Charge Repulsion Research Articles

Tables of equation-of-state, thermodynamic properties, and shock Hugoniot for hot dense fluid deuterium

We present computational results and tables of the equation-of-state, thermodynamic properties, and shock Hugoniot for hot dense fluid deuterium. The present results are generated using a recently developed chemical model that takes into account different high density effects such as Coulomb interactions among charged particles, partial degeneracy, and intensive short range hard core repulsion. Internal partition functions are evaluated in a statistical-mechanically consistent way implementing recent developments in the literature. The shock Hugoniot curve derived from the present tables is overall in reasonable agreement with the Hugoniot derived from the Nova-laser shock wave experiments on liquid deuterium, showing that deuterium has a significantly higher compressibility than predicted by the SESAME tables or by Path Integral Monte Carlo calculations. Computational results are presented as surface plots for the dissociated fraction, degree of ionization, pressure, and specific internal energy for densities ranging from 0.0001 to 40 g/cm3 and temperatures from 2000 to ∼106 K. Tables for values of the above mentioned quantities in addition to the specific heat at constant pressure, cp, ratio of specific heats, cp/cv, sound speed and Hugoniot curve (for a specific initial state) are presented for practical use.

Comprehensive Screen of Metal Oxide Nanoparticles for DNA Adsorption, Fluorescence Quenching, and Anion Discrimination

Although DNA has been quite successful in metal cation detection, anion detectioin remains challenging because of the charge repulsion. Metal oxides represent a very important class of materials, and different oxides might interact with anions differently. In this work, a comprehensive screen of common metal oxide nanoparticles (MONPs) was carried out for their ability to adsorb DNA, quench fluorescence, and release adsorbed DNA in the presence of target anions. A total of 19 MONPs were studied, including Al2O3, CeO2, CoO, Co3O4, Cr2O3, Fe2O3, Fe3O4, In2O3, ITO, Mn2O3, NiO, SiO2, SnO2, a-TiO2 (anatase), r-TiO2 (rutile), WO3, Y2O3, ZnO, ZrO2. These MONPs have different DNA adsorption affinity. Some adsorb DNA without quenching the fluorescence, while others strongly quench adsorbed fluorophores. They also display different affinity toward anions probed by DNA desorption. Finally, CeO2, Fe3O4, and ZnO were used to form a sensor array to discriminate phosphate, arsenate, and arsenite from the rest using linear discriminant analysis. This study not only provides a solution for anion discrimination using DNA as a signaling molecule but also provides insights into the interface of metal oxides and DNA.

Surfactant effect on the physicochemical characteristics of γ-oryanol-containing solid lipid nanoparticles

Abstract Surfactants employed in production of solid lipid nanoparticles (SLNs) play a major role on physical stability of nanoparticles, and the extent of drug dissolution and permeability into cells. In this study, cationic (cetyl pyridinium chloride), anionic (sodium dodecyl sulfate) and non-ionic (Tween 80) surfactants, as well as its combinations, were employed to formulate compritol based SLNs using γ-oryzanol as a model drug. The physicochemical properties of drug-loaded SLNs are influenced by the surfactant composition systems. Surfactant blend systems gave SLNs with superior stability compared with single surfactant systems due to enhanced incorporation of the drug within the solid lipid matrix. This may be due to synergic effects arising from electrostatic repulsion of the surfactant charges (ionic), the steric hindrance of surfactant structure (non-ionic), and ion-pairing effects between cationic and anionic ones. The cytotoxicity results demonstrated that positively charged SLNs can be uptaken into cells to a high extent. Nevertheless, the mixed surfactant systems (cationic/non-ionic and cationic/anionic/non-ionic), have remarkable advantages providing SLNs with high cell internalization capability, however, low cytotoxicity. This systematic investigation may give an insight into the practical integration of multi-surfactant systems to achieve non-toxic and highly effective drug delivery systems.

Precise colloids with tunable interactions for confocal microscopy.

Model colloidal systems studied with confocal microscopy have led to numerous insights into the physics of condensed matter. Though confocal microscopy is an extremely powerful tool, it requires a careful choice and preparation of the colloid. Uncontrolled or unknown variations in the size, density, and composition of the individual particles and interactions between particles, often influenced by the synthetic route taken to form them, lead to difficulties in interpreting the behavior of the dispersion. Here we describe the straightforward synthesis of copolymer particles which can be refractive index- and density-matched simultaneously to a non-plasticizing mixture of high dielectric solvents. The interactions between particles are accurately tuned by surface grafting of polymer brushes using Atom Transfer Radical Polymerization (ATRP), from hard-sphere-like to long-ranged electrostatic repulsion or mixed charge attraction. We also modify the buoyant density of the particles by altering the copolymer ratio while maintaining their refractive index match to the suspending solution resulting in well controlled sedimentation. The tunability of the inter-particle interactions, the low volatility of the solvents, and the capacity to simultaneously match both the refractive index and density of the particles to the fluid opens up new possibilities for exploring the physics of colloidal systems.

The activity of CouR, a MarR family transcriptional regulator, is modulated through a novel molecular mechanism.

CouR, a MarR-type transcriptional repressor, regulates the cou genes, encoding p-hydroxycinnamate catabolism in the soil bacterium Rhodococcus jostii RHA1. The CouR dimer bound two molecules of the catabolite p-coumaroyl–CoA (Kd = 11 ± 1 μM). The presence of p-coumaroyl–CoA, but neither p-coumarate nor CoASH, abrogated CouR's binding to its operator DNA in vitro. The crystal structures of ligand-free CouR and its p-coumaroyl–CoA-bound form showed no significant conformational differences, in contrast to other MarR regulators. The CouR–p-coumaroyl–CoA structure revealed two ligand molecules bound to the CouR dimer with their phenolic moieties occupying equivalent hydrophobic pockets in each protomer and their CoA moieties adopting non-equivalent positions to mask the regulator's predicted DNA-binding surface. More specifically, the CoA phosphates formed salt bridges with predicted DNA-binding residues Arg36 and Arg38, changing the overall charge of the DNA-binding surface. The substitution of either arginine with alanine completely abrogated the ability of CouR to bind DNA. By contrast, the R36A/R38A double variant retained a relatively high affinity for p-coumaroyl–CoA (Kd = 89 ± 6 μM). Together, our data point to a novel mechanism of action in which the ligand abrogates the repressor's ability to bind DNA by steric occlusion of key DNA-binding residues and charge repulsion of the DNA backbone.

Pyridinic nitrogen doped nanoporous graphene as desalination membrane: Molecular simulation study

Functionalized nanoporous graphene has shown great promise as emerging membrane. Herein, we computationally demonstrate the separation performance using pyridinic-like nitrogen doped nanoporous graphene as reverse osmosis desalination membrane. The water permeation and salt rejection of the functionalized graphene membranes with various nitrogen-doping levels were simulated and characterized. We show that all functionalized graphenes investigated in this work exhibit higher water flux and acceptable salt rejection. In particular, the NOH graphene membrane with partial hydroxyl group inclusion shows excellent desalination efficiency. The interfacial properties of water and ions, as well as their free energy landscapes in passing through the graphene nanopores have been simulated in order to explore the desalination mechanism. The moderate free energy barriers for water passages confirm larger water fluxes in the functionalized graphene membranes. It was revealed that the salt rejection for the functionalized graphenes is the pore size exclusion of hydrated ions and the charged repulsion from pore surfaces is not the main factor. Overall, our results indicate that pyridinic-like nitrogen doped graphenes have a significant potential as nanostructured desalination membranes.

A novel biomimetic logic gate for sensitive and selective detection of Pb(II) base on porous alumina nanochannels

A novel biomimetic logic gate sensor for Pb2 + is established using porous alumina membrane nanochannels modified with morpholino and DNA. It is based on electrochemical detection, and the current response from the diffusion flux of Fe(CN)63 − is influenced by the steric blockage and charge repulsion in nanochannels. A limit of detection (0.1 nM) and good linear range (0.1 nM–5 μM) for Pb2 + analysis are achieved in the tenth cycle. The sensing strategy shows prospective application in drug release, artificial ion channels, DNA logic gates for controlling biomolecule, and ion translocation.

Negativity of Fukui function of some isolated s and p block elements: The role of orbital relaxation effect

In the present article, we emphasize the correlation between orbital relaxation effect and nature of atomic Fukui functions. While doing so, f−(r¯) and f+(r¯) values are found to be positive and negative, respectively, for the chosen s block elements. Also, f−(r¯) and f+(r¯) values for chosen p block atoms are negative. Nodal nature of the highest occupied and lowest unoccupied orbitals, electron–electron repulsion and effective nuclear charge become handy in explaining the observed trends. Out of track results obtained for the inert atoms are also explained.

Phosphatidylglycerol Flip-Flop Suppression due to Headgroup Charge Repulsion.

The kinetics and thermodynamics of 1,2-distearoyl-sn-glycero-3-[phospho(1'-rac-glycerol)] (DSPG) flip-flop in 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) membranes were examined by sum-frequency vibrational spectroscopy (SFVS). The effect of DSPG concentration in the membrane and the influence of electrolyte concentration were examined in an attempt to decipher the role the anionic PG headgroup plays in dictating the dynamics of PG flip-flop for this biologically important lipid species. DSPG flip-flop dynamics and the activation barrier to exchange were found to be directly dependent on the amount of DSPG present in the bilayer. Analysis of the activation free energy for DSPG flip-flop in mixed DSPG + DSPC bilayers reveals that charge repulsion between neighboring PG headgroups modulates the free energy barrier and subsequently, the rate of translocation. Specifically, when DSPG comprises a small portion of the bilayer, the electrostatic potential of neighboring PG lipids are effectively shielded from each other under high ionic strength conditions and little to no charge repulsion occurs. When DSPG lipids are close enough to experience charge repulsion from neighboring PG lipids, as in bilayers containing a large fraction of DSPG, or for bilayers in low ionic strength solutions, the influence of charge repulsion on the energetics of lipid flip-flop are measurable. For biological membranes, where the concentration of PG is relatively low, the neighboring PG lipids are spaced far enough apart that their anionic charges are effectively shielded, such that under physiological conditions the charged nature of the headgroup does little to modulate its lipid flip-flop energetics and corresponding rate of translocation.

Influence of hydrothermal synthesis conditions on size, morphology and colloidal properties of Hematite nanoparticles

The synthesis, morphological characterization, and colloidal properties of Hematite nanoparticles (hemNP) with different sizes and shapes were presented. To prepare hemNP we used hydrothermal synthesis with different combinations of reaction conditions. Various characterization techniques involving XRD, TEM, AFM and DLS were employed to describe the dependence of the hemNP size, charge and morphology on different synthesis conditions. Spheroidal Hematite nanoparticles from 20 to 60 nm were obtained by varying the rate of addition of iron precursor solutions and the aging time (Δt) at ca. 100∘C. The transformation from Ferrihydrite to Hematite (α-Fe2O3) started with a burst nucleation of nanosized crystallites, followed by a first-order kinetic growth of primary grains up to a final threshold size of ca 30 nm. Colloidal behavior was also checked at pH 4 and 7, finding interparticle aggregation at circumneutral pH only for hemNP obtained for Δt<2 h. Electrophoretic ζ potential measurements yield positive values at pH 4 and negative at pH 7. The shifts in (IEP) at lower values depended on the adsorption of appreciable counterions on the particle surface and reduced at pH 7 the charge–charge repulsion, whereas the attractive forces became predominant. The results confirmed that the particle size of the Hematite nanoparticle critically affected their colloidal behavior with significant consequences for sorption and aggregation processes. Particularly for low particle sizes of the order of 10–20 nm, the surface charge and surface reactivity also could differ in important ways compared to 40–60 nm size particles.

Highly Sensitive CO₂-Responsive Polymeric Microgels That Respond Within Seconds.

In this work, polymeric microgels with swift response to CO2 are synthesized by polymerization of tertiary-amine containing methacrylate monomers (N,N-diethylaminoethyl methacrylate, DEAEMA) and polyethylene glycol monomethyl ether acrylate (PEGMA) as stabilizers. The obtained microgels are stable but very sensitive to CO2, which can rapidly swell and further collapse within 5 s upon bubbling of CO2, or within minutes in an atmosphere of gaseous CO2. The protonation of the tertiary amine groups in the presence of CO2 induces sensitive swelling and further irreversible collapse of the microgels due to the internal charge repulsion and relatively low cross-linking density in the core area of microgels. This rapid response to CO2 may find further applications in the fields of sensitive detection or responsive loading and release upon CO2 stimulus.

Analysis of a Four H-Bond Array Using Cyclic Voltammetry: Introducing a New Redox Center to Strengthen Dimerization

Hydrogen bonding arrays are useful building blocks for supramolecular chemistry due to their strong hydrogen bond interactions that can form dimers. The strength of the H-bond interactions can be perturbed by applying an external stimuli, in this case electron transfer. Recently, our group has modified Meijer’s four H-bond ureidopyrimidinone (UPy) array1 to make it electroactive by adding a dimethylaminophenyl group. The modified structure forms strong H-bond interactions in CH2Cl2, creating a dimer that can undergo a reversible 2e-oxidation, followed by an irreversible oxidation at more positive potentials. The latter results in the break-up of the dimer due to electrostatic repulsion. For this study, we aim to modify another four H-bond Upy array by introducing a methyl pyridinium redox center as shown below. We hypothesize that this array will prefer the tautomer that presents an ADAD H-bond motif in the starting oxidation state. This motif will form a dimer with relatively weak H-bond interactions due to electrostatic repulsions and also unfavorable secondary H-bond interactions. Upon 2e- reduction, where 1e- is gained for each methyl pyridinium redox center, the H-bond strength should increase due to the loss of repulsive charges and also allows the nitrogen to become a stronger hydrogen acceptor. Since the resulting nitrogen is now a better H-acceptor, there is a possibility of intermolecular proton transfer which will induce the tautomer to have an AADD motif that will further strengthen the H-bonds by favorable secondary H-bond interactions. 1. Beijer, F.H.; Sijbesma, R.P.; Kooijman, H.; Spek, A.L.; Meijer, E.W. J. Am. Chem. Soc., 1998, 120 (27), pp 6761–6769. Figure 1

High performance separation of xylose and glucose by enzyme assisted nanofiltration

An integrated membrane system was investigated for the separation of mixtures of xylose and glucose. Separation of these sugars is extremely challenging due to their similar structure, size and charge. In order to enhance the xylose separation factor in nanofiltration (NF), we present an enzymatic process for converting glucose to gluconic acid followed by separation of xylose from gluconic acid by nanofiltration. Process conditions which favored the negative charge repulsions between gluconic acid and the NF270 membrane were examined. At the best conditions (9:1 feed molar ratio of xylose to gluconic acid, 0.15M total feed concentration, pH 9.5, 25°C and 4bar), we achieved a xylose separation factor of 34 and a throughput of 18.7Lm−2h−1. In comparison, the separation factor was only 1.4 for solutions of xylose and glucose at the same process conditions, thus demonstrating the huge potential of the integrated system. Full conversion of glucose to gluconic acid assisted by glucose oxidase (GOD) could be achieved by coupling a parallel reaction catalyzed by catalase (CAT), where H2O2 (GOD-inhibitor formed in the first reaction) was decomposed to water and oxygen. GOD has a high oxygen-demand and it was demonstrated that sufficient oxygen could be obtained by controlling the CAT-catalyzed reaction through initial H2O2 addition. The new strategy suggested in this study, integrating reaction and nanofiltration to enhance separation while obtaining another value-added stream, presents new options for separating compounds with similar molecular weights by nanofiltration.

Giant volume magnetostriction in the Y2Fe17 single crystal at room temperature

An investigation of the Y2Fe17 compound belonging to the class of intermetallic alloys of rare-earth and 3d-transition metals is presented. The magnetization, magnetostriction, and thermal expansion of the Y2Fe17 single crystal were studied. The forced magnetostriction and magnetostriction constants were investigated in the temperature range of the magnetic ordering close to the room temperature. The giant field induced volume magnetostriction was discovered in the room temperature region in the magnetic field up to 1.2 T. The contributions of both anisotropic single-ion and isotropic pair exchange interactions to the volume magnetostriction and magnetostriction constants were determined. The experimental results were interpreted within the framework of the Standard Theory of Magnetostriction and the Landau thermodynamic theory. It was found out that the giant values of the volume magnetostriction were caused by the strong dependence of the 3d-electron Coulomb charge repulsion on the deformations and width of the 3d-electron energy band.

Influence of Electrostatics on Small Molecule Flux through a Protein Nanoreactor.

Nature uses protein compartmentalization to great effect for control over enzymatic pathways, and the strategy has great promise for synthetic biology. In particular, encapsulation in nanometer-sized containers to create nanoreactors has the potential to elicit interesting, unexplored effects resulting from deviations from well-understood bulk processes. Self-assembled protein shells for encapsulation are especially desirable for their uniform structures and ease of perturbation through genetic mutation. Here, we use the MS2 capsid, a well-defined porous 27 nm protein shell, as an enzymatic nanoreactor to explore pore-structure effects on substrate and product flux during the catalyzed reaction. Our results suggest that the shell can influence the enzymatic reaction based on charge repulsion between small molecules and point mutations around the pore structure. These findings also lend support to the hypothesis that protein compartments modulate the transport of small molecules and thus influence metabolic reactions and catalysis in vitro.

RNA folding retrospective: lessons from ribozymes big and small.

RNA folding retrospective: lessons from ribozymes big and small.

On the theoretical description of weakly charged surfaces.

It is widely accepted that the Poisson-Boltzmann (PB) theory provides a valid description for charged surfaces in the so-called weak coupling limit. Here, we show that the image charge repulsion creates a depletion boundary layer that cannot be captured by a regular perturbation approach. The correct weak-coupling theory must include the self-energy of the ion due to the image charge interaction. The image force qualitatively alters the double layer structure and properties, and gives rise to many non-PB effects, such as nonmonotonic dependence of the surface energy on concentration and charge inversion. In the presence of dielectric discontinuity, there is no limiting condition for which the PB theory is valid.

Facile metal transfer method for fabricating unconventional metamaterial devices

We report a facile metal transfer method to create metal patterns on polydimethylsiloxane (PDMS) surface. The metals were first patterned on polyethylene terephthalate (PET) substrate, and then transferred to PDMS simply by oxygen plasma treatment. The transfer was enabled by weakened metal-PET adhesion due to undercut and charge repulsion. In this way, we got rid of adhesion layers and sacrificial layers that were required by conventional methods. Ours is a fast and convenient method as it takes place at room temperature with just a gentle contact of PET and PDMS. The resolution of this method was found to be in sub-micron range and the morphologies of different transferred patterns were characterized. With such a method, we successfully fabricated some unconventional metamaterial devices, including a double-sided broadband THz filter with a stop band bandwidth of 1.4 THz, and devices on paper, fabric, leaf and non-planar surfaces.

Promoted water transport across graphene oxide–poly(amide) thin film composite membranes and their antibacterial activity

Hybrid composite membranes have great potential for desalination applications since water transport can be favorably promoted by selective diffusion at the interface between matrix and reinforcement materials. In this paper, graphene oxide nano-sheets were successfully incorporated across 200nm thick poly(amide) films by interfacial polymerization to form novel thin-film composite membranes. The impact of the graphene oxide on the morphology, chemistry, and surface charge of the ultra-thin poly(amide) layer, and the ability to desalinate seawater was investigated. The graphene oxide nano-sheets were found to be well dispersed across the composite membranes, leading to a lower membrane surface energy and an enhanced hydrophilicity. The iso-electric point of the samples, key to surface charge repulsion during desalination, was found to be consistently shifted to higher pH values with an increasing graphene oxide content. Compared to a pristine poly(amide) membrane, the pure water flux across the composite membranes with 0.12wt.% of graphene oxide was also found to increase by up to 80% from 0.122 to 0.219L·μm·m−2·h−1·bar−1 without significantly affecting salt selectivity. Furthermore, the inhibitory effects of the composite membrane on microbial growth were evaluated and the novel composite membranes exhibited superior anti-microbial activity and may act as a potential anti-fouling membrane material.

Enhancement of immobilized lipase activity by design of polymer brushes on a hollow fiber membrane

A polymer brush possessing aminoethanol (AE) functional groups for lipase immobilization was grafted onto a hollow fiber membrane by radiation-induced graft polymerization. Almost the AE groups-grafted polymer brushes unfold through positive charge repulsion between the AE groups, enabling multi-layer immobilization of lipase. The hydroxyl groups in AE can also retain water molecules around hydrophilic part of the lipase. In this study, we controlled the length and density of the polymer brushes consisting of the glycidyl methacrylate (GMA) by changing the concentration of GMA monomer during radiation-induced graft polymerization. Immobilized lipase showed the highest activity on the grafted membrane when 5wt% of glycidyl methacrylate as monomer for the radiation-induced graft polymerization was used. Consequently high efficiency esterification (approximately 1600mmol/h/g-membrane) was achieved in five-layer lipase on AE polymer brush than that in monolayer lipase on the polymer brush possessing only hydroxyl groups. Moreover, the polymer brush possessing AE functional groups for lipase immobilization maintained high activity on the reuse for several times.