are close to the data known for cationic surfactant micelles. It is demonstrated, however, that in the case under study the traditional way of interpretation of the pKaapp values and estimation of the interfacial pH is misleading. Contrary to it, an alternative approach is used for estimating the interfacial ionic concentrations. This approach is tested using the literature data for ζ-potential of gas bubbles at various pH values in water.

Live Monitoring of Microenvironmental pH Based on Extracellular Acidosis around Cancer Cells with Cell-Coupled Gate Ion-Sensitive Field-Effect Transistor.

We demonstrated the live monitoring of cellular respiration using an ion-sensitive field-effect transistor (ISFET), focusing on different types of living cells, namely cancer and normal cells. In particular, we realized the label-free, real-time, and noninvasive monitoring of microenvironmental pH behavior based on extracellular acidosis around cancer cells in the long term and in situ. The change in interfacial pH (ΔpHint), which was analyzed based on the change in interfacial potential (Δ Vout) at the cell/gate nanogap interface gradually decreased for every cell-based ISFET. Moreover, the ΔpHint for cancer cells shifted by a factor of 5 to 6, which was larger than that for normal cells. This is because cancer cells cause dysbolism and are activated, thereby suppressing oxidative phosphorylation in mitochondria so as not to induce their apoptosis. Therefore, cancer cellular respiration proceeds via the glycolysis pathway, through which lactic acid is eventually released. Additionally, the pH sensitivity of the ISFET device was maintained even when the device was immersed into a cell culture medium for 24 h and 1 w; thus, the effect of nonspecific adsorption of proteins contained in the medium on the pH sensitivity of the ISFET device was negligible in the live monitoring of cellular respiration.

Electrochemical study on hydrogen evolution and CO2 reduction on Pt electrode in acid solutions with different pH

Hydrogen evolution reaction (HER) is the major cathodic reaction which competes CO2 reduction reaction (CO2 RR) on Pt electrode. Molecular level understanding on how these two reactions interact with each other and what the key factors are of CO2 RR kinetics and selectivity will be of great help in optimizing electrolysers for CO2 reduction. In this work, we report our results of hydrogen evolution and CO2 reduction on Pt(111) and Pt film electrodes in CO2 saturated acid solution by cyclic voltammetry and infrared spectroscopy. In solution with pH>2, the major process is HER and the interfacial pH increases abruptly during HER; COad is the only adsorbed intermediate detected in CO2 reduction by infrared spectroscopy; the rate for COad formation increases with the coverage of UPD-H and reaches maximum at the onset potential for HER; the decrease of COad formation under HER is attributed to the available limited sites and the limited residence time for the reduction intermediate (Had), which is necessary for CO2 adsorption and reduction.

Mechanistic Analysis of Cocrystal Dissolution, Surface pH, and Dissolution Advantage as a Guide for Rational Selection

The dissolution behavior of a dibasic drug ketoconazole under the influence of pH has been evaluated and compared to its three 1:1 cocrystals with diacidic coformers, fumaric acid, succinic acid (SUC), and adipic acid. Mass transport models were developed by applying Fick's law of diffusion to dissolution with simultaneous chemical reactions in the hydrodynamic boundary layer adjacent to the dissolving surface to predict the interfacial pH and flux of the parent drug and cocrystals. All 3 cocrystals have the ability to modulate the interfacial pH to different extents compared to the parent drug due to the acidity of the coformers. Dissolution pH dependence of ketoconazole is significantly reduced by the cocrystallization with acidic coformers. Due to the different dissolution pH dependence, there exists a transition pH where the flux of the cocrystal is the same as the parent drug. Below this transition pH, the drug flux is higher, but above it, the cocrystal flux is higher. The development of these mass transport models provide a mechanistic understanding of the dissolution behavior and help identify cocrystalline solids with optimal dissolution characteristics.

Hydrostatic pressure effects on hydrogen entry into A514 steel with cathodic deposits

Hydrostatic pressure effects on cathodic deposits and long-term hydrogen permeation in 0.2 mol/L NaOH, 3.5% NaCl and artificial seawater were characterized by the high-pressure Devanathan cell hydrogen permeation tests, electrochemical impedance spectroscopy, Raman spectroscopy, scanning electron microscopy, coupled focused ion beam lithography and energy dispersive X-ray spectroscopy. Hydrostatic pressure accelerates the hydrogen evolution reaction and causes hydrogen atoms to permeate into steel. Furthermore, hydrostatic pressure increases the interfacial pH; changes deposits from a mixture of brucite, aragonite, and calcite to brucite only; and inhibits hydrogen permeation into steel in artificial seawater. These competitive effects control the mechanisms of hydrogen entry into steel in deep seawater.

Quantification of Interfacial pH Variation at Molecular Length Scales Using a Concurrent Non-Faradaic Reaction.

We quantified changes in interfacial pH local to the electrochemical double layer during electrocatalysis by using a concurrent non-faradaic probe reaction. In the absence of electrocatalysis, nanostructured Pt/C surfaces mediate the reaction of H2 with cis-2-butene-1,4-diol to form a mixture of 1,4-butanediol and n-butanol with selectivity that is linearly dependent on the bulk solution pH value. We show that kinetic branching occurs from a common surface-bound intermediate, ensuring that this probe reaction is uniquely sensitive to the interfacial pH value within molecular length scales of the surface. We used the pH-dependent selectivity of this reaction to track changes in interfacial pH during concurrent hydrogen oxidation electrocatalysis and found that the local pH value can vary dramatically (>3 units) relative to the bulk value even at modest current densities in well-buffered electrolytes. This study highlights the key role of interfacial pH variation in modulating inner-sphere electrocatalysis.

Quantification of Interfacial pH Variation at Molecular Length Scales Using a Concurrent Non‐Faradaic Reaction

AbstractWe quantified changes in interfacial pH local to the electrochemical double layer during electrocatalysis by using a concurrent non‐faradaic probe reaction. In the absence of electrocatalysis, nanostructured Pt/C surfaces mediate the reaction of H2 with cis‐2‐butene‐1,4‐diol to form a mixture of 1,4‐butanediol and n‐butanol with selectivity that is linearly dependent on the bulk solution pH value. We show that kinetic branching occurs from a common surface‐bound intermediate, ensuring that this probe reaction is uniquely sensitive to the interfacial pH value within molecular length scales of the surface. We used the pH‐dependent selectivity of this reaction to track changes in interfacial pH during concurrent hydrogen oxidation electrocatalysis and found that the local pH value can vary dramatically (>3 units) relative to the bulk value even at modest current densities in well‐buffered electrolytes. This study highlights the key role of interfacial pH variation in modulating inner‐sphere electrocatalysis.

In situ measurement of autophagy under nutrient starvation based on interfacial pH sensing

In this study, we report a novel method for the in situ measurement of autophagy under nutrient starvation using a principle of semiconductor technology. A semiconductor-based field-effect transistor (FET) biosensor enables the direct detection of ionic or molecular charges under biological conditions. In particular, cellular respiration accompanied by the generation of carbon dioxide can be continuously and directly monitored as a change in pH at a cell/sensor interface. When autophagy was induced in HeLa cells on a FET biosensor under nutrient starvation, the surface potential increased more significantly for about 15 h than that for nonstarved cells. This positive shift indicates an increase in the number of hydrogen ions produced from the respiration of starved cells because the sensing surface was previously designed to be sensitive to pH variation. Therefore, we have found that cellular respiration is more activated by autophagy under nutrient starvation because the amino acids that decomposed from proteins in autophagic cells would have been rapidly spent in cellular respiration.

Spectroscopic Evidence of Size-Dependent Buffering of Interfacial pH By Cation Hydrolysis during CO2 Electroreduction

The nature of the electrolyte cation is known to affect the Faradaic efficiency and selectivity of CO2 electroreduction. Singh et al. (J. Am. Chem. Soc. 2016, 138, 13006–13012) recently attributed this effect to the buffering ability of cation hydrolysis at the electrical double layer. According to them, the pKa of hydrolysis decreases close to the cathode due to the polarization of the solvation water molecules between the cation’s positive charge and the negative charge on the electrode surface. We have tested this hypothesis experimentally, by probing the pH at the gold-electrolyte interface in situ using ATR-SEIRAS. The ratio between the integrated intensity of the CO2 and HCO3 - bands, which has to be inversely proportional to the pH, provided a means to determining the pH change in-situ during the electroreduction of CO2. Our results confirm that the magnitude of the pH increase at the interface follows the trend Li+ > Na+ > K+ > Cs+, adding strong experimental support to Singh’s et al.’s hypothesis. We show, however, that the pH buffering effect was overestimated by Singh et al., their overestimation being larger the larger the cation. Figure 1

Impact of Interfacial Composition on Lipid and Protein Co-Oxidation in Oil-in-Water Emulsions Containing Mixed Emulisifers.

The impact of interfacial composition on lipid and protein co-oxidation in oil-in-water emulsions containing a mixture of proteins and surfactants was investigated. The emulsions consisted of 5% v/v walnut oil, 0.5% w/v whey protein isolate (WPI), and 0 to 0.4% w/v Tween 20 (pH 3 and pH 7). The protein surface load, magnitude of the ξ-potential, and mean particle diameter of the emulsions decreased as the Tween 20 concentration was increased, indicating the whey proteins were displaced by this nonionic surfactant. The whey proteins were displaced from the lipid droplet surfaces more readily at pH 3 than at pH 7, which may have been due to differences in the conformation or interactions of the proteins at the droplet surfaces at different pH values. Emulsions stabilized by whey proteins alone had relatively low lipid oxidation rates when incubated in the dark at 45 °C for up to 8 days, as determined by measuring lipid hydroperoxides and 2-thiobarbituric acid-reactive substances (TBARS). Conversely, the whey proteins themselves were rapidly oxidized, as shown by carbonyl formation, intrinsic fluorescence, sulfhydryl group loss, and electrophoresis measurements. Displacement of whey proteins from the interface by Tween 20 reduced protein oxidation but promoted lipid oxidation. These results indicated that the adsorbed proteins were more prone to oxidation than the nonadsorbed proteins, and therefore, they could act as better antioxidants. Protein oxidation was faster, while lipid oxidation was slower at pH 3 than at pH 7, which was attributed to a higher antioxidant activity of whey proteins under acidic conditions. These results highlight the importance of interfacial composition and solution pH on the oxidative stability of emulsions containing mixed emulsifiers.