The interfacial electrochemistry of goethite (α-FeOOH) especially the effect of CO2 contamination

The surface charge density/pH function of goethite is shown to be very similar to the surface charge density/surface potential functions reported for mercury. This finding differs from all other reports and we attribute the difference to the experimental method. This paper describes a method of titrating oxide suspensions which reduces pH drifts and suspension and stirring effects to negligible values and also a method for effectively removing CO₂ from oxide suspensions. When goethite which had been stored as a suspension in contact with air was titrated in N₂-purged NaI solution the point of zero charge (pzc) was within the range found by previous workers, i.e. pH 8.75 and the σ/pH function did not resemble reported σ/ψ₀ functions of mercury but when goethite from the same preparation was purged with N₂ for 2 months and then titrated in N₂-purged NaI the pzc was pH 9.3 and the σ/pH function did resemble the σ/ψ₀ function of mercury.     

Approximate analytic solution of the nonlinear Poisson–Boltzmann equation for spherical colloidal particles immersed in a general electrolyte solution

By means of polynomial approximation and iteration procedure using linearization solution as initial solution, the non-linear Poisson–Boltzmann equation describing a spherical colloidal particle immersed in an arbitrary valence and mixed electrolyte solution is solved analytically, and analytical expressions for electrical potential distribution ψ(r) and surface charge density/surface potential relationship (σ/ψ ₀) are acquired. The σ/ψ ₀ expression performs very well for the entire range of a reduced colloidal radius x ₀ only if a reduced surface potential y ₀ is lower than 15.5; particularly, in the case of x ₀ >10, the σ/ψ ₀ expression applies over the entire range of y ₀, and maximum of the absolute value of percent relative error (PRE) is not larger than 2 if one does not come across a parameter combination as of an extreme case of x ₀?<?0.9 and y ₀?>?15.5. The ψ(r) expression outperforms the linearization solution greatly and performs very well for the domain of x ₀?≤?1.5 and y ₀?≤?4.5 with maximum of the absolute value of PRE not larger than 3, whereas that of the linearization solution may stand 22 for the same parameter range. It is concluded that the present σ/ψ ₀ expression is the first one valid for entire range of x ₀, given that y ₀ is not unreasonably high, and the present paper proposes, to some extent, a “universal” way for solving nonlinear differential equations.     

The electrical double layer of micelles in ionic surfactant solutions in the presence of a background electrolyte: 2. Moderately concentrated micellar solutions of sodium dodecyl sulfate

To continue previous studies [1], a model is developed for calculating electrostatic potentials ¹ψ₀ and ¹ψ_d corresponding to the surface of micelle core and the Stern layer, respectively, and the degree ¹β of binding of counterions in moderately concentrated micellar solutions of ionic surfactants. By the example of sodium dodecyl sulfate micellar solutions, dependences of ¹ψ₀ and ¹ψ_d on overall concentration n varying over the range from 0.1 to 0.9 M are determined at different NaCl concentrations in the system. The absolute values of ¹ψ₀ and ¹ψ_d potentials are found to grow with a rise in the degree of micellization and a reduction in the average intermicellar distances. This growth is greatly pronounced at a low content of the background electrolyte, whereas, in concentrated NaCl solutions (of approximately 0.1 M), potentials ¹ψ₀ and ¹ψ_d remain practically constant. Dependence 1 ? ¹β(n) is monotonically decreasing function.     

Fine grained sampling of residue characteristics using molecular dynamics simulation

In a fine-grained computational analysis of protein structure, we investigated the relationships between a residue's backbone conformations and its side-chain packing as well as conformations. To produce continuous distributions in high resolution, we ran molecular dynamics simulations over a set of protein folds (dynameome). In effect, the dynameome dataset samples not only the states well represented in the PDB but also the known states that are not well represented in the structural database. In our analysis, we characterized the mutual influence among the backbone ?,ψ angles with the first side-chain torsion angles (χ₁) and the volumes occupied by the side-chains. The dependencies of these relationships on side-chain environment and amino acids are further explored. We found that residue volumes exhibit dependency on backbone 2° structure conformation: side-chains pack more densely in extended β-sheet than in α-helical structures. As expected, residue volumes on the protein surface were larger than those in the interior. The first side-chain torsion angles are found to be dependent on the backbone conformations in agreement with previous studies, but the dynameome dataset provides higher resolution of rotamer preferences based on the backbone conformation. All three gauche^?, gauche⁺, and trans rotamers show different patterns of ?,ψ dependency, and variations in χ₁ value are skewed from their canonical values to relieve the steric strains. By demonstrating the utility of dynameomic modeling on the native state ensemble, this study reveals details of the interplay among backbone conformations, residue volumes and side-chain conformations.     

Über den Einfluß der elektrostatischen Wechselwirkungen auf die α-Helixbildung von basischen Poly-[α-aminosäuren]

A theoretical treatment of the electrostatic interactions influencing the conformation of poly-[α-aminoacids] containing ionogenic side-chains is reported. Using the Gouy-Chapman-equation for the relation between the density of surface chargeσ _e and surfacepotentialψ the latter one will be determined on the surface of anα-helix of a basic poly-[α-aminoacid] in an aqueous electrolyte solution. As it could be shown,ψ decreases with the electrolyte concentrationc. The equilibrium constants for the propagation step of theα-helix-formation is expressed as a function ofψ and a parameterK ₀ containingΔS₀ andΔH₀ dependent onc, considering the electrostatic energy of neighboured basic groups. The temperature at the mid-point of the transitionT _m is a function of the reciproce ionic strengthI. In a solution of an 1-1-electrolyte,T _m depends linearly on 1/√c. The experimental data for Poly-[L-lysine] are in a very good agreement with the straight line given by this relation. However, in the case of a 1∶1 copolymer ofL-lysine andL-leucine the calculated values are much lower than the measured ones, obviously due to neglecting the hydrophobic interactions of the leucine side chains. Furthermore, a relation for the change of theα-helix-contentf _H with 1/T is given. In this case, in the well-known relation (?f _H/? (1/T))=?ΔH⁰/4Rσ ^1/2, the cooperative parameterσ is substituted by a modified parameterΣ depending on the ionic strength. According to this equation and agreeing with the experimental results,T _m increases with the ionic strength, because the electrostatic potentialψ at theα-helix-surface decreases. However, in contrast to reality, the inclination of thef _H-T-curves increases withI orc, respectively. This may be due to an increase ofσ with the electrolyte concentration.     

Experimental evaluation of single ion activities

When an ion-exchange membrane separates two electrolyte solutions having two different co-ions and a common counterion a bi-ionic potential (bi-co-ionic potential) appears across the membrane. If the membrane is ideally permselective for counterions and the activities of counterions on both sides of the membrane are equal to each other, the membrane potential Δψ becomes zero. We selected KCl as a reference electrolyte because of a symmetrical electrolyte in aqueous solutions. Then, the mean activity of ions in KCl may be assumed to be equal to each single ion activity at low molalities. Single ion activity in a test electrolyte containing K⁺ ion or Cl⁻ ion was estimated from the molality of KCl at Δψ=0 by interpolating bi-co-ionic potential to zero for KCl/membrane/LiCl, NaCl, C₆H₅COOK, or p-CH₂CHC₆H₄SO₃K systems. The values of single ion activity coefficients estimated in this work were fairly different not only from the mean activity coefficients of ions but also from the single ion activity coefficients estimated by Debye–Hückel formula.     

Determination of formal charge transfer coefficients in adsorption processes in the system Hg,Tl, Tl+, H2O: II. Electrocapillary curves of the second kind

The determination of the dependence of the formal charge transfer coefficient n₂ on the amalgam solution potential difference ψ from the equilibrium electrocapillary curves of the 2nd kind (taken under the condition μ_Tl=const) shows that there exists a discontinuity on the n₂, ψ curve, i.e. a transition from n₂=∞ to n₂=?∞ with increasing ?ψ. The cause of this anomaly is the existence of a minimum on the surface tension of an uncharged amalgam surface vs. μ_Tl curve, which is observed in the case of the Tl-amalgam/vacuum interface as well.     

Use of a size-consistent energy functional in many electron theory for closed shells

If the ground state wave-function ψ_gr is written as ψ_gr = Φ_o+X, withX as the correlation part satisfying (φ_o¦x) = 0, andx expressed as an expansion in terms of pair, pair-pair etc. cluster functions, then the expectation value of the energyE = (ψ_gr¦H¦ψ_gr)/(ψ_gr¦ψ_gr) has the property that the normalization term in the denominator completely cancels the unlinked part of the numerator, as noted by Sinanoglu. We use Cizek's coupled-pair ansatz ψ_gr = exp(T ₂)Φ₀ for transcribing Sinanoglu's expansion in a many-body language to study the behaviour of the size-consistent (linked) energy functional thus generated. For calculating the matrix-elements of the cluster components ofT, we use two recipes: (1) a variational determination of the cluster components using Euler's principle for the energy functional akin in spirit to the Varied Portion Approach (VPA) of Sinanoglu and (ii) a nonvariational determination of the cluster components using the conventional coupled-cluster theory. Results are presented for model test systems and are compared with variational CI and nonvariational coupled-cluster values. It has been observed that the values obtained from the size-consistent energy functional from the cluster components obtained from methods (i) and (ii) are quite close and both compare well with the nonvariational coupled-cluster results. Some useful simplifications afforded by the VPA are also indicated. A brief perspective of the method vis-a-vis other related theories is also given.     

Open circuit potential measurements with Ti/TiO2 electrodes

The open circuit potential (OCP) vs pH response of Ti/TiO₂ electrodes prepared by thermal and electrochemical oxidation of metallic titanium was measured in KNO₃ aqueous solutions in order to establish whether the slopes of the OCP-pH curves can be used as a measure of the variation of surface potential (ψ_o) of TiO₂ with the pH of the aqueous solution. For comparison purposes, ψ_o—pH slopes were also evaluated from surface charge-pH data obtained by acid-base potentiometric titrations of TiO₂ dispersions.Ti/TiO₂ electrodes showed a linear OCP-pH response in the range of ph 5–10 with slopes of −0.039 ± 0.005 V ph⁻¹. The OCP-pH dependence of Ti/TiO₂ electrodes was lower than that predicted thermodynamically. According to the triple-layer model, used to describe the oxide/aqueous solution interface, the OCP-pH slopes obtained for Ti/TiO₂ electrodes cannot be identified with the variation of ψ_o with the pH. Calculations with this model indicated that neither the single nor the double extrapolation methods used to evaluate the intrinsic ionization constant of oxide surface sites render realistic values of these parameters for the TiO₂ surface.The analysis of surface charge—pH data and the use of the triple-layer model to predict experimental values indicated that the ψ_o—pH slopes of TiO₂ surfaces must be greater (in absolute magnitude) than 0.041 V pH⁻¹.     

Determination of pH value of isoelectric solution and identification of passive iron surface phases using immersion method

Using the method of phase modeling, the pH values of solutions corresponding to the uncharged surface of passive iron and ferric oxide γ-Fe₂O₃ (pH₀) are compared. According to the theory of connected places, the charge of metal oxide surface is determined by the adsorption or desorption of hydrogen ions leading to a change in the potential drop at the oxide/solution interface. Preliminarily passivated iron electrode was washed with twice-distilled water and placed into 0.5 M NaNO₃ solution with various pH values; the variation in the potential (ΔE) with time was studied. The pH₀ value for passive electrode under the open-circuit conditions was determined by the dependence of ΔE on the pH value (pH₀ 6.2 ± 0.1). The pH₀ value was close to that for γ-Fe₂O₃ (pH₀ 6.2), which was determined by the method of potentiometrical titration of oxide suspension in the nitrate solution. The introduction of surface-active ions Ba²⁺ and Cl^? changes the charge of passive iron surface: Ba²⁺ ions increase the electrode potential, while Cl^? ions decrease it. Comparing the pH₀ values for passive electrode and metal oxides, one can identify the composition of passive electrode surface.     

Novel route in the synthesis of ψ[CH2NH] amide bond surrogate

An alternative method for the synthesis of pseudopeptides containing a ψ[CH₂NH] amide bond surrogate is reported. The synthetic approach is based on a nucleophilic displacement of the chiral N-protected β-iodoamines with conveniently protected amino acid esters. The compatibility of this method with both conventional and microwave-assisted peptide synthesis should increase the potentiality of the ψ[CH₂NH] peptide bond isostere in peptide chemistry.     

A new fluorescent pH probe for extremely acidic conditions

A novel turn-off fluorescent probe based on coumarin and imidazole moiety for extremely acidic conditions was designed and developed. The probe with pK_a = 2.1 is able to respond to very low pH value (below 3.5) with high sensitivity relying on fluorescence quenching at 460 nm in fluorescence spectra or the ratios of absorbance maximum at 380 nm to that at 450 nm in UV–vis spectra. It can quantitatively detect pH value based on equilibrium equation, pH = pK_a − log[(I_x − I_b)/(I_a − I_x)]. It had very short response time that was less than 1 min, good reversibility and nearly no interference from common metal ions. Moreover, using ¹H NMR analysis and theoretical calculation of molecular orbital, we verified that a two-step protonation process of two N atoms of the probe leaded to photoinduced electron transfer (PET), which was actually the mechanism of the fluorescence quenching phenomenon under strongly acidic conditions. Furthermore, the probe was also applied to imaging strong acidity in bacteria, E.coli and had good effect. This work illustrates that the new probe could be a practical and ideal pH indicator for strongly acidic conditions with good biological significance.     

Optical potential calculations for molecular collisions

The Feshbach optical formalism is applied to elastic, nonreactive atom-diatom scattering on a single potential energy surface. The optical potential depends on GQ, the resolvent of E-QHQ, where Q projects onto open as well as closed channels. A method for generating GQ is developed which goes beyond the free-space approximation by partitioning the radial part of the intermolecular separation into a set of intervals, on each of which the projected interaction QVQ is represented by a constant diagonal form. The resulting GQ is used in calculations on a model collinear system. The calculations are carried out with various approximations on the full nonlocal optical potential equation for Pψ Emphasis is placed on two of these, one of which is characterized by a local homogeneous equation for Pψ, and the other by a local inhomogeneous equation for Pψ.     

Fast cation-exchange separation of proteins in a plastic microcapillary disc

A novel disposable adsorbent material for fast cation-exchange separation of proteins was developed based on plastic microcapillary films (MCFs). A MCF containing 19 parallel microcapillaries, each with a mean internal diameter of 142 μm, was prepared using a melt extrusion process from an ethylene-vinyl alcohol copolymer (EVOH). The MCF was surface functionalised to produce a cation-exchange adsorbent (herein referred as MCF-EVOH-SP). The dynamic binding capacity of the new MCF-EVOH-SP material was experimentally determined by frontal analysis using pure protein solutions in a standard liquid chromatography instrument for a range of superficial flow velocities, u_LS = 5.5–27.7 cm s⁻¹. The mean dynamic binding capacity for hen-egg lysozyme was found to be approximately 100 μg for a 5 m length film, giving a ligand binding density of 413 ng cm⁻². The dynamic binding capacity did not vary significantly over the range of u_LS tested. The application of this novel material to subtractive chromatography was demonstrated for anionic BSA and cationic lysozyme at pH 7.2. The chromatographic separation of two cationic proteins, lysozyme and cytochrome-c, was also performed with a view to applying this technology to the analysis or purification of proteins. Future applications might include separation based on anion exchange and other modes of adsorption.     

Crystal growth of CaCO3 induced by monomethylitaconate grafted polymethylsiloxane

We report the preparation of a new monomethylitaconate grafted polymethylsiloxane (CO₂H-PMS) copolymer and its effect as template for crystal growth of CaCO₃. The in vitro crystallization of CaCO₃ was carried out using the gas diffusion method at different pH values at room temperature for 24 h. The CO₂H-PMS was prepared using polydimethylsiloxane-co-methylhydrogensiloxane (PDMS-co-PHMS), obtained through cationic ring opening polymerization, from cyclic monomers and monomethyltaconate (MMI) via hydrosilylation reactions with platinum complex as catalyst. FTIR results are in an agreement with the proposed template structure and confirmed that the hydrosilylation was complete. Experimental results from pH values and SEM analysis showed that the carboxylate groups of CO₂H-PMS alter the nucleation, growth and morphology of CaCO₃ crystals. SEM revealed single-truncated (ca. 5 μm) modified at pH 7-9, aggregated-modified (ca. 20 μm) at pH 10-11, and donut-shaped crystals at pH 12. These morphologies reflect the electrostatic interaction of carboxylic moieties with Ca²⁺ modulated by CO₂H-PMS adsorbed onto the CaCO₃ particles. EDS confirmed the presence of Si atoms on the crystals surface. XRD analysis showed the existence of only two polymorphs: calcite and vaterite revealing a selective control of CaCO₃ polymorphisms. In summary, the use of grafted polymethylsiloxane template offer a good alternative for polymer controlled crystallization and a convenient approach for understanding the biomineralization process useful for the design of novel materials.     

pH-responsive deoxyribonucleic acid capture/release by polydopamine functionalized magnetic nanoparticles

Polydopamine functionalized magnetic nanoparticles (PDA@Fe₃O₄) were prepared and characterized by transmission electron microscopy, scanning electron microscopy, zeta potential and vibrating sample magnetometry. They were found to enable highly efficient capture of genomic deoxyribonucleic acid (DNA). The adsorption capacity of PDA@Fe₃O₄ for genomic DNA can reach 161 mg g⁻¹. The extraction protocol used aqueous solutions for DNA binding to and releasing from the surface of the magnetic particles based on the pH inducing the charge switch of amino and phenolic hydroxyl groups on PDA@Fe₃O₄. The extracted DNA with high quality (A₂₆₀/A₂₈₀ = 1.80) can be directly used as templates for polymerase chain reaction (PCR) followed by capillary electrophoresis (CE) analysis. None of the toxic chemical reagents and PCR inhibitors was used throughout the whole procedure. PDA@Fe₃O₄ based magnetic solid phase extraction (MSPE) method was superior to those using commercial kit and traditional phenol–chloroform extraction methods in yield of DNA. The developed PDA@Fe₃O₄ based MSPE-PCR-CE method was applied for simultaneous and fast detection of Listeria monocytogenes and Escherichia coli O157:H7 in milk.     

Simultaneous spectrophotometric flow-through measurements of pH, carbon dioxide fugacity, and total inorganic carbon in seawater

An autonomous multi-parameter flow-through CO₂ system has been developed to simultaneously measure surface seawater pH, carbon dioxide fugacity (fCO₂), and total dissolved inorganic carbon (DIC). All three measurements are based on spectrophotometric determinations of solution pH at multiple wavelengths using sulfonephthalein indicators. The pH optical cell is machined from a PEEK polymer rod bearing a bore-hole with an optical pathlength of ∼15 cm. The fCO₂ optical cell consists of Teflon AF 2400 (DuPont) capillary tubing sealed within the bore-hole of a PEEK rod. This Teflon AF tubing is filled with a standard indicator solution with a fixed total alkalinity, and forms a liquid core waveguide (LCW). The LCW functions as both a long pathlength (∼15 cm) optical cell and a membrane that equilibrates the internal standard solution with external seawater. fCO₂ is then determined by measuring the pH of the internal solution. DIC is measured by determining the pH of standard internal solutions in equilibrium with seawater that has been acidified to convert all forms of DIC to CO₂. The system runs repetitive measurement cycles with a sampling frequency of ∼7 samples (21 measurements) per hour. The system was used for underway measurements of sea surface pH, fCO₂, and DIC during the CLIVAR/CO₂ A16S cruise in the South Atlantic Ocean in 2005. The field precisions were evaluated to be 0.0008 units for pH, 0.9 μatm for fCO₂, and 2.4 μmol kg⁻¹ for DIC. These field precisions are close to those obtained in the laboratory. Direct comparison of our measurements and measurements obtained using established standard methods revealed that the system achieved field agreements of 0.0012 ± 0.0042 units for pH, 1.0 ± 2.5 μatm for fCO₂, and 2.2 ± 6.0 μmol kg⁻¹ for DIC. This system integrates spectrophotometric measurements of multiple CO₂ parameters into a single package suitable for observations of both seawater and freshwater.     

Fabrication and kinetic studies of a novel silver nanoparticles-glucose oxidase bioconjugate

In this study, a new effective, pH and thermally stable glucose oxidase (GOX)-silver nanoparticles (AgNPs) bioconjugate was designed. AgNPs were synthesized based on the reduction of silver nitrate (AgNO₃) by sodium borohydride (NaBH₄) using two simple procedures. Periodic acid was used for oxidation of the GOX and emission of Lucifer yellow (LyCH) was monitored by spectrofluorometer for evaluation of the oxidation properties of the GOX. The oxidized GOX (Ox-GOX) was immobilized on AgNPs by its sugar moieties via 6-aminohexanoic acid (6AHA) as linker. A sample of the synthesized bioconjugate was loaded on 7.5% non-denaturing polyacrylamide gel electrophoresis (PAGE) to confirm its structural and physical stability. The results from enzymatic activity assay showed that the bioconjugate, GOX and Ox-GOX were similar in stability and activity in acidic and basic pH (optimum pH = 7.0-8.0). Based on the results from thermal stability assay, it was found that the activity of the bioconjugate was found to be higher at lower temperatures. The V_max of the bioconjugate, GOX, and Ox-GOX was estimated as 28.6, 6.2, and 6 IU μg⁻¹ enzyme and the K_m was calculated as 2.7, 9, and 9.5 mM, respectively. It was found that the immobilization method improves the activity and stability of the GOX in different pH and temperatures. As a conclusion, the proposed method opens up the way to the development of a new bioconjugate with potential use in sensing, and many find potential applications in clinical diagnostics, medicine, and industries.     

Towards surface acid–base property of the carboxylic multi-walled carbon nanotubes by zero current potentiometry

The surface acid–base property of carboxylic multi-walled carbon nanotubes (MWNTs) is investigated by zero current potentiometry with a new electrochemical measurement system. The pH dependent interface potential variation at the interface of carboxylic MWNTs/solution is investigated by measuring zero current potential E_zcp. In the pH range of 1–11, the pH response of carboxylic MWNTs exhibits two linear relationships according to the following equations: E_zcp = 0.791–0.0535 pH (pH 1–5.1) and E_zcp = 0.643–0.0241 pH (pH 5.1–11), respectively. The intersection at pH 5.1 of two regions indicates the surface pK_a value of carboxylic group terminated MWNTs.     

Electrochemical and XPS measurements on thin oxide films on zirconium

The potentiodynamic growth of thin oxide films on zirconium electrodes was investigated by coulometric and simultaneous impedance measurements, as a function of the electrode potential (0 V ⩽ E ⩽ 9 V), the pH (0 ⩽ pH ⩽ 14) and the surface preparation (electropolishing, etching and mechanical polishing). The initial film thickness d₀ is at least 4–6 nm; with increasing potential, the oxide grows irreversibly by 2.6 nm/V (pH 0.3) up to 3.2 nm/V (pH 14). In Cl⁻- and ClO⁻₄-containing solutions the oxide growth is limited by localized corrosion. The oxide behaves like a typical insulator with a donor concentration N_D < 10¹⁹ cm⁻³ and a dielectric constant D = 31. Below −0.5 V (vs. SHE) only, th film behaves like an n-type semiconductor with N_D ≈ 3 × 10¹⁹ cm⁻³. From photoelectrochemical measurements a direct and an indirect transition with band gap energies of E_g = 5 eV and E_g = 2.8 eV could be derived. Anodic electron-transfer reactions (ETRs) are blocked at the homogeneous oxide surface, but cathodic ETRs are possible at larger overvoltages. Near the flatband potential E_fb ≈ −1.3 ± 0.2 V (vs. SHE) hydrogen evolution takes place with a simultaneous increase of the capacity which may be attributed to hydrogen incorporation. With XPS measurements the stoichiometry of the oxide film was determined as ZrO₂ at all the pH values examined, but a thin outer layer contained some hydroxide. Components of the forming electrolyte could not be detected (sulphate, borate and perchlorate < 1%), but etching in HF caused accumulation of F⁻ at the inner boundary.