The Influence of Polyvalent Ions on Interfacial Impedance

Preferential adsorption of a polyvalent ion may give rise to a series resistive component in the interfacial impedance when the ion is a minor component of the system. Studies of the potential dependence of this component by modulation polarography suggest that the adsorption of polyvalent cations at a negatively charged mercury electrode is dependent on both the nature of the polyvalent cation and on the identity of the univalent cation when the concentration of the latter ion greatly exceeds that of the polyvalent ion. Results for chloride solutions indicate that polyvalent cations are less strongly adsorbed when the electrode is positively charged than simple theory would predict.     

Impact of N-Terminal Tags on De Novo Vimentin Intermediate Filament Assembly

Vimentin, a type III intermediate filament protein, is found in most cells along with microfilaments and microtubules. It has been shown that the head domain folds back to associate with the rod domain and this association is essential for filament assembly. The N-terminally tagged vimentin has been widely used to label the cytoskeleton in live cell imaging. Although there is previous evidence that EGFP tagged vimentin fails to form filaments but is able to integrate into a pre-existing network, no study has systematically investigated or established a molecular basis for this observation. To determine whether a tag would affect de novo filament assembly, we used vimentin fused at the N-terminus with two different sized tags, AcGFP (239 residues, 27 kDa) and 3 × FLAG (22 residues; 2.4 kDa) to assemble into filaments in two vimentin-deficient epithelial cells, MCF-7 and A431. We showed that regardless of tag size, N-terminally tagged vimentin aggregated into globules with a significant proportion co-aligning with β-catenin at cell–cell junctions. However, the tagged vimentin aggregates could form filaments upon adding untagged vimentin at a ratio of 1:1 or when introduced into cells containing pre-existing filaments. The resultant filament network containing a mixture of tagged and untagged vimentin was less stable compared to that formed by only untagged vimentin. The data suggest that placing a tag at the N-terminus may create steric hinderance in case of a large tag (AcGFP) or electrostatic repulsion in case of highly charged tag (3 × FLAG) perhaps inducing a conformational change, which deleteriously affects the association between head and rod domains. Taken together our results shows that a free N-terminus is essential for filament assembly as N-terminally tagged vimentin is not only incapable of forming filaments, but it also destabilises when integrated into a pre-existing network.     

Adsorption of diquat, paraquat and methyl green on sepiolite: experimental results and model calculations

Adsorption of the divalent organic cations paraquat (PQ), diquat (DQ) and methyl green (MG) on sepiolite was determined experimentally and investigated with an adsorption model. The largest amounts of DQ, PQ and MG adsorbed were between 100% and 140% of the cation exchange capacity (CEC) of sepiolite. In previous experiments with monovalent organic cations (dyes), the largest amounts of dyes adsorbed were about 400% of the CEC of sepiolite. Consequently, it was proposed that most of this adsorption was to neutral sites of the clay. The large differences between the adsorption of these divalent organic cations and the monovalent dyes may indicate that there is almost no interaction between DQ, PQ and MG and the neutral sites of sepiolite. This assumption was confirmed by infrared (IR) spectroscopy measurements, that did not show changes in the peaks arising from the vibrations of external Si---OH groups of the clay when the divalent organic cations were added. Adsorption results were compared with calculations of an adsorption model that combines the Gouy–Chapman solution and specific binding in a closed system. The model considers cation adsorption on neutral sites of the clay, in addition to adsorption to mono- or divalent negatively charged sites, forming neutral or charged complexes. The model could adequately simulate the adsorption of the divalent organic cations DQ and PQ when added alone, and could yield good fit for the competitive adsorption experiment between the monovalent dye methylene blue and DQ. In competitive adsorption experiments, when total cationic charges exceeded the CEC, monovalent organic cations were preferentially adsorbed on the clay at the expense of the divalent cations.     

The effect of polycations on early cement paste

This paper studies the possibility for improving the ductility of cement based materials by means of oligocationic additives. Actually, the setting of cement is due to ionic correlation forces between highly negatively charged C-S-H nanoparticles throughout a calcium rich solution. The main drawback of this strong attraction is its very short range that results in low elastic deformation of hydrated cementitious materials. A way to enlarge the attraction range between C-S-H particles would be to add cationic oligomers that would compete with calcium ions modifying the ionic correlation forces via a bridging mechanism of longer range, which could lead to a more ductile material. The studied parameters were the polymerization degree, the separation distance between the charged monomers and the balance between oligocations and monovalent and divalent cations in the solution. The results, both experimental and numerical by Monte Carlo (MC) simulations, demonstrate that cationic oligomers can compete with calcium cations as counterions to the C-S-H surface. The cohesive forces between C-S-H surfaces, calculated by MC simulations, show an interesting behaviour where range and magnitude can be tuned with oligomer concentration, polymerization degree and line charge density. Thus, it seems possible to modulate the ductility and critical strain of cement by addition of cationic oligomers.     

Adsorption des cations a des potentiels anodiques

As is known from the literature, the nature of the cation has a marked influence upon the kinetics of the anodic process of the formation of persulphuric acid at a platinum electrode, occurring at such potentials where the adsorption of cations would not seem to be of essential importance. The study of the dependence of the rate of evolution of oxygen in HClO₄ solutions on the concentration and radius of the cation has shown that the adsorption of cations is connected with the covering of the electrode surface with chemisorbed oxygen resulting in a discontinuous rise of oxygen overvoltage. This adsorption effect can be explained by the dipole nature of the platinum/oxygen bond.

The phenomena observed during a change in the properties of surface oxides in the region of considerable anodic polarization are somewhat similar to those found in the initial stage of the oxidation of platinum, which was shown to impart to the electrode the ability to adsorb cations.

The absorption of organic cations at positively charged metal/solution interfaces depends to a large extent on the presence of primarily adsorbed surface active anions and on the interaction of the π-electrons of the aromatic nuclei with the metal surface. The latter effect is demonstrated by the difference of the adsorbabilities of anilinium and cyclohexylammonium cations on positively charged mercury.

Some inorganic cations, for instance thallium on mercury, or thallium and cadmium on platinum, display a specific adsorbability similar to the adsorbability of bromide or iodide anions.     

STABILITY OF LEAD GLASSES AND POLARIZATION OF IONS*

The current theories of glass formation deal mainly with inorganic substances containing oxygen as the negatively charged component. A distinction has been made between those positively charged components which are considered as essential for the formation of the continuous network and those which fit into the interstices of the network and have only a secondary modifying effect on the properties of the glass. While the small and highly charged ions B³+, Si⁴⁺, and P⁵⁺, which exert a strong electrical attraction on oxygen, have been classified as typical network-formers, the larger, only singly and doubly charged alkali and alkaline earth cations were considered as typical network-modifiers. However, the behavior of Be²⁺ and Al³⁺ is intermediate between these extremes and shows that the classification is not precise. Furthermore, the following facts prove that the strength of the forces between the positive and negative components is not the decisive factor for glass formation: S⁶⁺ is not a glassformer, whereas PbO favors glass formation even though Pb²⁺ resembles Sr²⁺ in many other respects. It is suggested in the present paper that in the case of inorganic substances, the mutual polarization of ions is the common factor responsible for the stability of glasses. In those silicates which contain only cations of noble gas type, the decisive factor is the polarization of O^2- by the strong field of the central cation Si⁴⁺. The high polarizability of the non-noble gas type cation Pb²⁺ is considered responsible for the stability of lead glasses. Strong polarization effects diminish the symmetry of the constituent parts which have to move against each other in the process of crystallization. The lack of symmetry of the field of forces around the ions of inorganic substances as well as around the molecules of organic compounds like glycerol appears to be responsible for the high viscosity of the liquid at the solidification temperature and thus for the stability of the vitreous state. The tightness of the electronic shell of oxygen ions in silicates containing only cations of noble gas type is compared on the basis of the apparent Lorentz-Lorenz molar refraction per one O^2-. These values are derived from the molar refraction of the silicate by subtracting those of Si⁴⁺ and of the other cations in the free gaseous state. It is shown that the electronic shell of the oxygen ions becomes less tight in the series 2Be²⁺, Si⁴⁺, ⁴/₃Al³⁺, 2Mg²⁺, 4Li⁺, etc., which is that of decreasing field strength of the cation. Increase of the ratio of the numbers of O to Si atoms also leads to decreased tightness.     

Catalytic effect of metal cations on the formation of carbon nanotubes inside the channels of AlPO4-5 crystal

Single-walled 0.4 nm carbon nanotubes (SWNTs) were fabricated using various metal cation substituted AlPO₄-5 (MeAPO-5) molecular sieves as the template. The catalytic behavior varies with different metal incorporation. The incorporation of metal cations (Mn, Mg, Co) and Si gives rise to the formation of negatively charged frameworks and Brønsted acid sites. These frameworks thus play an important catalytic role in the pyrolysis and conversion of the organic molecules to SWNTs within the crystal channels. It is shown that the MeAPO-5 single crystals have a higher density of SWNTs than those crystals without the metal cations. The experimental results agree with the predictions of first-principles calculations, which show the metal-incorporated frameworks to be favorable for the SWNTs formation.     

Maximum de conductivite des solutions aqueuses d'electrolytes

The conductivities and specific masses of concentrated aqueous solutions of alkaline and alkaline-earth salts were measured at 93°C.The ratio of the number of moles of water to the number of moles of salt where the maximum conductivity occurs (R_max) was determined for all the investigated systems. The R_max values are primarily determined by the charge of the cation. In the case of singly charged cations R_max yields a value of about 8 while this ratio reaches a mean value of 20 for electrolytes with divalent cations. These R_max values are correlated to the primary hydration numbers of the ions of the electrolyte.For a given concentration, the conductivities and equivalent conductances of mixtures of water and electrolytes with divalent cation clearly depend on the nature of the anion.     

Electrochemical and AFM evaluation of hazard compounds-DNA interaction

The evaluation of the interaction of hazard compounds, divalent cations Pb, Cd, Ni and Pd, with double-stranded DNA (dsDNA), forming a metal-DNA complex, was studied by atomic force microscopy (AFM) and differential pulse voltammetry (DPV) on two carbon electrode surfaces, highly oriented pyrolytic graphite (HOPG) and glassy carbon (GC). The electrochemical behaviour of these metal-DNA complexes was related to the different adsorption patterns and conformational changes obtained by the AFM images on the HOPG surface. The dsDNA interaction was specific with each metal cation, inducing structural changes in the B-DNA structure, local denaturation of the double helix and oxidative damage. AFM images showed an increase of the electrode surface coverage for lead, cadmium and nickel DNA complexes. For cadmium and nickel-DNA complexes oxidative damage to DNA was electrochemically detected for the concentrations studied. Palladium interaction with dsDNA induced condensation of the dsDNA secondary structure, which led to the aggregation of helixes forming very compact and thick filaments in the network film. The voltammetric data for the palladium-DNA complex showed a sharp decrease of the guanosine and adenosine oxidation peak currents, consistent with the AFM results of DNA condensation in the presence of palladium, but no DNA oxidative damage was detected for the range of concentrations used.     

Influence of monovalent cations and CuO nanoparticles on X-nanozeolite in uranium anionic species separation from contaminated drinking water

The main purpose of this work was the modification of NaX nanozeolite using copper oxide nanoparticles and various monovalent cations such as K~+, Cs~+, and Ag~+in order to make the negatively charged zeolite surface accessible for anionic forms of uranium which are the most dominant species of uranium in the contaminated radioactive waters at natural p H. Various methods such as the X-ray fluorescence(XRF), X-ray powder diffraction(XRD), field emission scanning electron microscopy(FE-SEM), transmission electron microscopy(TEM), Fourier transform infrared spectroscopy(FT-IR), and atomic absorption spectroscopy(AAS) were used to characterize the final synthesized absorbents. Batch technique was used to study the adsorption behavior of uranium ions from polluted drinking water by Na X nanozeolite and its modified forms. In order to better understand the performance of them, the results were compared with those that were obtained for synthesizing bulk NaX zeolite and Na-form of clinoptilolite natural zeolite. Preliminary results indicated that uranium sorption increased as the loading level of CuO nanoparticles on NaX nanozeolite increased from 2.1 wt% to 11.2 wt%. In addition,from the obtained data, an increase in uranium removal efficiency resulted as charge/ionic radius ratio of exchanged cation decreased. Also, the effect of contact time, solid–liquid ratio, initial concentration and temperature on the adsorption process was studied. It is worth mentioning that, in this study, the sorption of uranium was performed under natural conditions of pH and the presence of competing cations and anions which are available in drinking waters.     

Interfacial Modeling of Fibrinogen Adsorption onto LiNbO3 Single Crystal–Single Domain Surfaces

For the development of next-generation protein-based biosensor surfaces, it is important to understand how functional proteins, such as fibrinogen (FBG), interact with polar substrate surfaces in order to prepare highly sensitive points of medical care diagnostics. FBG, which is a fibrous protein with an extracellular matrix, has both positively and negatively charged regions on its 3-dimensional surface, which makes interpreting how it effectively binds to polarized surfaces challenging. In this study, single-crystal LiNbO₃ (LNO) substrates that have surface charges were used to investigate the adsorption of FBG protruding polar fragments on the positively and negatively charged LNO surfaces. We performed a combination of experiments and multi-scale molecular modeling to understand the binding of FBG in vacuum and water-solvated surfaces of LNO. XPS measurements showed that the FBG adsorption on LNO increased with increment in solution concentration on surfaces independent of charges. Multi-scale molecular modeling employing Quantum Mechanics, Monte Carlo, and Molecular Mechanics addressed the phenomenon of FBG fragment bonding on LNO surfaces. The binding simulation validated the experimental observation using zeta potential measurements which showed presence of solvated medium influenced the adsorption phenomenon due to the negative surface potential.     

Effect of ionic environment on BSA filtration and the properties of BSA deposits

Although many current applications of membrane filtration involve the removal, concentration, or exchange of electrolytes, there is still no fundamental understanding of the effects of ionic composition and concentration on the performance of these filtration processes. We have obtained experimental data for the effect of ionic species on the hydraulic permeability of bovine serum albumin deposits formed during filtration. The permeability of the protein deposits decreased with increasing solution ionic strength at pH both above and below the protein isoelectric point but was relatively independent of ionic strength at the isoelectric point. The permeability of the BSA deposits at neutral pH was markedly smaller when measured with solutions of divalent cations than with solutions of monovalent cations but was unaffected by the valence of the anions. This dependence on ionic strength and ion valence was consistent with a simple model in which the protein layer permeability is determined by the balance between the compressive pressure associated with the filtration and the electrostatic repulsion between the negatively charged proteins in the deposit. These results were then combined with available data for the albumin diffusivity and osmotic pressure to examine the effects of solution ionic strength on the flux during protein filtration.     

牛血清白蛋白初级团聚体分散体在多价反离子作用下的再稳定性和分形结构

动态光散射的研究发现,牛血清白蛋白初级团聚体（BSA-PAs）的分散体在三价反离子钇离子（Y³⁺）作用下,团聚出现再稳定现象,即反离子浓度较低时BSA-PAs团聚速度较慢,增大反离子浓度团聚速度加快,再进一步增加反离子浓度其团聚速度反而又变慢了。经分析,BSA-PAs在高离子浓度下的再稳定性现象是由于过多的三价反离子与BSA-PAs表面结合导致表面偶极子增加所产生的短程排斥性水合力所致。小角光散射仪的静态光散射（SLS）和透射电镜（TEM）测试结果表明BSA-PAs团聚体在低和高反离子浓度时生成结构疏松的纤维状团聚体,在中间浓度时生成结构致密的球形团聚体,SLS和TEM进一步验证了高离子浓度时短程水合力的产生。这种短程水合力只在蛋白质带电或极性区域产生,它只能保护这些区域以免团聚发生,却不能保护两端的疏水性区域,所以BSA-PAs两端在反离子诱导团聚时相互连接生成纤维状团聚体。     

Role of Defect Interaction in Boundary Mobility and Cation Diffusivity of CeO2

Grain boundary mobility of CeO₂ containing 0.1% and 1.0% trivalent dopant cations (Sc, Yb, Y, Gd, and La, in order of increasing ionic radius) has been measured. At the lower dopant concentration (intrinsic regime), mobility is controlled by grain boundary diffusion of host cations, whereas at the higher dopant concentration (extrinsic regime), mobility is controlled by solute drag through the lattice. The effect of trivalent dopants is closely associated with their ability to provide and to interact with oxygen vacancies. Evidence consistent with an interstitial mechanism for cation diffusion has been found which is remarkably affected by the presence of oxygen vacancies. Ce diffusion is enhanced by free oxygen vacancies in the system, while dopant diffusion is suppressed if a dopantassociated oxygen vacancy is not present. A bare Sc cation, however, appears to be a fast-diffusing species, due to its highly distorted local environment, while Y at 1.0% emerges as the most effective grain growth suppressant.     

Sintering Behavior of Doped Lanthanum and Yttrium Manganite

The sintering behavior of doped manganite powders was found to be highly dependent on changes in calcination conditions and A/B cation ratio. Coarsening of combustion synthesized powders by calcination allowed for higher green densities in dry-pressed compacts, which resulted in higher sintered densities for powders calcined in the temperature range 800°-1200°C. Sintered densities decreased for calcination temperatures greater than 1200°C. Preparation of manganites with a deficiency of A-site cations improved the densification behavior substantially. This effect was attributed to an increased concentration of A-site vacancies which enhanced the diffusion of A-site cations during sintering. Modification of doped manganites by alteration of composition and calcination conditions allowed their sintering shrinkage to be "tailored" to more closely match the shrinkage of yttria-stabilized zirconia.     

A new charge alternating model of localization of bivalent cations in high silica zeolites with distantly placed aluminum atoms in the framework

The present communication reports an unexpected result on quantitative substitution in HZSM-5 of all acidic protons by bivalent zinc cations via chemical reaction with zinc vapor at 800 K. Based on this result we suggest a new model for localization of bivalent cations in high silica zeolites with distantly placed aluminum atoms in the framework. According to this model some of the bivalent cations can be localized at the isolated singly negatively charged aluminum occupied oxygen tetrahedra. In parallel the equivalent number of the isolated framework aluminum atoms remains without compensating protons or zinc cations. Their negative charges are compensated indirectly by coulomb interaction with the surrounding excessively charged cationic sites. The resulting Lewis sites with only partially compensated positive electric charges possess unusually high chemical activity. They very strongly perturb molecular hydrogen at 77 K and dissociatively adsorb it at temperatures only slightly above room temperature. Application of low temperature H₂ adsorption as a molecular probe enables us to distinguish such unusual sites from Zn²⁺ cations at the conventional ion exchange positions. The similar unusual Lewis sites distantly separated from basic oxygen have also been previously detected by low temperature hydrogen adsorption in the samples prepared by conventional incipient wetness impregnation of HZSM-5 with aqueous solution of zinc nitrate though in smaller amount.     

Quantitative determination of metal cations chemisorbed by polycaproamide fabrics

Studies were conducted to develop a method of quantitative determination of metal cations chemisorbed by fabrics made of polycaproamide fibres. A complexonometric method was selected to determine the cation content. Complexonometric methods, which are highly accurate, approaching the accuracy of gravimetric methods, require much less time for execution. A single method is proposed for determining the amount of different metal cations chemisorbed by synthetic materials. The reproducibility of the results is good.     

Strategies for the successful preparation of homoionic smectites

Many chemical, mechanical and thermal properties of smectites depend on the interlayer cations. Therefore, homoionic exchanged smectites are prepared in order to investigate the reaction exchange mechanisms and to improve their behavior as function of the interlayer cation. Presently no general protocol exists and many of the suggested procedures lack proof of achieved exchange. Thus, the aim of the present study was to depict strategies and to determine factors for successful preparation of homoionic smectites. Three natural bentonites and one purified bentonite were saturated by different compensator cations using different (Na⁺, Li⁺, K⁺, Ca^2 +, Mg^2 +, Cu^2 +, or Zn^2 +) chloride solutions. Concentration of the cations in solution varied between 1 and 20 times of CEC. The exchange was repeated one to three times. Extent of smectite interlayer saturation was studied by measurements of cation exchange capacity (CEC) and subsequent determination of the exchanged interlayer cations.Chemical properties of used solution salts influence pH and the surface properties of the smectites. As a result CEC of the smectites was altered by changing contribution from edge charges. It is established that divalent cation exchange was more successful than monovalent cation exchange (Bergaya et al., 2006). Moreover, fractions of high charged 2:1 layers could also hinder homoionic exchange. Finally, successful cation exchange reaction was strongly influenced by soluble accessory minerals and experimental setup.It was found that there is no simple recipe which is equally applicable for all smectites and bentonites, but there is a general protocol considering the following parameters: mineral composition of the sample, smectite CEC and/or layer charge, chemical and mechanical pretreatment, initial interlayer cations, type and concentration of saturating salt, solid–liquid ratio, reaction time, number of repetitions and washing procedure. Finally, proof of interlayer occupation is mandatory.     

Bioadsorption of Pb(II), Cd(II), and Cr(VI) on activated carbon from aqueous solutions

The bioadsorption of Pb(II), Cd(II), and Cr(VI) using bacteria and activated carbon has been studied. Preliminary studies yielded the chemical and textural characterization of the carbons. The adsorption of bacteria on the activated carbons modified their surface characteristics, reducing the volume of pores and the pH of the point of zero charge, with a resulting increase in the density of the negative charge of their surface. The adsorption of the above metals was studied in both static and dynamic conditions and in the absence and presence of bacteria (Escherichia coli). The presence of bacteria in aqueous solution enhances the adsorption of Pb(II) and Cd(II) and reduces the adsorption of Cr(VI). These results can be explained by changes in the surface charge density of the carbons when bacteria are adsorbed, and by considering the structural and chemical characteristics of the bacterial cell walls. Investigation of the effect of electrolytes on the bioadsorption of these metals showed, in general, a resulting reduction in the amount of metal adsorbed, mainly in the presence of divalent cations. According to the divalent cation bridging theory, these results derive from competition between the Pb(II) or Cd(II) cations and the electrolyte cations for the negatively charged functional groups of extracellular polymeric substances.