Specific Adsorption Of Cations Research Articles

The Role of Alkali Cations on the Selectivity of 5-Hydroxymethylfurfural Electroreduction on Glassy Carbon.

In the past decade, organic electrosynthesis has emerged as an atom- and energy-efficient strategy for harvesting renewable electricity that provides exceptional control over the reaction parameters. A profound and fundamental understanding of electrochemical interfaces becomes imperative to advance the knowledge-based development of electrochemical processes. The major strategy toward an efficient electrochemical system is based on the advancement in material science for electrocatalysis. Studies on the complex interplay among electrode surface, electrolyte, and transformation intermediates have only recently started to emerge. It involves acquiring atomic-scale insights into the electrochemical double layer, for which the identity and concentration of composing ions play a crucial role. In this study, we present how the identity and concentration of alkali cations impact the selectivity of aldehyde functionality electroreduction. As a case-study transformation, we set the electrochemical conversion of 5-hydroxymethylfurfural (HMF), a promising biomass-derived feedstock for the sustainable production of polymer or fuel precursors. Our findings reveal a consistent trend of the selectivity shift towards 2,5-bis(hydroxymethyl)furan (BHMF) as a function of cation size and concentration, rationalized by specific cation adsorption at the glassy carbon (GC), followed by the increase in the electrode surface charge density.

Specifically adsorbed ferrous ions modulate interfacial affinity for high-rate ammonia electrosynthesis from nitrate in neutral media

The electrolysis of nitrate reduction to ammonia (NRA) is promising for obtaining value-added chemicals and mitigating environmental concerns. Recently, catalysts with high-performance ammonia synthesis from nitrate has been achieved under alkaline or acidic conditions. However, NRA in neutral solution still suffers from the low yield rate and selectivity of ammonia due to the low binding affinity and nucleophilicity of NO3-. Here, we confirmed that the in-situ-generated Fe(II) ions existed as specifically adsorbed cations in the inner Helmholtz plane (IHP) with a low redox potential. Inspired by this, a strategy (Fe-IHP strategy) was proposed to enhance NRA activity by tuning the affinity of the electrode-electrolyte interface. The specifically adsorbed Fe(II) ions [SA-Fe(II)] greatly alleviated the electrostatic repulsion around the interfaceresulting in a 10-fold lower in the adsorption-free energy of NO3- when compared to the case without SA-Fe(II). Meanwhile, the modulated interface accelerated the kinetic mass transfer process by 25 folds compared to the control. Under neutral conditions, a Faraday efficiency of 99.6%, a selectivity of 99%, and an extremely high NH3 yield rate of 485.8 mmol h-1 g-1 FeOOH were achieved. Theoretical calculations and in-situ Raman spectroscopy confirmed the electron-rich state of the SA-Fe(II) donated to p orbitals of N atom and favored the hydrogenation of *NO to *NOH for promoting the formation of high-selectivity ammonia. In sum, these findings complement the textbook on the specific adsorption of cations and provide insights into the design of low-cost NRA catalysts with efficient ammonia synthesis.

Specific Cation Adsorption: Exploring Synergistic Effects on CO2 Electroreduction in Ionic Liquids

AbstractIonic liquids (ILs) have gained attention as promising solvents as reaction media for the electrochemical conversion of CO2 into valued‐added products. ILs can co‐catalyze the CO2 reduction reaction (CO2RR), in particular, when Ag is applied as the primary metallic electrocatalyst. To broaden this co‐catalysis concept, we conducted a comprehensive study of a range of electrode materials exposed to three ILs, which differed in the chemical nature of their anions and cations. Here, we examine the activity of the electrode/IL combinations toward the CO2RR based on cyclic voltammetry data. The voltammetric experiments are complemented by gas chromatography analysis of the products, demonstrating that CO is the main product for most of the electrode/IL combinations. Our results demonstrate that the specific adsorption of the IL cation on the electrode plays a crucial role in the co‐catalytic effect of the ILs. Based on our analysis, we identify three classes of electrode materials, which differ in their synergistic interaction with the IL cations and their resulting CO2RR activity. The key criterion for this classification is the extent to which the CO2RR is superimposed on the reductive decomposition of the ILs.

Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum

AbstractUnderstanding the pH dependent shift of the oxidation peak of the underpotential deposited hydrogen (Hupd) in cyclic voltammograms on the Pt surface is of significance in terms of both the fundamentals of electrochemistry and the rational design of catalysts for the hydrogen oxidation/evolution reactions (HOR/HER). In this work, we provide compelling evidence that the pH dependent shift in the Hupdpeak on Pt surfaces is driven by the structure of interfacial water rather than the specific adsorption of cations on the electrode surface. Combined cyclic voltammetric and surface enhanced spectroscopic investigations using an organic cation and crown‐ether chelated alkali metal cations show that specific adsorption of metal and organic cations on the Pt surface at the conditions relevant to the HOR/HER is unlikely. The vibrational band corresponding to strongly bound water is monitored when the electrode potential is varied in the Hupdrange in both acid and base.

Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum.

Understanding the pH dependent shift of the oxidation peak of the underpotential deposited hydrogen (Hupd ) in cyclic voltammograms on the Pt surface is of significance in terms of both the fundamentals of electrochemistry and the rational design of catalysts for the hydrogen oxidation/evolution reactions (HOR/HER). In this work, we provide compelling evidence that the pH dependent shift in the Hupd peak on Pt surfaces is driven by the structure of interfacial water rather than the specific adsorption of cations on the electrode surface. Combined cyclic voltammetric and surface enhanced spectroscopic investigations using an organic cation and crown-ether chelated alkali metal cations show that specific adsorption of metal and organic cations on the Pt surface at the conditions relevant to the HOR/HER is unlikely. The vibrational band corresponding to strongly bound water is monitored when the electrode potential is varied in the Hupd range in both acid and base.

Behavior of TiO2 and CeO2 Nanoparticles and Polystyrene Nanoplastics in Bottled Mineral, Drinking and Lake Geneva Waters. Impact of Water Hardness and Natural Organic Matter on Nanoparticle Surface Properties and Aggregation

Intensive use of engineered nanoparticles (NPs) in daily products ineluctably results in their release into aquatic systems and consequently into drinking water resources. Therefore, understanding NPs behavior in various waters from naturel to mineral waters is crucial for risk assessment evaluation and the efficient removal of NPs during the drinking water treatment process. In this study, the impact of relevant physicochemical parameters, such as pH, water hardness, and presence of natural organic matter (NOM) on the surface charge properties and aggregation abilities of both NPs and nanoplastic particles is investigated. TiO2, CeO2, and Polystyrene (PS) nanoplastics are selected, owing to their large number applications and contrasting characteristics at environmental pH. Experiments are performed in different water samples, including, ultrapure water, three bottled mineral waters, Lake Geneva, and drinking water produced from Lake Geneva. Our findings demonstrate that both water hardness and negatively charged natural organic matter concentrations, which were measured via dissolved organic carbon determination, are playing important roles. At environmental pH, when negatively charged nanoparticles are considered, specific cation adsorption is promoting aggregation so long as NOM concentration is limited. On the other hand, NOM adsorption is expected to be a key process in NPs destabilization when positively charged PS nanoplastics are considered.

Combined Sum Frequency Generation and Thin Liquid Film Study of the Specific Effect of Monovalent Cations on the Interfacial Water Structure.

Some salts have been recently shown to decrease the sum frequency generation (SFG) intensity of the hydrogen-bonded water molecules, but a quantitative explanation is still awaited. Here, we report a similar trend for the chloride salts of monovalent cations, that is, LiCl, NaCl, and CsCl, at low concentrations. Specifically, we revealed not only the specific adsorption of cations at the water surface but also the concentration-dependent effect of ions on the SFG response of the interfacial water molecules. Our thin-film pressure balance (TFPB) measurements (stabilized by 10 mM of methyl isobutyl carbinol) enabled the determination of the surface potential that governs the surface electric field affecting interfacial water dipoles. The use of the special alcohol also enabled us to identify a remarkable specific screening effect of cations on the surface potential. We explained the concentration dependency by considering the direct ion-water interactions and water reorientation under the influence of surface electric field as the two main contributors to the overall SFG signal of the hydrogen-bonded water molecules. Although the former was dominant only at the low-concentration range, the effect of the latter intensified with increasing salt concentration, leading to the recovery of the band intensity at medium concentrations. We discussed the likelihood of a correlation between the effect of ions on reorientation dynamics of water molecules and the broad-band intensity drop in the SFG spectra of salt solutions. We proposed a mechanism for the cation-specific effect through the formation of an ionic capacitance at the solution surface. It explains how cations could impart the ion specificity while they are traditionally believed to be repelled from the interfacial region. The electrical potential of this capacitance varies with the charge separation and ion density at the interface. The charge separation being controlled by the polarizability difference between anions and cations was identified using the SFG response of the interfacial water molecules as an indirect probe. The ion density being affected by the absolute polarizability of ions was tracked through the measurement of the surface potentials and Debye-Hückel lengths using the TFPB technique.

Improvement of calcium mineral separation contrast using anionic reagents: electrokinetics properties and flotation

The flotation separation of salt type calcium minerals is problematic, due to the similarities in their same active Ca2+ related site for interaction with anionic collectors and similar physicochemical characteristics such as solubility, zero-point charge, surface speciation and Ca-site density. The work was performed to achieve effective and selective separation of the calcium-minerals using pure minerals samples: orange calcite with Mg impurities, optic calcite with impurities level and an apatite. The pure samples surface was examined using techniques sensitive near-surface like infrared spectroscopy (FTIR) and chemical composition was obtained by ICPMS. The isoelectric point (IEP) and point of zero charge (PZC) in electrolyte were recorded using electrophoresis method at different ionic strengths of the solution. Mechanisms of charge development at the mineral-water interface are discussed. The time of contact as important parameter for the charge equilibrium was deduced from kinetics study and fixed to 30 minutes. The difference in the values obtained between IEP and PZSE can be explained by the presence of a specific adsorption of cations and anions on the surface. The effect of pure anionic collectors such as oleic and linoleic acid were studied. At low pH, both collectors lead to a good recovery for the calcites. The flotation recovery of optic calcite at pH 9 with sodium oleate is higher than with sodium linoleate. At alkaline pH, apatite showed a better recovery with sodium linoleate.

Impact of environmental conditions on aggregation kinetics of hematite and goethite nanoparticles

Hematite and goethite nanoparticles were used as model minerals to investigate their aggregation kinetics under soil environmental conditions in the present study. The hydrodynamic diameters of hematite and goethite nanoparticles were 34.4 and 66.3 nm, respectively. The positive surface charges and zeta potential values for goethite were higher than for hematite. The effective diameter for goethite was much larger than for hematite due to anisotropic sticking of needle-shaped goethite during aggregation. Moreover, the critical coagulation concentration (CCC) values of nanoparticles in solutions of NaNO3, NaCl, NaF, and Na2SO4 were 79.2, 75.0, 7.8, and 0.5 mM for hematite and they were 54.7, 62.6, 5.5, and 0.2 mM for goethite, respectively. The disparity of anions in inducing hematite or goethite aggregation lay in the differences in interfacial interactions. NO3 − and Cl− could decrease the zeta potential and enhance aggregation mainly through increasing ionic strength and compressing electric double layers of hematite and goethite nanoparticles. F− and SO4 2− highly destabilized the suspensions of nanoparticles mainly through specific adsorption and then neutralizing the positive surface charges of nanoparticles. Specific adsorption of cations could increase positive surface charges and stabilize hematite and goethite nanoparticles. The Hamaker constants of hematite and goethite nanoparticles were calculated to be 2.87 × 10−20 and 2.29 × 10−20 J−1, respectively. The predicted CCC values based on DLVO theory were consistent well with the experimentally determined CCC values in NaNO3, NaCl, NaF, and Na2SO4 systems, which demonstrated that DLVO theory could successfully predict the aggregation kinetics even when specific adsorption of ions occurred.

Quantification of permanent and variable charges in reference soils of the State of Pernambuco, Brazil

The electrical charges in soil particles are divided into structural or permanent charges and variable charges. Permanent charges develop on the soil particle surface by isomorphic substitution. Variable charges arise from dissociation and association of protons (H+), protonation or deprotonation, and specific adsorption of cations and anions. The aim of this study was to quantify the permanent charges and variable charges of Reference Soils of the State of Pernambuco, Brazil. To do so, 24 subsurface profiles from different regions (nine in the Zona da Mata, eight in the Agreste, and seven in the Sertão) were sampled, representing approximately 80 % of the total area of the state. Measurements were performed using cesium chloride solution. Determination was made of the permanent charges and the charges in regard to the hydroxyl functional groups through selective ion exchange of Cs+ by Li+ and Cs+ by NH4+, respectively. All the soils analyzed exhibited variable cation exchange capacity, with proportions from 0.16 to 0.60 and an average of 0.40 when related to total cation exchange capacity.

The Effect of Specific Adsorption of Cations and Their Size on the Charge-Compensation Mechanism in Carbon Micropores: The Role of Anion Desorption

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The Effect of Specific Adsorption of Cations and Their Size on the Charge-Compensation Mechanism in Carbon Micropores: The Role of Anion Desorption

Combined application of cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) technique reveals a complicated interplay between the adsorption of ammonium and lower molecular weight tetraalkyl ammonium cations and desorption of Cl(-) anions inside carbon micropores at low surface charge densities, which results in failure of their permselectivity. Higher negative surface charge densities induce complete exclusion (desorption) of the Cl(-) co-ions, which imparts purely permselective behavior on the carbon micropores. The second fundamental effect discovered herein relates to the dominant role of anion desorption (as compared to cation adsorption), that is, overwhelming failure of permselectivity extends to high negative charge densities of the electrode in the presence of bulky tetraalkyl ammonium cations, which tend to be confined in the micropores of the carbon. The results obtained are important for advancement of high power density carbon-based supercapacitors, nanofiltration technologies with porous carbon membranes, and studies of ionic transport across biological membranes.

Electrical double-layer interaction between oppositely charged dissimilar oxide surfaces with charge regulation and Stern–Grahame layers

The force of double-layer interaction between dissimilar flat oxide surfaces charged oppositely at infinite separation and with Stern–Grahame layers was calculated as a function of the surface separation under the conditions of charge regulation. The interaction force showed a monotonic attraction with respect to the surface separation, lying between constant surface potential and constant surface charge conditions. The variations of surface potentials and surface charge densities of respective double layers were also presented as functions of the surface separation. When the negative diffuse-layer potential at infinite separation changed its sign to positive as a result of specific adsorption of cations on the negatively changed surface, the double-layer interaction force showed a repulsive force barrier followed by an attraction at some particular separation.

Role of Multivalent Cations in the Self-Assembly of Phospholipid−DNA Complexes

In view of efficient and nontoxic delivery of genes to cells, complexes made of phospholipids (noncationic) and DNA are assembled through the mediation of multivalent cations. The association of lipids with DNA is explained through the charge reversal of lipid headgroups by specific adsorption of cations. The ion binding is quantified by the Gouy-Chapman-Stern theory which provides a good estimate for the minimal concentration of cations required to produce complexes. Coarse-grained Monte Carlo calculations support X-ray diffraction experiments in the sense that lipids form inverted micelles around hexagonally arranged DNA rods, with cations in between to maintain the cohesion. The complexes are more cohesive in terms of total free energy as the cation valence increases. The presented methodology may help develop predictive models for biomolecular self-assembled systems.

Analysis of the Interfacial Properties of Fibrillated and Nonfibrillated Oral Streptococcal Strains from Electrophoretic Mobility and Titration Measurements: Evidence for the Shortcomings of the ‘Classical Soft-Particle Approach'

Chemical and structural intricacies of bacterial cells complicate the quantitative evaluation of the physicochemical properties pertaining to the cell surface. The presence of various types of cell surface appendages has a large impact on those properties and therefore on various interfacial phenomena, such as aggregation and adhesion. In this paper, an advanced analysis of the electrophoretic mobilities of fibrillated and nonfibrillated strains (Streptococcus salivarius HB and Streptococcus salivarius HB-C12, respectively) is performed over a wide range of pH and ionic strength conditions on the basis of a recent electrokinetic theory for soft particles. The latter extends the approximate formalism originally developed by Ohshima by solving rigorously the fundamental electrokinetic equations without restrictions on the bacterial size, charge, and double layer thickness. It further allows (i) a straightforward implementation of the dissociation characteristics, as evaluated from titration experiments, of the ionogenic charged groups distributed throughout the bacterial cell wall and/or the surrounding exopolymer layer and (ii) the inclusion of possible specific interactions between the charged groups and ions from the background electrolyte other than charge-determining ions. The theory also enables an estimation of possible swelling/shrinking processes operating on the outer polymeric layer of the bacterium. Application of the electrokinetic model to HB and HB-C12 clearly shows a significant discrepancy between the amount of surface charges probed by electrophoresis and by protolytic titration. This is ascribed to the specific adsorption of cations onto pristine charged sites in the cell wall. Physicochemical parameters pertaining to the hydrodynamics (softness degree) and electrostatics of the bacterial cell wall (HB-C12) and soft polymeric layer (HB) are quantitatively derived.

Effect of the surface conductance on electrokinetic potential calculated from electrophoretic mobilities of the diamond particles in ethanol electrolyte solutions

Effect of the surface conductance on the ζ -potential of synthetic diamond particles (with sizes of 0.5–1 μm) determined from their electrophoretic mobility in ethanol solutions of 1:1 electrolytes over the range C =10 −5 –10 −2 M was studied. The conductivity of dilute diamond suspensions at the variable particle volume fraction was measured and the dependencies of surface conductivity ( K s ) on the electrolyte concentration were calculated by expression of the Wagner theory. It was found that K s has excessively high values, continually increasing with the growth of C and having high non-zero values at the points of a reversal of ζ -potential sign to positive, found at C =5×10 −4 M CsBr and C =10 −2 M NaClO 4 . Such character of the dependency K s (log C ) is explained by the preferable specific adsorption of cations and the main contribution of Stern layer to the surface conductance. It was found that the plots ζ (log C ), derived using Smoluchowski's equation, show a weak dependencies on C in the whole concentration range and disconcerting maxima. Incorporation of the experimental K s values in the Henry's equation brings to a linear character of the corresponding plots ζ (log C ) with the slope 20–25 mV per decade.

Electrokinetic study of specific adsorption of cations on synthetic goethite

The sign of the electrokinetic potential of goethite in the presence of 10−4 mol dm−3 of cetyltrimethylammonium bromide is positive over the entire pH range. The same concentration of La cations induces a shift in the IEP of goethite to high pH by one pH unit. The influence of divalent metal cations (Zn, Ca) on the IEP of goethite is negligible. Probably the adsorption of one divalent metal cation on goethite is accompanied by release of two protons. Thus, the net electric charge within the slipping plane is not affected by the adsorption and this explains the absence of a shift in the IEP.

Electrokinetic Effects in Poly(ethylene glycol)-Coated Capillaries Induced by Specific Adsorption of Cations

Capillaries coated with poly(ethylene glycol) (PEG) exhibit unique electrokinetic properties because of the specific adsorption of cations at the solid/liquid interface. The adsorption of cations present in methanol- or acetonitrile-containing electrolytes can induce a positive surface charge on PEG-coated capillaries. This results in an adjustable anodic electroosmotic flow (EOF) in nonaqueous electrolytes, whereas a reduced cathodic EOF is observed in aqueous electrolytes. The EOF is dependent on the electrolyte constitution, namely, on the type of solvent used and especially on the abilities of the background cations to interact with polyethers such as PEG. The dependency of the EOF on the electrolyte concentration in nonaqueous electrolytes can be explained by two counterbalancing mechanisms: (i) the increase in surface charge density and (ii) the decrease in double-layer thickness. Using different alkali metal and alkaline earth metal cations dissolved in methanol the EOF can be varied over a wide range. The magnitude of the induced anodic EOF was found to vary in the order Ba2+ > Ca2+ > Mg2+ for alkaline earth metal ions and K+ > Cs+ > Na+ for alkali metal ions, whereas with lithium ions, a cathodic EOF was observed. The magnitude of the anodic EOF of PEG-coated capillaries in nonaqueous electrolytes was found to be dependent on the coating thickness.

Specific cation adsorption on protein-covered particles and its influence on colloidal stability

Protein coated particles present an anomalous colloidal stability at high ionic strength when the classical theory (DLVO) predicts aggregation. This observed deviation from DLVO behaviour appears for electrolyte concentrations above some critical bulk value. As we have suggested in previous publications the existence of an additional short-range repulsive ‘hydration force’ due to specific hydrated cation adsorption could explain this anomalous stability. The overlap of the hydration layers when two particles approach should provoke this repulsive force. New evidence of this mechanism has been observed when electrophoretic mobilities of protein-carrying latex particles were measured at various concentrations of sodium and calcium chloride. In the latter case a sign reversal of zeta-potential was found, probably due to the specific adsorption of Ca 2+ ions on protein molecules. The adsorption increases with the medium pH. These results have been analyzed following the treatment proposed by Ohshima and co-workers for large charged colloidal particles coated with a layer of protein. This study shows an increase in the positive fixed-charge density on the protein caused by the adsorption of cations.

Cooperative Effect of Adsorbed Cations and Iodide on the Interception of Back Electron Transfer in the Dye Sensitization of Nanocrystalline TiO2

Specific adsorption of cations (H+, Li+, ...) on TiO2 nanocrystalline particles is known to control the energetics of the conduction band and therefore the ability for molecular sensitizers to inject electrons into the semiconductor upon irradiation. In photoelectrochemical energy conversion devices employing dye-sensitized titanium dioxide mesoporous electrodes, back electron transfer is generally intercepted by the use of the iodide/triiodide couple as a charge mediator. Kinetics of the oxidation of I- by the oxidized state of cis-RuII(dcbpy)2(NCS)2 sensitizer adsorbed on TiO2 was measured by flash photolysis in propylene carbonate. The rate of this reaction was found to depend on the nature and concentration of added cations such as Mg2+, Li+, Na+, and K+. A brusque acceleration of the process was in particular observed at a critical concentration. Electrophoretic measurements showed that this step in the dye regeneration reaction kinetics corresponds to the reversal of particle surface charge upon adsorption of potential-determining species, which causes I- to efficiently adsorb onto the oxide. These observations strongly suggest that the specific adsorption of cations on TiO2 nanoparticles governs the formation of (I-, I-) ion pairs on the surface, and allows the more energetically favorable and faster mechanism involving oxidation of I- to I2•- radical to take place.