Charge Neutralization Process Research Articles

Interactions between polycationic and polyanionic layers: Changes in rigidity, charge and adsorption kinetics

Interactions between poly-electrolytes on solid substrates and the subsequent formation of films with desirable characteristics is a relatively new technology that strongly depends on the mechanical and chemical properties of the new “two-dimensional” material. We show that binding between positively-charged poly( l-lysine) (PLL) and negatively charged poly(styrene sulfonate) (PSS) result in a significantly more rigid aggregate film than that for non-neutralized layers (∼4 times more rigid). Thus, providing the basis for mechanically stabilized 2D films. The value for the adsorption rate constant, k a, estimated from the random sequential adsorption (RSA) model for the first PLL layer (adsorption onto gold) was ∼0.020 cm/min where as that for the second layer (adsorption on preformed PLL–PSS layer) was ∼0.016 cm/min. The k a, value for the first PSS layer was ∼0.018 cm/min and increased to ∼0.021 cm/min for the second adsorption layer. Using a negatively charged tethered probe with exposed carboxylic acid groups (–COO − at pH 7.4), we could easily follow the charge neutralization process of the adsorbed layers with chemical force microscopy. Sequential layer-by-layer assembly exhibits similar changes in rigidity and adhesion force for the two bi-layers suggesting good reproducibility in physico-chemical properties of the multilayer.

Charge neutralization process of mobile species developed during potentiodynamic conditions. Part 2: Simulation and fit of probe beam deflection experiments

The theoretical model presented in Part 1 is employed to simulate and fit experimental probe beam deflection (PBD) data of Fe(CN) 6 3 - / Fe(CN) 6 4 - and Fe 3+/Fe 2+ couples collected under voltammetric conditions. The dependence of beam deviation on the diffusion coefficients and on the charges of involved species is described. The fit of voltammetric curves was performed to get the values for the standard potential of each system as well as diffusion coefficients of electroactive species. The data were then employed to fit a set of experimental voltadeflectometric profiles collected at different beam-electrode distances. The least-squares Simplex algorithm was employed to fit proposed functions to experimental data. The data obtained from the fitted voltadeflectometric curves were consistent with values previously reported for chronodeflectometry profiles, supporting the hypothesis that the refraction index not only should be considered a function of the concentration of the neutral salts dissolved, but also depends on the concentration gradient of each soluble species. The effects of the scan rate and the beam-electrode distance are discussed in detail. In relation to this, simple relationships between these adjustable parameters and the angle of the deflected laser beam are also provided. Thus, it is now possible to perform a quantitative analysis on the response of techniques such as PBD when the heterogeneous charge transfer of electroactive species is studied with a cyclic linear potential scan.

Charge neutralization process of mobile species developed during potentiodynamic conditions. Part 1: Theory

This is theoretical work in which a general expression for the diffusion and migration contribution of electroactive and non-electroactive species is provided. The model is employed to simulate concentration and concentration gradient profiles of electroactive and non-electroactive species, driven by an electrochemical process under potentiodynamic conditions. The effects of experimental parameters such as diffusion coefficients, charges of redox species and supporting electrolyte (SE) ions are examined and discussed. The curves included in this work are mostly related to systems where electroactive and non-electroactive species have different diffusion coefficients. The proposed model can explain extreme cases related to asymmetrical diffusion of involved species that are more difficult to understand, as well as simpler cases where all species would have similar diffusion coefficients. The data measurement related to concentration gradients would be more sensitive to determine concentration changes than those linearly related to concentration. If a given technique were applied to detect the concentration changes of a particular species, then this model could be used to estimate the profiles of the other species involved. Concentration gradients of electroactive and SE species are mostly interesting to describe profiles of in situ techniques like probe beam deflection. Relatively low scan rates are considered to describe these profiles. This is because, the curves of (∂ C j /∂ x) x ≠0 are similar to the respective cyclic voltammogram and thus, it is possible to compare the position of anodic and cathodic peaks. However, this resemblance between (∂ C j /∂ x) x ≠0 and the response of current disappears when the probe-electrode distance or the potential scan rate is increased. A method for obtaining linear dependence between the square root of the forward potential peak of (∂ C j /∂ x) and the distance Δ x is provided. This method can be used to determine Δ x by employing experimental data only.

Stabilization in Water of Polymer/Cu2+ Complexes Using Poly(sodium acrylate)‐graft‐poly(N,N‐dimethylacrylamide) Graft Copolymers

AbstractThe formation of complexes between Cu2+ ions and graft copolymers built from a PANa backbone and non‐ionic hydrophilic PDMAM side chains is investigated by means of turbidimetry, viscometry, DLS, ζ‐potential measurements, and UV‐vis spectrophotometry. The macroscopic phase separation which accompanies complexation is gradually depressed with increasing PDMAM content x, and water‐soluble complexes are formed for x = 70 mol‐%. The polymer/Cu2+ complexation is a charge neutralization process that leads to the formation of water‐soluble, compact, hybrid polymer/metal ion nanostructures. UV‐vis data indicate that this graft copolymer could possibly serve as a water‐soluble sensor for Cu2+ ions.magnified image

Charge Neutralization Process of Mobile Species at Any Distance from the Electrode/solution Interface. 1. Theory and Simulation of Concentration and Concentration Gradients Developed during Potentiostatic Conditions

Charge Neutralization Process of Mobile Species at Any Distance from the Electrode/solution Interface. 1. Theory and Simulation of Concentration and Concentration Gradients Developed during Potentiostatic Conditions

Charge Neutralization Process of Mobile Species at Any Distance from the Electrode/Solution Interface. 1. Theory and Simulation of Concentration and Concentration Gradients Developed during Potentiostatic Conditions

The theoretical framework of a general model for the simulation of concentration profiles of electroactive and nonelectroactive species, driven by an electrochemical process under potentiostatic conditions, is presented. Based on this analysis, finite differences simulations are performed to calculate the actual profiles under different experimental conditions. Furthermore, the effect of experimental parameters (diffusion coefficients of the ions of the redox couple or the supporting electrolyte, charge of the different species, etc.) on the concentration profiles is also examined. The results obtained when low and high concentrations of supporting electrolyte are compared aid understanding of the effect of the electrolyte on the measurements. The presented model also underlines the role of supporting electrolyte species when nonspecific techniques are employed to measure the concentration changes produced by electroactive species. On the other hand, if a highly specific technique were used to detect changes in the concentration or concentration gradient of a given species, then it would be possible to estimate the respective profiles of the other species. The simulations suggest that techniques measuring concentration gradients are more sensitive to determining concentration changes than those involving a measurable linearly related to concentration.

Charge Neutralization Process of Mobile Species at Any Distance from the Electrode/Solution Interface. 2. Concentration Gradients during Potential Pulse Experiments

The theoretical model presented in part 1 of this work is employed to simulate and fit experimental probe beam deflection (PBD) data of Fe(CN)6(3-)/Fe(CN)6(4-) and Fe3+/Fe2+ couples. Current and beam deviation dependency on time at constant potential (chronoamperometry and chronodeflectometry) is analyzed via a new treatment based on the migration and diffusion properties of all the species involved. The diffusion coefficients of electroactive species are obtained by fitting chronoamperometric curves. Those coefficients are then employed to simulate the respective chronodeflectometric profiles. The experimental data and the theoretical function are fitted by the minimum squares Simplex algorithm. The effect of working with systems in which both electroactive species are charged is discussed in detail. Specifically, the possibility of quantitative analysis of nonspecific techniques data is analyzed when a relative high concentration of supporting electrolyte is used. Such analysis widens the scope of techniques as PBD since in many cases the effect of supporting electrolyte species could be negligible as compared to the response of electroactive species. The variation of the refraction index with the concentration gradient of each soluble species is also discussed.

Water-Soluble Stoichiometric Polyelectrolyte Complexes Based on Cationic Comb-Type Copolymers

Summary: The interaction between the cationic comb-type copolymer poly(acrylamide-co-MAPTAC)-graft-polyacrylamide, synthesized by free-radical polymerization, and the anionic homopolymer NaPA has been investigated. Water-soluble PECs are formed through a charge neutralization process. They adopt a compact structure, and form nanoparticles consisting of a hydrophobic PEC core and a protective hydrophilic PAM corona. By increasing the composition of the graft copolymers in neutral PAM side chains, the aggregation number and the size of the nanoparticles decrease. Moreover, increasing the ionic strength of the solution favours the dissociation of the complexes. A schematic representation of the PEC nanoparticles formed at NNaPA = 0.5.

A TTF–TCNQ Electrode as a Voltammetric Analogue of an Ion‐Selective Electrode

AbstractA TTF‐TCNQ/PVC composite electrode is proposed as a voltammetric cation and anion sensor. The electrode relies on the principle that, during redox processes involving the TCNQ0/− couple for cations and the TTF+/0 couple for anions, electrolyte ions are included into lattice sites in the charge neutralization process. This voltammetric ion‐sensor provides results that are similar to those of sensors based on two electrodes (viz. one modified with TCNQ for cations and another modified with TTF for anions) but with some practical advantages over them.

H1 Family Histones in the Nucleus: CONTROL OF BINDING AND LOCALIZATION BY THE C-TERMINAL DOMAIN

H1 histones bind to DNA as they enter and exit the nucleosome. H1 histones have a tripartite structure consisting of a short N-terminal domain, a highly conserved central globular domain, and a lysine-and arginine-rich C-terminal domain. The C-terminal domain comprises approximately half of the total amino acid content of the protein, is essential for the formation of compact chromatin structures, and contains the majority of the amino acid variations that define the individual histone H1 family members. This region contains several cell cycle-regulated phosphorylation sites and is thought to function through a charge-neutralization process, neutralizing the DNA phosphate backbone to allow chromatin compaction. In this study, we use fluorescence microscopy and fluorescence recovery after photobleaching to define the behavior of the individual histone H1 subtypes in vivo. We find that there are dramatic differences in the binding affinity of the individual histone H1 subtypes in vivo and differences in their preference for euchromatin and heterochromatin. Further, we show that subtype-specific properties originate with the C terminus and that the differences in histone H1 binding are not consistent with the relatively small changes in the net charge of the C-terminal domains.

Water-Soluble Complexes through Coulombic Interactions between Bovine Serum Albumin and Anionic Polyelectrolytes Grafted with Hydrophilic Nonionic Side Chains

The interaction between bovine serum albumin (BSA) and the anionic graft copolymers poly(sodium acrylate-co-sodium 2-acrylamido-2-methyl-1-propanesulfonate)-graft-poly(N,N-dimethylacrylamide) (P(NaA-co-NaAMPS)-g-PDMAMx) was investigated within the acid pH region, 2 < or = pH < or = 7. The weight percentage, x, of the poly(N,N-dimethylacrylamide) (PDMAM) side chains varied from 0 up to 75% (w:w). When BSA and P(NaA-co-NaAMPS)-g-PDMAMx are oppositely charged, i.e., when pH is lower than the isoelectric point of BSA, the two macromolecules associate through Coulombic attractions. When the anionic graft copolymer is rich enough to the nonionic PDMAM side chains, x > or = 50% w:w, the associative phase separation is practically prevented, as revealed by the turbidimetric study of the BSA/P(NaA-co-NaAMPS)-g-PDMAMx mixtures in aqueous solution vs pH. In addition, the viscosity measurements support the formation through a charge neutralization process of a rather compact protein-polyelectrolyte complex stabilized by the hydrophilic PDMAM side chains grafted onto the anionic copolymer backbone.

Images of complex interactions of an intense ion beam with plasma electrons

Ion beam propagation in a background plasma is an important scientific issue for many practical applications. The process of ion beam charge and current neutralization is complex because plasma electrons move in the strong electric and magnetic fields of the beam. Computer simulation images of plasma interaction with an intense ion beam pulse are presented.

Ion-molecule reactions of primary radical cations in the liquid-phase radiolysis of n-alkanes: An EPR study

Radiation-chemical yields the liquid-phase radiolysis of C5–C12n-alkanes were measured using the spin trap technique. The yields of n-alkyl radicals depended only slightly on the chain length in C5–C9 alkanes and amounted up to 30% of the total yield of trapped radicals; they were inhibited by the addition of charge scavengers. An analysis of the experimental results together with data on radicals in irradiated crystalline alkanes and radical cations in freon matrices showed that n-alkyl radicals results from the ion-molecule reactions of primary radical cations, whereas the protonated ions RH2+ as products of these reactions are a source of sec-alkyl radicals. At least 60% of primary radical cations are consumed via these reaction pathways. A part of sec-alkyl radicals is due to gauche-conformers. The relative amount of primary alkyl radicals formed in the degradation of excited states and the subsequent charge neutralization processes should be insignificant.

An analogy of an ion-selective electrode sensor based on the voltammetry of microcrystals of tetracyanoquinodimethane or tetrathiafulvalene adhered to an electrode surface.

The voltammetry of solid 7,7,8,8-tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (ITF) at an electrode-microparticle-aqueous (electrolyte) interface generates characteristic current-potential profiles associated with solid-solid-phase transformations. During the reactions, electrolyte ions are included into the TCNQ (cations) and TTF (anions) lattice sites as part of the charge neutralization process. Consequently, electrolyte ion concentration is associated with the reversible potential of the TCNQ0/- and TTF0/+ reactions, making these processes candidates for the development of novel voltammetric cation and anion sensors, respectively. Electrode potential-analyte ion concentration dependence studies exhibited highly reproducible potential shifts of 45 (+/- 1) mV/decade change in ion analyte concentration for both the TCNQ cation sensor and the TTF anion sensor. When presented with mixed-analyte solutions, both ion-sensing systems exhibited a degree of ion selectivity. Ion selectivity trends may be modeled using equations based on a Nicolsky-type selectivity relationship, in accordance with the concept that these are the voltammetric analogies of potentiometric ion-selective membrane electrodes.

Generalized theory of steady-state voltammetry without a supporting electrolyte. Effect of product and substrate diffusion coefficient diversity.

A generalized theory of the steady-state voltammetric response of a microelectrode in the absence of supporting electrolyte and for any values of diffusion coefficients of the substrate and the product of an electrode process is presented. The treatment applies to any reasonable combination of the charge numbers of the substrate, its counterion, and the product. A way to incorporate the activation polarization into the model is also demonstrated. It has been shown that the height, position, and shape of the migrational voltammogram are affected by the ratio of the product to substrate diffusivity (theta). In particular, for the electrode processes with sign retention, unequal diffusivities of electroactive species influence both characteristic points of the voltammogram (the limiting current and the half-wave potential). For charge neutralization processes (uncharged product), the changes in theta parameter are accompanied only by a shift in the half-wave potential. The most dramatic changes in the I-E relation can be observed for the charge reversal processes. In this case, a consecutive increase in theta results in the transition of the voltammogram shape from rapid exponential growth (theta < 1), through ramp shape (theta = 1), to common wave shape (theta > 1). On the basis of the expressions derived for the limiting current (exact and linearized), a possibility of the determination of the diffusion coefficient of the electrode reaction product is demonstrated. In addition, the ranges of theta where the assumption of equal diffusivities of the substrate and the product is obeyed within an insignificant error have been determined quantitatively. The theory has been experimentally verified using voltammetric oxidation of hexacyanoferrate(II).

Ion-beam plasma neutralization interaction images

Neutralization of the ion beam charge and current is an important scientific issue for many practical applications. The process of ion beam charge and current neutralization is complex because the excitation of nonlinear plasma waves may occur. Computer simulation images of plasma neutralization of the ion beam pulse are presented.

Biophysical Characterization of the DNA Binding and Condensing Properties of Adenoviral Core Peptide μ (mu)

Cationic peptides containing Lys and Arg residues interact with DNA via charge-charge interactions and are known to play an important role in DNA charge neutralization and condensation processes. In this paper, we describe investigations of the interaction of the cationic adenovirus core complex peptide mu with a dodecameric ODN (12 bp) and pDNA (7528 bp) using a combination of fluorescence spectroscopy, circular dichroism spectroscopy, isothermal titration calorimetry, and photon correlation spectroscopy. Comparisons are made with protamine, a cationic peptide well-known for DNA charge neutralization and condensation. Equilibrium dissociation constants are derived independently by both CD and ITC methods for the interaction between protamine or mu with pDNA (K(d) = 0.6-1 microM). Thermodynamic data are also obtained by ITC, indicating strong charge-charge interactions. The interaction of protamine with pDNA takes place with decreasing entropy (-28.7 cal mol(-1) K(-1)); unusually, the interaction of mu with pDNA takes place with increasing entropy (Delta S degrees (bind) = 11.3 cal mol(-1) K(-1)). Although protamine and mu appear to destabilize pDNA double helix character to similar extents, according to CD thermal titration analyses, PCS studies show that interactions between mu and pDNA result in the formation of significantly more size-stable condensed particles than protamine. The enhanced flexibility and size stability of mu-DNA (MD) particles (80-110 nm) compared to protamine counterparts suggest that MD particles are ideal for use as a part of new nonviral gene delivery vectors.

Electrical mobility and size distribution of aged 212Pb nanometer carriers in nitrogen gas

Radon and thoron progeny are positively charged clusters of radioactive atoms and other molecules and are neutralized through several mechanisms when they are aged. The charge status of radon clusters has been shown to be a major factor influencing their size distribution. In the present work, we attempted to determine simultaneously the activity size and charge distributions of thoron progeny using an electrical mobility spectrometer and a graded diffusion battery. These measurements allow us to study the dynamics of the charge neutralization process. Our data showed that 212Pb generated in dry N 2 atmospheres (RH < 3.4%) had a bimodal distribution. The small cluster mode with a size range of 0.5–2.5 nm was comprised of mainly neutral progeny, whereas a significant portion of the large nucleation mode (2.5–10 nm) consisted of charged progeny (36–72% based on activity) depending on the aging time. The relative humidity (RH) had a major effect on the charge neutralization process. As the RH increased from 3.4 to 17%, the proportion of charged progeny decreased steadily indicating charge neutralization. At the same time, the fraction of the nucleation mode decreased. At RH ⩾ 13%, the progeny consisted of essentially neutral molecular clusters with a single size mode about 1 nm. Therefore, the mean particle size decreased in the neutralization process, consistent with previous observations. As the concentration of the nucleation fraction decreased, the charge fraction of these nanometer particles increased from 72% to essentially 100%. These results show the importance of charged progeny in the formation and disappearance of nucleation mode.

Tetrabutylammonium cation expulsion versus perchlorate electrolyte anion uptake in the electrochemical oxidation of microcrystals of [(C 4 H 9 ) 4 N][Cr(CO) 5 I] mechanically attached to a gold electrode: a voltammetric and quartz crystal microbalance study

The electrochemistry of microcrystals of [(C4H9)4N][Cr(CO)5I] attached to a gold electrode which is placed in aqueous (lithium or tetrabutylammonium perchlorate) electrolyte media has been studied in detail by chronoamperometric, voltammetric and electrochemical quartz crystal microbalance (ECQCM) techniques. Whilst chronoamperometric and voltammetric measurements show that the expected one-electron oxidation of microcrystalline [Cr(CO)5I]− solid to Cr(CO)5I occurs at the solid-electrode-solvent (electrolyte) interface, the ECQCM measurements reveal that charge neutralization does not occur exclusively via the expected ejection of the tetrabutylammonium cation. Rather, uptake of ClO4 − occurs under conditions where the solubility of sparingly soluble [(C4H9)4N]ClO4 is exceeded. This is the first time that uptake of an anion rather than loss of a cation has been detected in association with an oxidation during electrochemical studies of microcrystals attached to electrode surfaces. It is therefore now emerging that analogous charge neutralization processes to those encounted in voltammetric studies on conducting polymers are available in voltammetric studies of microcrystals attached to electrodes which are placed in contact with solvent (electrolyte) media. In the presence of LiClO4 as the electrolyte, an ion exchange process occurs leading to formation of Li[Cr(CO)5I] . X H2O which then slowly dissolves in water at a rate that is strongly influenced by the electrolyte concentration, the relatively hydrophobic nature of the [(C4H9)4N]+ cation and the poor solubility of [(C4H9)4N]ClO4.

Ions in the terrestrial atmosphere and in interstellar clouds

AbstractIons are formed in the terrestrial atmosphere (TA) and in interstellar clouds (ISC) by the action of solar/stellar UV and cosmic rays on the ambient neutral atmospheres. The precursor ions so formed develop largely via gas‐phase ion chemistry from the simple ions characteristic of the ambient atmospheres to the observed, more complex ions. In the upper TA and in ISC, only bimolecular ion chemistry occurs. In the tenuous upper TA, the ion types remain simple, O+, O, and NO+ being dominant, and similarly in the tenuous diffuse ISC, ions such as H+, H22+, C+, and CH+ are dominant. At the high pressures of the lower TA, termolecular association reactions are important, and the initial ions, e.g., O and N formed by cosmic ray ionization are quickly converted to very complex cluster ions of the type H+(H2O)n (bases)m with bases including NH3 and CH3CN, and a parallel negative ion chemistry develops from the primary negative ions O− and O producing ions like NO(H2O)n(acid)m with acids HNO3 and H2SO4. In the dense ISC, the ion chemistry proceeds to produce polyatomic ions from the most important initial ions C+, H, and CH, and the process of radiative association, the bimolecular analog of termolecular association, is important in producing, for example, ions such as CH M, where M are the common observed interstellar molecules such as the carboxy, cyano, and amino molecules, which are formed via this ion chemistry. The major ion neutralization process in the upper TA and in ISC is positive ion–electron recombination, which limits the degree of ionization in these regions, whereas in the lower TA, where negative ions balance the positive ions, the charge neutralization process is ion–ion recombinarion. © 1996 John Wiley &amp; Sons, Inc.