Number Of Counterions Research Articles

Cation-Induced Disruption of the Local Structure of Water in Faujasite

The changes in the structure of water confined in faujasite were resolved by a series of ab initio molecular dynamic simulations of explicit water molecules in faujasite with varying Si/Al ratio and counterions present. Radial distribution functions of water oxygen atom pairs, as well as counterions to Al pairs, were calculated to quantify the structure of water in faujasite and the distribution of counterions in the water-filled faujasite pores. Radial distribution functions reveal that confinement without counterions results in more water molecules hydrogen bound to each other. In hydrogen-exchanged faujasite, the increase in aluminum content led to the disruption of the confined water structure as charge-compensating protons dissociated from Al tetrahedra and entered the liquid water in the pores. In addition, a potential of mean force of proton transfer shows that protons are thermodynamically favored to be in the liquid water rather than bound on Al tetrahedra. Exchanged Na+ and La3+ counterions, which remain closer to Al tetrahedra in liquid water than H+, also disordered the confined water structure. Two La3+ ions disrupted water less than did six Na+ ions, suggesting that, for the alkali and lanthanide ions, the number of counterions in FAU has a larger effect on the structure of water than the charge of the ion itself.

In-Depth Molecular Dynamics Study of All Possible Chondroitin Sulfate Disaccharides Reveals Key Insight into Structural Heterogeneity and Dynamism.

GAGs exhibit a high level of conformational and configurational diversity, which remains untapped in terms of the recognition and modulation of proteins. Although GAGs are suggested to bind to more than 800 biologically important proteins, very few therapeutics have been designed or discovered so far. A key challenge is the inability to identify, understand and predict distinct topologies accessed by GAGs, which may help design novel protein-binding GAG sequences. Recent studies on chondroitin sulfate (CS), a key member of the GAG family, pinpointing its role in multiple biological functions led us to study the conformational dynamism of CS building blocks using molecular dynamics (MD). In the present study, we used the all-atom GLYCAM06 force field for the first time to explore the conformational space of all possible CS building blocks. Each of the 16 disaccharides was solvated in a TIP3P water box with an appropriate number of counter ions followed by equilibration and a production run. We analyzed the MD trajectories for torsional space, inter- and intra-molecular H-bonding, bridging water, conformational spread and energy landscapes. An in-house phi and psi probability density analysis showed that 1→3-linked sequences were more flexible than 1→4-linked sequences. More specifically, phi and psi regions for 1→4-linked sequences were held within a narrower range because of intra-molecular H-bonding between the GalNAc O5 atom and GlcA O3 atom, irrespective of sulfation pattern. In contrast, no such intra-molecular interaction arose for 1→3-linked sequences. Further, the stability of 1→4-linked sequences also arose from inter-molecular interactions involving bridged water molecules. The energy landscape for both classes of CS disaccharides demonstrated increased ruggedness as the level of sulfation increased. The results show that CS building blocks present distinct conformational dynamism that offers the high possibility of unique electrostatic surfaces for protein recognition. The fundamental results presented here will support the development of algorithms that help to design longer CS chains for protein recognition.

Electro-osmotic flow through nanochannel with different surface charge configurations: A molecular dynamics simulation study

Electro-osmotic flow behavior through rectangular graphene nanochannels with different charge (negative in nature) configurations is discussed in detail using non-equilibrium molecular dynamics (MD) simulations. Alternate patterning of charged and neutral stripes on the surface of the nanochannel lowers the water permeance and electro-osmotic flow velocity through the nanochannel. For all of the charge configurations, water permeance and electro-osmotic velocity through the nanochannel increase as surface charge density (σ) increases from 0.005 to 0.025 C m−2. This can be attributed to the increase in the number of counterions (Na+ ions) near the surface of the nanochannel. However, with further increase in σ, water permeance and electro-osmotic velocity through the nanochannel gradually decrease despite the increase in the number of counterions near the surface of the nanochannel. This is because of the significant increase in electrostatic interaction between the water molecules and the surface of the nanochannel. At a lower value of σ (σ≤0.025 C m−2), the overall interaction between the water molecules and the surface of the nanochannel is significantly dominated by van der Waals (vdW) interactions (electrostatic/vdW ≤0.40). The slip velocity of water molecules in the charged stripe portion of the wall (SlipCharge) is higher as compared to the slip velocity of water molecules in the neutral stripe portion (SlipNeutral) except at σ=0.2 cm−2. This difference between SlipCharge and SlipNeutral is highest at σ=0.025 C m−2 with SlipCharge > SlipNeutral, for all of the charge configurations.

One-dimensional colloidal model with dielectric inhomogeneity.

We consider a one-dimensional model allowing analytical derivation of the effective interactions between two charged colloids. We evaluate exactly the partition function for an electroneutral salt-free suspension with dielectric jumps at the colloids' position. We derive a contact relation with the pressure that shows there is like-charge attraction, whether or not the counterions are confined between the colloids. In contrast to the homogeneous dielectric case, there is the possibility for the colloids to attract despite the number of counterions (N) being even. The results are shown to recover the mean-field prediction in the limit N→∞.

Putting the Squeeze on Molecule-Based Magnets: Exploiting Pressure to Develop Magneto-Structural Correlations in Paramagnetic Coordination Compounds

The cornerstone of molecular magnetism is a detailed understanding of the relationship between structure and magnetic behaviour, i.e., the development of magneto-structural correlations. Traditionally, the synthetic chemist approaches this challenge by making multiple compounds that share a similar magnetic core but differ in peripheral ligation. Changes in the ligand framework induce changes in the bond angles and distances around the metal ions, which are manifested in changes to magnetic susceptibility and magnetisation data. This approach requires the synthesis of a series of different ligands and assumes that the chemical/electronic nature of the ligands and their coordination to the metal, the nature and number of counter ions and how they are positioned in the crystal lattice, and the molecular and crystallographic symmetry have no effect on the measured magnetic properties. In short, the assumption is that everything outwith the magnetic core is inconsequential, which is a huge oversimplification. The ideal scenario would be to have the same complex available in multiple structural conformations, and this is something that can be achieved through the application of external hydrostatic pressure, correlating structural changes observed through high-pressure single crystal X-ray crystallography with changes observed in high-pressure magnetometry, in tandem with high-pressure inelastic neutron scattering (INS), high-pressure electron paramagnetic resonance (EPR) spectroscopy, and high-pressure absorption/emission/Raman spectroscopy. In this review, which summarises our work in this area over the last 15 years, we show that the application of pressure to molecule-based magnets can (reversibly) (1) lead to changes in bond angles, distances, and Jahn–Teller orientations; (2) break and form bonds; (3) induce polymerisation/depolymerisation; (4) enforce multiple phase transitions; (5) instigate piezochromism; (6) change the magnitude and sign of pairwise exchange interactions and magnetic anisotropy, and (7) lead to significant increases in magnetic ordering temperatures.

Protonation-Driven Aqueous Lyotropic Self-Assembly of Synthetic Six-Tail Lipidoids.

We report the aqueous lyotropic mesophase behaviors of protonated amine-based "lipidoids," a class of synthetic lipid-like molecules that mirrors essential structural features of the multitail bacterial amphiphile lipid A. Small-angle X-ray scattering (SAXS) studies demonstrate that the protonation of the tetra(amine) headgroups of six-tail lipidoids in aqueous HCl, HNO3, H2SO4, and H3PO4 solutions variably drives their self-assembly into lamellar (Lα) and inverse micellar (III) lyotropic liquid crystals (LLCs), depending on acid identity and concentration, amphiphile tail length, and temperature. Lipidoid assemblies formed in H2SO4(aq) exhibit rare inverse body-centered cubic (BCC) and inverse face-centered cubic (FCC) micellar morphologies, the latter of which unexpectedly coexists with zero mean curvature Lα phases. Complementary atomistic molecular dynamics (MD) simulations furnish detailed insights into this unusual self-assembly behavior. The unique aqueous lyotropic mesophase behaviors of ammonium lipidoids originate in their dichotomous ability to adopt both inverse conical and chain-extended molecular conformations depending on the number of counterions and their identity, which lead to coexisting supramolecular assemblies with remarkably different mean interfacial curvatures.

The apparent viscosity of poly(acryloyloxyethyltrimethyl ammonium chloride-acrylamide) with serial molecular weights in the aqueous salt solutions

In order to study the relationship between the structure and solution property of poly(acryloyloxyethyltrimethyl ammonium chloride-acrylamide)(P(DAC-AM)), the P(DAC-AM) with cationicity of 50% and serial molecular weights were synthesized by aqueous solution polymerization, and characterized by gel permeation chromatography (GPC). Then the effects of inorganic salts on the apparent viscosity (ηa) of P(DAC-AM) were investigated. The results showed that the value of ηa increased with the increase of the molecular weight when the molecular weights of P(DAC-AM) synthesized were 1.67 × 106, 2.16 × 106 and 2.90 × 106 g/mol. The ηa firstly decreased to the inflection point, then remained the value or increased with the increase of the salt concentration. It was discovered that the multivalent salt ions had a great effect on the ηa due to the larger number of counter ions and the larger volume of the complex formed by themselves. Furthermore, the effect of anions on the ηa of P(DAC-AM) was larger than that of cations. The above researches laid a foundation for the further study of the relationship among the structure, property and performance of P(DAC-AM) with serial cationicity.

Физико-химические и селективные свойства гетерогенных ионообменных мембран МК-40 и МА-40 после электродиализа природных вод

Установлено влияние структурных изменений и образования осадков после электродиализа природных вод на равновесные и селективные свойства гетерогенных ионообменных мембран МК-40 и МА-40. Образование малорастворимых осадков на поверхности и в объеме мембран, извлеченных из электродной секции реверсивного электродиализатора, приводит к блокировке функциональных групп и транспортных каналов, уменьшению обменной емкости, влагосодержания и чисел переноса противоионов, затруднению процессов переноса. Для мембраны из электродиализатора-концентратора выявлено увеличение макропористости, которое является основной причиной роста влагосодержания на фоне потери обменной емкости и селективности.

Charge Melting of Liposome Colloidal Crystals

Recently we measured the charge dependence of electrostatic interparticle repulsions in liposome colloidal crystals (Cohen, Wei & Ou-Yang, ms. in preparation). The liposomes were made from binary mixtures of charged and neutral phospholipids at various ratios. The shear modulus (rigidity) of each liposome suspension in deionized water was measured by laser-tweezer methods. As a function of increasing liposome charge, suspension rigidity first increased, peaked at ∼20% charged-lipid content, then decreased, becoming watery near 100% charged lipids. By analogy to colloidal-crystal “shear melting,” we call this phenomenon “charge melting.” The effect was calculated quantitatively by use of the Wigner-Seitz (WS) model, which requires solving the electrostatics problem of a radially symmetric charged sphere and its counterions in an electroneutral spherical WS shell whose radius R is determined by the liposome concentration. Electroneutrality means the entire H+ screening cloud surrounding each liposome is compressed into its WS cell. The compression creates an osmotic pressure related to the electrostatic potential ψ(R) at the WS-cell surface. The osmotic pressure is a measure of the interparticle interaction strength, which is what stiffens the sample. “Melting” as a function of particle charge Z arises because electroneutrality requires (a) as Z increases, so does the number of counterions, which increases osmotic pressure and ψ(R). This effect is linear in Z. (b) More counterions also cause more screening of Z, which decreases ψ(R) and osmotic pressure. This effect is exponential in Z, so it ultimately dominates. We solve the WS electrostatics problem analytically in the Debye-Hückel approximation and numerically with the full Poisson-Boltzmann equation. The problem must be solved self-consistently for a closed system with no reservoir, a counterion-only electrolyte, surface H+ binding with charge regulation, and zero electric fields at the WS-cell surface and liposome center.

Preparation, Characterization and Transport Properties of Novel Cation-Exchange Nanocomposite Membrane Containing BaFe12O19 Nanoparticles

A new type of ion-exchange nanocomposite membranes was prepared by addition of barium ferrite nanoparticles to a blend containing sulfonated poly (2,6-dimethyl-1,4-phenylene oxide) and sulfonated polyvinylchloride via a simple casting method. Hard magnetic BaFe12O19 nanoparticles were synthesized via a facile sonochemical-assisted reaction. Nanoparticles and nanocomposites were then characterized using scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction and alternating gradient force magnetometer. Various characterizations revealed that the addition of different amounts of inorganic fillers could affect the membrane performance. The inorganic nanoparticles not only created extra pores and water channels that led to improve ion conductivity, but also provided higher permselectivity and transport number of counter-ions.

On dissociation in weakly doped ice

Currently, there is some ambiguity in the problem of decay of a single donor into charged fragments. Thus, in the well-known Ostwald approximation used for semiconductors (ice being one of them) the donor dissociation degree of tends to its maximum value (i.e., unity) as the doping impurity concentration approaches zero. At the same time, the statistical theory of atom reveals within the Thomas–Fermi (or Debye–Hückel) approximation the existence of a thermodynamically equilibrium state of a single multi-electron atom (donor) where charged nucleus keeps the number of counterions just necessary for its neutralization. These scenarios do not show the atom dissociation at all. Discussed in the present paper is the alternative between the full dissociation of a single donor (i.e., dissociation degree equals unity) in a semiconducting media (ice, water, semiconductor) and zero dissociation degree.

Two-dimensional nanocrystals of molecular Janus particles.

This paper describes a rational strategy to obtain self-assembled two-dimensional (2D) nanocrystals with definite and uniform thickness from a series of molecular Janus particles based on molecular nanoparticles (MNPs). MNPs are 3D framework with rigid shapes. Three different types of MNPs based on derivatives of polyhedral oligomeric silsesquioxane (POSS), [60]fullerene (C60), and Lindqvist-type polyoxometalate (POM) are used as building blocks to construct these amphiphilic molecular Janus particles by covalently connecting hydrophobic crystalline BPOSS with a charged hydrophilic MNP. The formation of 2D nanocrystals with an exact thickness of double layers of molecules is driven by directional crystallization of the BPOSS MNP and controlled by various factors such as solvent polarity, number of counterions, and sizes of the MNPs. Strong solvating interactions of the ionic MNPs in polar solvents (e.g., acetonitrile and dimethylformamide) are crucial to provide repulsive interactions between the charged outlying ionic MNPs and suppress further aggregation along the layer normal direction. The number of counterions per molecule plays a major role in determining the self-assembled morphologies. Size matching of the hydrophobic and ionic MNPs is another critical factor in the formation of 2D nanocrystals. Self-assembly of rationally designed molecular Janus particles provides a unique "bottom-up" strategy to engineer 2D nanostructures.

How Counterions Affect the Solution Structure of Polyoxoaurates: Insights from UV/Vis Spectral Simulations and Electrospray Mass Spectrometry

AbstractThe solution structure of the two presently known polyoxoaurates, [Au4As4O20]8– (Au4As4) and [Au4Se4O16]4– (Au4Se4), was elucidated by a combination of time‐dependent density‐functional‐theory (TD‐DFT) calculations, experimental UV/Vis spectroscopy, and electrospray ionization mass spectrometry (ESI‐MS). The TD‐DFT‐predicted electronic spectra for molecular models including different numbers of counterions, along with the main Au4As4 and Au4Se4 structural motifs, were analyzed and compared with the experimental UV/Vis spectra of the two polyanions. The performed analysis revealed important structural differences between the arsenate and the selenite derivatives in aqueous solution. For the selenite oxoaurate, the bare Au4Se4 polyanion itself shows very good agreement between predicted and experimental excitation energies. In contrast, the solution state of the arsenate oxoaurate appears to be best described by a dimeric assembly of the type [Na5Au8As8O40]11– (Na5Au8As8) with a stoichiometry and structure very similar to those reported for the solid state. Indirect experimental evidence that Na5Au8As8 is the dominant species in aqueous solution is provided by ESI‐MS measurements. Our work demonstrates that the combination of experimental UV/Vis spectroscopy and TD‐DFT calculations complemented with electrospray mass spectrometry may serve as an alternative strategy for rationalizing the solution chemistry of polyoxometalates, which cannot be studied by conventionally used spectroscopic techniques, such as NMR or EPR spectroscopy.

Electrochemical studies on zirconium phosphoborate based heterogeneous membranes

Electrode potential measurements have been applied to study electrical characteristics like transport numbers, permselectivity & fixed charged density of zirconium phosphoborate ion exchange membranes. The potential measurements were made across the cation exchange membrane maintained at 27±0.1 °C, using halide and nitrate salts of alkali and alkaline earth metals as electrolytes. The membrane potentials, transport numbers and permselectivity values increase with increase in average concentration from 0.0055 M to 0.0495 M for 1:1 and 1:2 electrolytes. With the increase in concentration of the electrolyte, the number of counter ions interacting with the membrane surface increases leading to enhanced Donnan exclusion responsible for the increase of transport numbers. Fixed charge density of the membrane (X) for 1:2 electrolytes is higher in magnitude than for 1:1 electrolytes indicating that the cation exchange is taking place as hydrated species. This hypothesis is supported by higher transport numbers for alkaline earth metal ions than alkali metal ions throughout the range of concentration.

Pyridinium and imidazolium 1,3,4-oxadiazole ionic liquid crystals: a thermal and photophysical systematic investigation

In this study we present the synthesis and complete structural characterization of twenty five 1,3,4-oxadiazole molecules (ionic and non-ionic) with some systematic structural variations, such as number of counterions, number of alkoxy chains, alkyl chain size, type of counterion and its position on the molecule. The thermal and photophysical properties for all compounds were investigated allowing a complete and interesting simultaneous study of the effect of structural changes on these properties. We show that every parameter studied affected both the thermal and photophysical properties. For these bent core molecules, the liquid crystalline behavior was observed only for compounds containing one counterion, SmA (monolayer or bilayer) being the predominant phase. The neutral (non-ionic) compounds and the imidazolium derivatives showed emission at around 360 nm and a high quantum yield, while most of the pyridinium derivatives showed emission at around 530 nm, a low quantum yield and a complex photophysical behavior. In all cases, the quantum yield for the different counterions follows the sequence ClO4− > BF4− > DS− > NO3− > Br−(I−).

The effect of aprotic solvent on the selectivity of ion-exchange membranes

The effect of N,N-dimethylacetamide on the selectivity of heterogeneous (MK-40, MA-40, and MA-41) and homogeneous (MF-4SK) ion-exchange membranes is studied for the first time. Concentration dependences of electrical conductivity and diffusion permeability of the membranes were measured experimentally over wide lithium chloride concentrations; on their basis, electrodiffusion coefficients of the co- and counterions were calculated. The interrelation between the electrodiffusion coefficients and the specific moisture capacity of the heterogeneous and homogeneous membranes (which affect their selectivity) is revealed. The calculated electrodiffusion coefficients were used in the calculations of the electromigration transport numbers of counterions in the initial membranes and those processed in mixed solvent. It is shown that the heterogeneous membrane selectivity either increased under the action of the aprotic solvent to polymer material (MA-40, MA-41) or remained practically unchanged (MK-40); the selectivity of homogeneous perfluorinated membranes (e.g., MF-4SK) decreased, thus approaching that of the studied heterogeneous anion-exchange membranes.

Effects for the Incorporation of Five-atom Thioacetamido Nucleic Acid (TANA) Backbone on Hybridization Thermodynamics and Kinetics of DNA Duplexes

Thermodynamic studies of nucleic acids serve not only to widen our understanding on the nature and strength of forces that stabilize nucleic acids in a myriad of structural states they assume but also to facilitate the development of databases that could be used to predict the stability and selectivity of probe/primer-sets required in a broad range of nucleic acid-based diagnostic and therapeutic protocols. In the current study, we investigated the effect of a novel, backbone-modified "thioacetamido linkage" (TANA) on thermodynamics of hybridization, binding kinetics, and conformation of a DNA duplex. The modification comprises of an extended five-atom amide (N3'-CO-CH2-S-CH2) linker, as opposed to four-atom phosphodiester linker backbone present in DNA. One to three TANA-substitutions have been introduced in the linker backbone of two thymidine residues of one of the strand of the DNA duplex. Using spectroscopic and calorimetric techniques, we observed that TANA destabilizes the DNA helix by lowering the favorable enthalpy parameter of duplex formation. TANA x DNA duplexes were found to adopt a conformation toward an A-type duplex as shown by circular dichroism spectroscopy studies. Analysis of differential scanning calorimetry data indicated a nonzero heat capacity change, deltaCp, accompanying the duplex formation. The average deltaCp, change per duplex was found to be 832.5 cal mol(-1) K(-1), giving an average base-pair change of 59.5 cal (mol of base pairs)(-1) K(-1). Hybridization kinetic measurements using surface plasmon resonance indicated a decrease in binding affinity parameter (KA) that originates from higher dissociation rate constants (k(d)). Furthermore, optical melting studies showed that increasing the number of modifications results in a modest change in the number of counterions taken up during duplex formation.

Electric transport properties of ultra-and nanoporous glasses in electrolyte solutions

Electric transport characteristics (conductivity, specific surface conductance, and transport numbers of counterions) for nano-and ultraporous glass membranes with pore radii of 1.3–160 nm are studied and compared for chloride solutions containing single-, double-, and triple-charged cations.

Counterions in layer-by-layer films—Influence of the drying process

The amount of counterions, measured by means of X-ray photoelectron spectroscopy (XPS), in layer-by-layer (LbL) films of poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS), prepared from solutions with various NaCl concentrations, is shown to be greatly influenced by the film drying process: a smaller amount of counterions is observed in films dried after adsorption of each layer, when compared with films that were never dried during the film preparation. This is attributed to the formation of NaCl nanocrystals during the drying process which dissolve when the film is again immersed in the next polyelectrolyte solution. The presence of bonded water molecules was confirmed in wet films indicating that the counterions near the ionic groups are immersed in a water network. The number of counterions is dependent on the amount of salt in polyelectrolyte solutions in such a way that for a concentration of 0.2 M the relative amount of counterions attains saturation for both dried and wet samples, indicating that the process which leads the aggregation of counterions near of the ionic groups is not influenced by the drying process. Moreover, it is proven for wet samples that the increase in salt concentration leads to a decrease in the number of PAH ionized groups as predicted by the Muthukumar theory [J. Chem. Phys. 120 (2004) 9343] accounting for the counterion condensation on flexible polyelectrolytes.

Electrophoretic properties of highly charged colloids: A hybrid molecular dynamics∕lattice Boltzmann simulation study

Using computer simulations, the electrophoretic motion of a positively charged colloid (macroion) in an electrolyte solution is studied in the framework of the primitive model. In this model, the electrolyte is considered as a system of negatively and positively charged microions (counterions and coions, respectively) that are immersed into a structureless medium. Hydrodynamic interactions are fully taken into account by applying a hybrid simulation scheme, where the charged ions (i.e., macroion and electrolyte), propagated via molecular dynamics, are coupled to a lattice Boltzmann (LB) fluid. In a recent electrophoretic experiment by Martin-Molina et al. [J. Phys. Chem. B 106, 6881 (2002)], it was shown that, for multivalent salt ions, the mobility mu initially increases with charge density sigma, reaches a maximum, and then decreases with further increase of sigma. The aim of the present work is to elucidate the behavior of mu at high values of sigma. Even for the case of monovalent microions, a decrease of mu with sigma is found. A dynamic Stern layer is defined that includes all the counterions that move with the macroion while subjected to an external electrical field. The number of counterions in the Stern layer, q(0), is a crucial parameter for the behavior of mu at high values of sigma. In this case, the mobility mu depends primarily on the ratio q(0)/Q (with Q the valency of the macroion). The previous contention that the increase in the distortion of the electric double layer (EDL) with increasing sigma leads to the lowering of mu does not hold for high sigma. In fact, it is shown that the deformation of the EDL decreases with the increase of sigma. The role of hydrodynamic interactions is inferred from direct comparisons to Langevin simulations where the coupling to the LB fluid is switched off. Moreover, systems with divalent counterions are considered. In this case, at high values of sigma the phenomenon of charge inversion is found.