The structure of the electrochemical double layer: Ag(111) in alkaline electrolyte

The structure of the electrochemical double layer at the interface between a Ag(111) electrode and 0.1 M KOH electrolyte has been probed using in-situ surface X-ray scattering (SXS). Detailed modeling of the SXS data at negative potential (E = − 1.0 V versus SCE) is consistent with the presence of an hydrated K⁺ cation layer at a distance of 4.1 ± 0.3 Å from the Ag surface and at positive potential (E = − 0.2 V), indicates that the presence of OH_ad stabilizes the hydrated K⁺ cations through a non-covalent interaction forming a compact double layer structure in which the Ag-K⁺ distance is reduced to 3.6 ± 0.2 Å.     

Electrosurface Properties of Schungite III Particles in Aqueous Electrolyte Solutions

The kinetics of electrolyte extraction into water and the electrosurface properties (adsorption of potential-determining ions H⁺ and OH⁻ and ζ potential) of five fractions of schungite III (particle sizes of < 5, 50–100, 160–400, 400–1000, and 1600–2500 µm) are studied in aqueous NaCl, CaCl₂, and AlCl₃ solutions at different pH values. It is shown that, in water and NaCl and CaCl₂ solutions, the point of zero charge (PZC) of the particles with sizes of 50–100 and 160–400 µm is observed at pH 4.0 and is independent of the electrolyte concentration. The isoelectric point (IEP) for small (<5 µm) schungite III particles is observed at pH 2.8. The IEP position is independent of CaCl₂ concentration, but it shifts to pH 2.4 when NaCl concentration increases to 0.1 M. The disclosed differences in the PZC and IEP values may be caused by different compositions of particles of different fractions. In a 10⁻⁵ M AlCl₃ solution, schungite particles demonstrate three IEPs (pH 3.0, 4.5, and 7.4) due to different degrees of AlCl₃ hydrolysis at different pH values.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 4, 2005, pp. 450–457.Original Russian Text Copyright © 2005 by Aleinikov, Lorentsson, Chernoberezhskii.     

Preparation of ultra-and nanoporous glasses and study of their structural and electrokinetic characteristics in 1: 1 electrolyte solutions

Nano-and ultraporous glass membranes with pore radii of 4.5–150 nm are prepared from sodium borosilicate glasses of various compositions. Structural parameters (structure resistance coefficient, volume porosity, and filtration factor) and electrokinetic characteristics (conductivity, counterion transport numbers, and electrokinetic potential ζ _α ^* ) of membranes are determined at various KCl and NaCl solution concentrations (10^?4?10^?1 M) in a neutral pH region. The passage from nano-to ultraporous glasses is accompanied by an increase in |ζ _α ^* | values, which is apparently related to a decrease in the thickness of a gel layer due to the removal of ion-permeable secondary silica from pore channels. The comparison of electrokinetic characteristics of glass membranes (counterion transport numbers, efficiency coefficients, and electrokinetic potentials) measured in NaCl and KCl solutions indicates a higher specificity of K⁺ counterions as compared to Na⁺ ions.     

Counterion effects on the isotopic properties of the Cl<Superscript>−</Superscript> and Li<Superscript>+</Superscript> ions in aqueous solutions

The reduced partition function ratios between isotopic forms (β-factors) were calculated by the ab initio RHF/6-311++G**(3df, 3p) and MP2/6-311++G**(3df, 3p) quantum-chemical methods for hydrated chloride ion and ion pair hydrates NaCl·nH₂O and LiOH · nH₂O. The influence of the Na⁺ cation on the β-factor value and the chlorine isotope separation factor in the precipitation of NaCl from concentrated aqueous solutions was found to be substantial. At the same time, the presence of OH^? counterions had no noticeable effect on the β-factor of the hydrated Li⁺ cation.     

Proton Transfer Accompanied by the Oxidation of Adenosine

Despite numerous experimental and theoretical studies, the proton transfer accompanying the oxidation of 2′-deoxyadenosine 5′-monophosphate 2’-deoxyadenosine 5’-monophosphate (5’-dAMP, A ) is still under debate. To address this issue, we have investigated the oxidation of A in acidic and neutral solutions by using transient absorption (TA) and time-resolved resonance Raman (TR³) spectroscopic methods in combination with pulse radiolysis. The steady-state Raman signal of A was significantly affected by the solution pH, but not by the concentration of adenosine (2–50 mm ). More specifically, the A in acidic and neutral solutions exists in its protonated ( A H⁺(N1+H⁺)) and neutral ( A ) forms, respectively. On the one hand, the TA spectral changes observed at neutral pH revealed that the radical cation ( A ^.+) generated by pulse radiolysis is rapidly converted into A ^.(N6−H) through the loss of an imino proton from N6. In contrast, at acidic pH (<4), A H^.2+(N1+H⁺) generated by pulse radiolysis of A H⁺(N1+H⁺) does not undergo the deprotonation process owing to the pK_a value of A H^.2+(N1+H⁺), which is higher than the solution pH. Furthermore, the results presented in this study have demonstrated that A , A H⁺(N1+H⁺), and their radical species exist as monomers in the concentration range of 2–50 mm . Compared with the Raman bands of A H⁺(N1+H⁺), the TR³ bands of A H^.2+(N1+H⁺) are significantly down-shifted, indicating a decrease in the bond order of the pyrimidine and imidazole rings due to the resonance structure of A H^.2+(N1+H⁺). Meanwhile, A ^.(N6−H) does not show a Raman band corresponding to the pyrimidine+NH₂ scissoring vibration due to diprotonation at the N6 position. These results support the final products generated by the oxidation of adenosine in acidic and neutral solutions being A H^.2+(N1+H⁺) and A ^.(N6−H), respectively.     

Electrosurface properties of microcrystalline cellulose dispersions in aqueous solutions of aluminum chloride, nitrate, and sulfate

The method of microelectrophoresis is employed to study the dependence of the ζ-potential of microcrystalline cellulose particles on the concentration (10^?6–10^?3 M) and pH (2–11) of aqueous aluminum chloride, nitrate, and sulfate solutions. It is shown that, in the absence of aluminum salts, the isoelectric point (IEP) of the particles is independent on the nature of acid anions and is observed at pH 3.2. The addition of aluminum salts in concentrations as low as 2 × 10 ^?6 M for chloride and nitrate and 1 × 10^?5 M for sulfate causes a shift of IEP to a less acidic region (pH 3.8), the value of which is virtually independent of the nature of the salt. As the concentration of salts is increased, the ζ-potential becomes positive, rises with an increase in pH to a maximum magnitude at pH 5.0–6.0, and decreases further until the second IEP (pH ～ 6.5–7.0) is reached. At higher pH values, the ζ-potential becomes negative again. The observed ζ(pH) dependences are explained by the formation of hydrolyzed aluminum species exhibiting different adsorbabilities on microcrystalline cellulose particles. It is shown that positively charged hydroxocomplexes formed in aluminum sulfate solutions are characterized by a lower adsorbability than hydroxocomplexes formed in chloride and nitrate solutions.     

Interfacial Capacitance of Graphene Oxide Films Electrodes: Fundamental Studies on Electrolytes Interface Aiming (Bio)Sensing Applications

The understanding of bidimensional materials dynamics and its electrolyte interface equilibrium, such as graphene oxide (GO), is critical for the development of a capacitive biosensing platform. The interfacial capacitance (C_i) of graphene-based materials may be tuned by experimental conditions such as pH optimization and cation size playing key roles at the enhancement of their capacitive properties allowing their application as novel capacitive biosensors. Here we reported a systematic study of C_i of multilayer GO films in different aqueous electrolytes employing electrochemical impedance spectroscopy for the application in a capacitive detection system. We demonstrated that the presence of ionizable oxygen-containing functional groups within multilayer GO film favors the interactions and the accumulation of cations in the structure of the electrodes enhancing the GO C_i in aqueous solutions, where at pH 7.0 (the best condition) the C_i was 340 μF mg⁻¹ at −0.01 V vs Ag/AgCl. We also established that the hydrated cation radius affects the mobility and interaction with GO functional groups and it plays a critical role in the Ci, as demonstrated in the presence of different cations Na⁺=640 μF mg⁻¹, Li⁺=575 μF mg⁻¹ and TMA⁺=477 μF mg⁻¹. As a proof-of-concept, the capacitive behaviour of GO was explored as biosensing platform for standard streptavidin-biotin systems. For this system, the C_i varied linearly with the log of the concentration of the targeting analyte in the range from 10 pg mL⁻¹ to 100 ng mL⁻¹, showing the promising applicability of capacitive GO based sensors for label-free biosensing.     

QM and QM/MM umbrella sampling MD study of the formation of Hg(II)–thymine bond: Model for evaluation of the reaction energy profiles in solutions with constant pH

The formation of the Hg–N3(T) bond between the 1-methylthymine (T) molecule and the hydrated Hg²⁺ cation was explored with the combined quantum mechanics/molecular mechanics (QM/MM) method including Free Energy Perturbation corrections. The thermodynamic properties were determined in the whole pH range, when these systems were explicitly investigated and considered as the QM part: (1) T + [Hg(H₂O)₆]²⁺, (2) T + [Hg(H₂O)₅(OH)]⁺, (3) T + Hg(H₂O)₄(OH)₂, and (4) N3-deprotonated T + Hg(H₂O)₄(OH)₂. The MM part contained only solvent molecules and counterions. As a result, the dependence of Gibbs-Alberty reaction free energy on pH was obtained along the reaction coordinate. We found that an endoergic reaction in acidic condition up to pH < 4–5 becomes exoergic for a higher pH corresponding to neutral and basic solutions. The migration of the Hg²⁺ cation between N3 and O4/2 positions in dependence on pH is discussed as well. For the verification, DFT calculations of stationary points were performed confirming the qualitative trends of QM/MM MD simulations and NMR parameters were determined for them.     

Computer Simulation of Water Clusterization on Chlorine Ions: 2. Microstructure

The molecular structure of Cl^–(H₂O) _n clusters, n = 1–60, in equilibrium with vapor, and the cluster with n = 500 was studied by the Monte Carlo method. The first hydrated layer of a cluster is formed in unsaturated water vapors. The second hydrated layer begins to be formed in saturated vapor. The position of hydrated layers is not changed with an increase in cluster size and coincides with the position of the hydrated layers of ions in aqueous solutions of weak electrolytes. Orientational order in a cluster also has the layered structure. The orientation of molecules between the layers is random. The stability of the first layer is ensured only due to direct interactions with ions, whereas the stability of subsequent layers is due to cooperative interactions between molecules and between molecules and ions. As temperature decreases, the effect of ion displacement to the cluster surface becomes stronger.     

Characterization of the transient species generated by the photoionization of Berberine: A laser flash photolysis study

Using 266 nm laser flash photolysis it has been demonstrated that Berberine (BBR) in aqueous solution is ionized via a mono-photonic process giving a hydrated electron, anion radical that formed by hydrated electron react with steady state of BBR, and neutral radical that formed from rapid deprotonation of the radical cation of BBR. The quantum yield of photoionization is determined to be 0.03 at room temperature with KI solution used as a reference. Furthermore utilizing pH changing method and the SO₄⁻ radical oxidation method, the assignment of radical cation of BBR was further confirmed, the pK_a value of it was calculated, and the related set up rate constant was also determined.     

Indenyl and fluorenyl transition metal complexes : V. Study of reversible isomerization of η6- and η5-fluorenylchromium tricarbonyl anions by IR and PMR spectroscopy

IR and PMR spectroscopy was used to study the reversible isomerization of η⁶- and η⁵-fluorenylchromium tricarbonyl anions (I and II, respectively) in solutions. Consideration was given to the dependence of the equilibrium position and nature of the resulting particles on the type of cation (Li⁺, Na⁺, K⁺, Ph₃PCH₃⁺, n-Bu₄N⁺), the solvent and the presence of solvating cation additives (dicyclohexyl-18-crown-6 (IV), DMSO). The η⁵-isomer exists in THF solution as an equilibrium mixture of solvent-separated and contact ion pairs (SSIP and CIP, respectively), the cation in the latter being located near the oxygen atom of one of the carbonyl groups. The existence of such an equilibrium is also shown for the salts of the C₅H₅(CO)₃Cr^? anion. The dependence of the equilibrium constants for I ? II on the above factors was determined. For the K salt in THF solution at 28°C both IR and PMR spectroscopy gave fairly coincident values of K_eq equal to 7.15 and 7.35, respectively. IR spectroscopy was applied to the kinetic studies of the reversible isomerization of K salts of I and II at 0—28°C with and without crown ether IV. The process is shown to be entropy controlled. The equilibrium position is governed by the type of cation, the degree of its solvation and the temperature. The factors promoting the formation of SSIP's lead to the equilibrium being shifted towards I.     

Preparation of a cobalt hexacyanoferrate film-modified aluminum electrode by chemical and electrochemical methods: enhanced stability of the electrode in the presence of phosphate and ruthenium(III)

Modification of an aluminum electrode by means of a thin film of cobalt hexacyanoferrate (CoHCF) using electroless and electrochemical procedures is described. The modification conditions of the aluminum surface, including the electroless deposition of metallic cobalt on the electrode surface from CoCl₂+NaF solution and the chemical derivatization of the deposited cobalt to give a CoHCF film in 0.25 M KCl+0.25 M K₃[Fe(CN)₆] solution, have been determined. The modified Al electrodes prepared under optimum conditions show one or two well-defined redox couples in phosphate buffer solutions of pH 7.2, depending on the preparation procedure, due to the [Co^IIFe^III/II(CN)₆]^–/2– system. The effect of pH, alkali metal cations, and anions of the supporting electrolyte on the electrochemical characteristics of the modified electrode were studied. Diffusion coefficients of hydrated Na⁺ in the film, the transfer coefficient, and the transfer rate constant for electrons were determined. The stability of the modified electrodes under various experimental conditions was studied and their high stability in the sodium phosphate buffer solutions was confirmed. Enhanced stability was observed when the modified electrode was scanned in fresh solutions of RuCl₃ between 0 and 1 V for at least 20 cycles, due to the formation of mixed hexacyanoferrates of cobalt and ruthenium. Electronic Publication     

Capillary affinity electrophoresis and ab initio calculation studies of valinomycin complexation with Na+ ion

In a combined experimental and theoretical approach, the interactions of valinomycin (Val), macrocyclic depsipeptide antibiotic ionophore, with sodium cation Na⁺ have been investigated. The strength of the Val–Na⁺ complex was evaluated experimentally by means of capillary affinity electrophoresis. From the dependence of valinomycin effective electrophoretic mobility on the sodium ion concentration in the BGE (methanolic solution of 20 mM chloroacetic acid, 10 mM Tris, 0–40 mM NaCl), the apparent binding (stability) constant (K_b) of the Val–Na⁺ complex in methanol was evaluated as log K_b = 1.71 ± 0.16. Besides, using quantum mechanical density functional theory (DFT) calculations, the most probable structures of the nonhydrated Val–Na⁺ as well as hydrated Val–Na⁺·H₂O complex species were proposed. Compared to Val–Na⁺, the optimized structure of Val–Na⁺·H₂O complex appears to be more realistic as follows from the substantially higher binding energy (118.4 kcal/mol) of the hydrated complex than that of the nonhydrated complex (102.8 kcal/mol). In the hydrated complex, the central Na⁺ cation is bound by strong bonds to one oxygen atom of the respective water molecule and to four oxygens of the corresponding C=O groups of the parent valinomycin ligand.     

Density functional theory study of calix[4]arene‐N‐azacrown‐5, calix[4]arene‐N‐phenyl‐azacrown‐5, and their complexes with alkali‐metal cations: Na+, K+, and Rb+

Theoretical studies of 1,3‐alternate‐25,27‐bis(1‐methoxyethyl)calix[4]arene‐azacrown‐5 ( L₁ ), 1,3‐alternate‐25,27‐bis(1‐methoxyethyl)calix[4]arene‐N‐phenyl‐azacrown‐5 ( L₂ ), and the corresponding complexes M⁺/ L of L₁ and L₂ with the alkali‐metal cations: Na⁺, K⁺, and Rb⁺ have been performed using density functional theory (DFT) at B3LYP/6‐31G* level. The optimized geometric structures obtained from DFT calculations are used to perform natural bond orbital (NBO) analysis. The two main types of driving force metal–ligand and cation–π interactions are investigated. The results indicate that intermolecular electrostatic interactions are dominant and the electron‐donating oxygen offer lone pair electrons to the contacting RY* (1‐center Rydberg) or LP* (1‐center valence antibond lone pair) orbitals of M⁺ (Na⁺, K⁺, and Rb⁺). What's more, the cation–π interactions between the metal ion and π‐orbitals of the two rotated benzene rings play a minor role. For all the structures, the most pronounced changes in geometric parameters upon interaction are observed in the calix[4]arene molecule. In addition, an extra pendant phenyl group attached to nitrogen can promote metal complexation by 3D encapsulation greatly. In addition, the enthalpies of complexation reaction and hydrated cation exchange reaction had been studied by the calculated thermodynamic data. The calculated results of hydrated cation exchange reaction are in a good agreement with the experimental data for the complexes. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Complexation thermodynamics of some alkali-metal cations with 1,13-bis(8-quinolyl)-1,4,7,10,13-pentaoxatridecane in Acetonitrile

The interaction of 1,13-bis(8-quinolyl)-1,4,7,10,13-pentaoxatridecane (Kryptofix5) with alkali-metal cations (Li⁺, Na⁺, K⁺) in aprotic medium (acetonitrile) has been investigated. Conductance measurements demonstrated that 1:1 metal cation:ligand stoichiometries are found with these cations in this solvent. ⁷Li and ²³Na NMR experiments were carried out by titration of the metal cation solutions with Kryptofix5 solution in CD₃CN + CH₃CN at 298 K. Thermodynamic parameters of complexation for this ligand and alkali-metal cations in acetonitrile at 278–308 K were derived from titration conductometry. The highest stability is found for sodium complex. The complexation sequence, based on the value of log K at 278–308 K was found to be Na⁺ > K⁺ > Li⁺.     

Analytical Study of New Creatininium and Tetramethyl-ammonium Cation Selective Membrane Electrodes

Abstract

Two new liquid membrane electrodes which respond to creatininium and tetramethylammonium cations are described. The creatininium cation electrode exhibits rapid and near Nernstian response to creatininium cation activity, at pH 3, in the 10^?3?10^?1 mol/L range. The useful concentration range extends to 10^?4 mol/L. The tetramethyl-ammonium cation electrode exhibits rapid and near Nernstian response to tetramethylammonium cation activity, at pH 2–11.5, in the 2x10^?5? 10^?1 mol/L range. Major interferences for the creatininium electrode are Na⁺, K⁺, NH⁺ ₄ and creatine. The pK_a of the creatininium cation was calculated. A method is described for the potentiometric precipitation titration of tetramethylammonium cation with sodium tetraphenylboron. Amounts of tetramethylammonium in the range 20–200 μol have been determined using Gran's plots, with an average error of about 0.6%.     

Determination of the Structure and Effective Dielectric Constant of Hydrated Ions

Abstract

Debye's equation for the salting in or out of nonpolar compounds, such as benzene, in aqueous salt solutions was expanded so as to determine the effective dielectric decrement and constant of the hydrated domain of an ion. For ions having an electrostatic charge per surface area less than or equal to that of the K⁺ or Cl^? ions, this domain consists of a single layer of water molecules loosely or negatively hydrated to the ion; i.e., the domain consists of a mono-molecular B region. For ions having an electrostatic charge per unit surface area approximately equal to that of the Na⁺ and F^? ions, there exists no B region and only one layer of tightly bound or positively hydrated water (a monomolecular A region). Since the electrostatic field does not appreciably influence water molecules beyond this A region, such ions have an effective dielectric constant that is near zero, as in relatively inert molecules such as hydrocarbons. For all other ions, such as H⁺, Li⁺, Mg²⁺⁰,Cr²⁺, Sr²⁺, Ba²⁺, and other multivalent ions, there exists only one monomolecular A region followed by one monomolecular B region. The effective value of the dielectric constant of such an ion is obtained from its B region, since its A region cannot be penetrated. The effective dielectric decrement or constant of any B region as measured by benzene solubility goes through a maximum as the electrostatic charge per unit surface area (C/A) is decreased because a large C/A restricts the orientation of the hydrated water molecules and a low value of C/A allows competitive interaction between surrounding water molecules. Thus both small and large values of C/A decrease the solubility of benzene, i.e., decrease i t s ability to penetrate into the medium. A decrease in the macroscopic dielectric constant of water upon the addition of salt is due to the destruction of the clusters of water by the ions, or to the addition of ions which have effective dielectric constants less than that of water, or both. All hydrated ions o r molecules which salt-in or salt -out benzene have, respectfully, effective dielectric constants greater or less than that of water.     

Interaction between tetracycline and smectite in aqueous solution

The fate and transport of commonly used antibiotics in soil and groundwater have attracted renewed studies due to increased sensitivities of analytical instruments and thus frequent detections of these compounds even in treated wastewater. Smectite, an important soil component, has large surface area and high cation exchange capacity, while tetracycline (TC) can exist in different forms and charges under different pH conditions. Thus, the interaction between smectite and TC in aqueous systems is of great importance. This research focused on elucidating the mechanisms of TC uptake by smectite, in terms of TC adsorption, cation desorption, and pH changes associated with TC adsorption by smectite and intercalation in smectite. TC adsorption onto smectite was a relatively fast process even though most of the adsorption sites were in the interlayer position involved in intercalation as confirmed by the expansion of d₀₀₁ spacing. The TC adsorption capacity was equivalent to 0.74–1.11 times the cation exchange capacity for three of the four smectite minerals studied. Accompanying TC adsorption was simultaneous adsorption of H⁺, resulting in protonation of TC on the dimethylamine group. At higher TC input concentrations further adsorption of H⁺ resulted in the ratio of H⁺ adsorbed to TC adsorbed greater than one, suggesting that additionally adsorbed H⁺ could serve as counterions to partially offset the negative charges on the tricarbonyl or phenolic diketone functional groups. The positive correlations between cations desorbed and TC adsorbed, as well as TC adsorbed and H⁺ adsorbed, provided a first time evidence to confirm cation exchange as the main mechanism of TC uptake, even under neutral pH conditions.     

The adsorption of CaOH+ on (001) basal and (010) edge surface of Na‐montmorillonite: a DFT study

Ca²⁺ cations were generally added to facilitate the coagulation of stable fine clay mineral dispersion due to the specific adsorption of their first hydrolysis CaOH⁺ species at pH near 10. The adsorption of CaOH⁺ on dry and hydrated (001) basal surface and (010) surface of Na‐montmorillonite was investigated by using density functional theory method combined with the periodic slab model method. The adsorption energies and geometries, Mulliken charge, electron density difference, and density of state were presented and discussed. It was found that the adsorption energy of CaOH⁺ on (010) edge surface of Na‐montmorillonite (?328.8 kJ/mol) was much larger than that (?126.9 kJ/mol) on (001) basal surface. The presence of waters could increase the adsorption energy of CaOH⁺ on (001) surface but affect that on (010) surface slightly. The protons in Si–OH and Al–OH₂ groups as well as the OH₂ ligands in Al–OH₂ group on (010) edge surface were easily dissociated and coordinated to CaOH⁺ to form new waters. CaOH⁺ was the most steady adsorption species among CaOH⁺, Ca²⁺ cation, and H₂O molecule on both (001) and (010) surfaces. Copyright © 2016 John Wiley & Sons, Ltd.