Ionic Strength Of Aqueous Solution Research Articles

Rheology of plant protein–polysaccharide gel inks for 3D food printing: Modeling and structure–property relations

Plant protein–polysaccharide composite hydrogels and emulsion gels are attractive materials for production of edible inks for 3D food printing. Their ability to satisfy the printability requirements is conventionally evaluated in small- and large-amplitude oscillatory shear tests. A model is derived that describes experimental data in linear and nonlinear rheological tests on protein gels in a unified manner. The model is based on the population-balance approach and involves a reasonably small number of adjustable parameters. Fitting observations on soy protein isolate–xanthan gum, peanut protein–gellan gum, peanut protein–carrageenan, pea protein–carrageenan and pea protein isolate–flaxseed gum gels demonstrates that material parameters evolve consistently with composition of the gels. The governing equations describe adequately the effects of pH and ionic strength of aqueous solutions on the mechanical properties of the gels and predict the stress–strain Lissajous curves and the crossover amplitudes for transition from the solid-like to fluid-like response. Based on the analysis of observations in large-amplitude oscillatory tests with various frequencies, a mechanism is proposed for transition from the type-I (strain thinning) to type-III (weak strain overshoot) response of protein–polysaccharide gels.

Protein analysis using capillary electrophoresis coupled to mass spectrometry through vibrating sharp-edge spray ionization.

Capillary electrophoresis (CE) interfaced to mass spectrometry (MS) with electrospray ionization typically incorporates acidic additives or organic solvents to assist in ionization. Vibrating sharp-edge spray ionization (VSSI) is a voltage-free method to interface CE and MS that does not require these additives, making it appealing for protein analyses. CE-VSSI nanoflow sheath separations are performed with low ionic strength aqueous solutions in the sheath to reduce suppression. Serine is also included in the sheath to reduce analyte adduction. Proteins are detected in the 2.5-10µM range, corresponding to an injected mass range of 0.1-1.2ng. The anionic proteins β-lactoglobulin and transferrin are resolved using an unmodified fused silica capillary because they do not exhibit nonspecific surface adsorption. Conversely, separations of cationic proteins cytochrome c, ribonuclease A, and α-chymotrypsinogen A in an unmodified capillary require acidic background electrolytes to overcome adsorption. Alternatively, a semipermanent coating comprised self-assembled lipids overcomes surface adsorption at a neutral pH. Separations with zwitterionic and hybrid cationic coatings are complete within 15 or 6min, respectively. The dimeric form of triosephosphate isomerase was observed at a 60µM, corresponding to a mass of 19ng, by dropping the temperature of the MS inlet.

Geochemistry of fluorine in halo waters of ore fields of the southeastern part of the Dnipro-Donetsk avlakogen

The geochemical features of fluorine and its possible sources in the waters of the hydrothermal ore fields of the Dnipro-Donetsk avlakogen (DDA) are considered. It was established that the formation of anomalies occurs due to the interaction of infiltration waters of the zone of free water exchange and waters of deep formation, which are unloaded along the zones of deep faults. It is substantiated that the water migration of fluorine is determined by the geochemical type of groundwater, the degree of its mineralization, acid-base (pH) and oxidation-reduction (Eh) potentials, the presence of complex-forming elements with which fluorine forms mobile complex compounds, etc. Formulation of the problem. Fluorine is a typomorphic chemical element that is widely present in groundwater. It forms large-scale, contrasting hydrogeochemical anomalies in various types of hydrothermal ore fields, which are controlled by zones of deep faults - fluorite, mercury, polymetallic, and others. Presenting main material. Anomalies of the element are also formed in those fault zones, where the processes of modern heat and mass transfer are observed, manifested in the upward discharge of endogenous fluids against the background of increased intensity of the geothermal field. This indicates the probable arrival of fluorine in endogenous fluid flows, which are mixed with formation waters during their upward migration. The most contrasting aureoles of fluorine scattering are established in the zone of hypergenic weathering of fluorite of the Pokrovo-Kyreevsky deposit and in the aureole waters of the Mykytiv mercury ore field, in the rocks of which fluorite mineralization is absent. In them, fluorine, together with other trace elements, forms multicomponent anomalies and is part of the hydrogeochemical association of elements-indicators of hidden mercury mineralization - Hg, As, B, F, (Sb). Conclusions. 1. Geochemical features of fluorine in the processes of hydrothermal mineralization are caused by both hypogenic and hypergenic factors of its migration and concentration in the hydrolithosphere of the Dnipro-Donetsk avlakogen. 2. The high migration activity of fluorine in fluid systems and, in particular, in groundwater, is determined by numerous mobile forms of the element that exist under different geochemical conditions - in the form of gas, simple anions, complex compounds with metals, silicon, manganese, boron and other chemical elements Fluorine migrates best in alkaline sodium bicarbonate and sodium chloride waters, in which Сl–, Na+ and ОН– ions not only act as a complexing agent, but also provide high ionic strength of aqueous solutions. 3. The high content of fluorine in the waters of the Pokrovo-Kyreivsky fluorite deposit is provided by two different geochemical processes: a) hypergenic physicochemical destruction of CaF2 by gravitational infiltration waters of the zone of free water exchange; b) the influx of fluorine into deep formation waters together with endogenous fluids that are discharged along fault structures at the post-hydrothermal stage of their tectonic activation. 4. Abnormally high concentrations of fluorine in the hydrothermal mercury ore fields of the Mykytivskyi and Druzhkivsko-Kostiantynivskyi deposits, in the rocks of which there is practically no fluorite mineralization, are due to the influx of the element into the mineralization zones from deep fluid flows and underground waters of deep horizons. 5. The main natural sources of fluorine in the waters of hydrothermal deposits of the region are: a) products of hypergenic weathering of hydrothermal mineralization of fluorite; b) flows of endogenous postheterothermal fluids; c) pore solutions of sedimentary rocks of marine origin.

Facile and scalable preparation of ZIF-67 decorated cotton fibers as recoverable and efficient adsorbents for removal of malachite green

Recently, metal–organic frameworks (MOFs) have received considerable attention as highly efficient adsorbents for dye wastewater remediation. However, the immobilization of MOFs on the substrate surfaces to fabricate easy recyclable adsorbents via a facile route is still a challenge. In this work, ZIF-67/cotton fibers as adsorbents for dye removal were prepared in a large-scale using a simple coordination replication method. The successful fabrication of the ZIF-67/cotton fibers was confirmed by FTIR, XRD, XPS, SEM and BET analysis, respectively. As expected, the as-prepared ZIF-67/cotton fibers exhibited high adsorption capacity of 3787 mg/g towards malachite green (MG). Meanwhile, the adsorption kinetics and isotherm obeyed the pseudo-second-order kinetics and Langmuir model, respectively. Moreover, its removal efficiency towards MG was not significantly influenced by the pH and ionic strength of aqueous solution. Most importantly, the ZIF-67/cotton fibers can remove MG from synthetic effluents, and it can be easily regenerated without filtration or centrifugation processes, with the regeneration efficiency remaining over 90% even after 10 cycles. Additionally, the ZIF-67/cotton fibers presented excellent antimicrobial performance against E. coli and S. aureus. Hence, the distinctive features of the as-prepared ZIF-67/cotton fibers make it promisingly applicable for the colored wastewater treatment.

Equilibrium swelling of multi-stimuli-responsive copolymer gels

Copolymer gels prepared by polymerization of thermo-responsive and anionic monomers demonstrate strong sensitivity to several triggers such as temperature, pH and ionic strength of aqueous solutions. For biomedical applications of these materials (as on–off switches in controlled drug delivery and release), fine tuning of their volume phase transition temperature (VPTT) and a sharp decay in degree of swelling upon transition from the swollen to the collapsed state are needed. These requirements are fulfilled under swelling of copolymer gels and microgels in water under acidic conditions, but are violated when tests are conducted under alkaline conditions or in aqueous solutions of salts with physiological salinity.A model is developed for equilibrium swelling of multi-stimuli-responsive copolymer gels in aqueous solutions with arbitrary pH and molar fractions of a monovalent salt. Unlike conventional approaches, the model accounts for secondary interactions between chains (hydrogen bonding) to describe the kinetics of aggregation of hydrophobic segments above VPTT. Material constants are determined by fitting experimental swelling diagrams on poly(N-isopropylacrylamide–co–sodium acrylate) gels with various molar fractions of ionic monomers. The effects of temperature, pH and molar fraction of salt on the equilibrium degree of swelling below and above VPTT are studied numerically.

The effects of pH and ionic strength on the volume phase transition temperature of thermo-responsive anionic copolymer gels

Applications of thermo-responsive (TR) gels in biomedicine and biotechnoligy require modulation of their volume phase transition temperature Tc in a rather wide interval. This is conventionally performed by incorporation of hydrophilic or hydrophobic monomers in the feed. The most pronounced growth of Tc occurs when TR monomers are copolymerized with anionic, cationic or ampholyte monomers.The effect of ionic monomers on the volume phase transition temperature of copolymer gels is traditionally analyzed in equilibrium swelling tests in distilled water. To evaluate how pH and ionic strength of aqueous solutions affect Tc, a constitutive model is developed. Adjustable parameters are found by fitting observations on poly(N-isopropylacrylamide-co-acrylic acid) and poly(N-isopropylacrylamide-co-itaconic acid) macro- and microgels below and above their critical temperature. The ability of the model to predict Tc is validated by comparison of experimental data with results of simulation. An analytical formula is derived that describes the influence of molar fraction of comonomers, pH and molar fraction of salt in aqueous solutions on the volume phase transition temperature of TR anionic gels.

Rapid Entrapment of Phenazine Ethosulfate within a Polyelectrolyte Complex on Electrodes for Efficient NAD+ Regeneration in Mediated NAD+-Dependent Bioelectrocatalysis.

Over the past two decades, the designs of redox polymers have become critical to the field of mediated bioelectrocatalysis and are used in commercial glucose biosensors, as well as other bioelectrochemical applications (e.g., energy harvesting). These polymers are specifically used to immobilize redox mediators on electrode surfaces, allowing for self-exchange-based conduction of electrons from enzymes far from the electrode to the electrode surface. However, the synthesis of redox polymers is challenging and results in large batch-to-batch variability. Herein, we report a rapid entrapment of mediators for NAD+-dependent bioelectrocatalysis within reverse ionically condensed polyelectrolytes. A high ionic strength aqueous solution of oppositely charged polyelectrolytes, composed of cationic polyguanidinium (PG) chloride and anionic sodium hexametaphosphate (P6), undergoes phase inversion into a solid microporous polyelectrolyte complex (PEC) when introduced into a low ionic strength aqueous solution. The ionic strength-triggered phase inversion of PGP6 solutions was investigated as a means to entrap mediators on the surface of electrodes for mediated bioelectrocatalysis. Compared to the traditional cross-linked immobilizations using redox polymers, this phase inversion takes place within seconds and requires up to 60 min for complete stabilization. In this work, redox mediator phenazine ethosulfate (PES) was entrapped within PGP6 on electrode surfaces for nicotinamide adenine dinucleotide (NAD+)-dependent bioelectrocatalysis. In the bulk solution, NAD+-dependent dehydrogenase enzymes catalyze the oxidation of the substrate while reducing NAD to reduced nicotinamide adenine dinucleotide (NADH). The resulting NADH is reoxidized to NAD+ by the entrapped PES that gets reduced on the electrode, completing the NAD+-regeneration-based bioelectrocatalysis. To show the use of these new materials in an application, biofuel cells were evaluated using four different anodic enzyme systems (alcohol dehydrogenase, lactate hydrogenase, glycerol dehydrogenase, and glucose dehydrogenase).

FUNCTIONALIZED HALLOYSITE NANOTUBES FOR ENHANCED REMOVAL OF Hg2+ IONS FROM AQUEOUS SOLUTIONS

Water is essential for humans, animals, and plants; pollutants, usually derived from anthropogenic activities, can have a serious effect on its quality. Heavy metals are significant pollutants and are often highly toxic to living organisms, even at very low concentrations. Among the numerous removal techniques proposed, adsorption onto suitable adsorbent materials is considered to be one of the most promising. The objective of the current study was to determine the effectiveness of halloysite nanotubes (HNT) functionalized with organic amino or thiol groups as adsorbent materials to decontaminate polluted waters, using the removal of Hg2+ ions, one of the most dangerous heavy metals, as the test case. The effects of pH, ionic strength (I), and temperature of the metal ion solution on the adsorption ability and affinity of both materials were evaluated. To this end, adsorption experiments were carried out with no ionic medium and in NaNO3 and NaCl at I = 0.1 mol L−1, in the pH range 3–5 and in the temperature range 283.15–313.15 K. Kinetic and thermodynamic aspects of adsorption were considered by measuring the metal ion concentrations in aqueous solution. Various equations were used to fit experimental data, and the results obtained were explained on the basis of both the adsorbent’s characterization and the Hg2+ speciation under the given experimental conditions. Thiol and amino groups enhanced the adsorption capability of halloysite for Hg2+ ions in the pH range 3–5. The pH, the ionic medium, and the ionic strength of aqueous solution all play an important role in the adsorption process. A physical adsorption mechanism enhanced by ion exchange is proposed for both functionalized materials.

The performance of activated carbon/NiFe2O4 magnetic composite to retain heavy metal ions from aqueous solution

A novel magnetic activated carbon composite (AC/NiF) was synthesized by a precipitation method and applied in retention of Cu(II), and Zn(II) ions from aqueous solutions. The impact of different sorption parameters such as: equilibration time, solution pH value, competing cations and ionic strength on the amount sorbed of Cu(II), and Zn(II) was clarified. Results illustrated that the magnetic composite had retention ability towards both metal ions significantly higher than that of activated carbon (AC). The magnetic composite exhibited an affinity to adsorb Cu(II) higher than Zn(II) ions. The maximum sorption capacities (Qmax) of the applied magnetic composite (AC/NiF) towards Cu(II) and Zn(II) were 105.8 and 75.1 mg·g−1, respectively. Retention of Cu(II) and Zn(II) was proposed to be achieved though an ion exchange and surface adsorption in neutral conditions, while precipitation was believed to be the relevant mechanism in their removal from basic solutions. The kinetic studies showed that sorption process followed the kinetics of pseudo-second-order reactions with rate constant of 3 × 10−3 and 2 × 10−3 min−1 for sorption of Cu(II) and Zn(II) onto AC/NiF composite. Removal of Cu(II) slightly decreased with increasing the ionic strength of aqueous solution, using NaCl as a background electrolyte. In contrast, presence of Mn(II), Mg(II) and Co(II) in reaction solutions highly depressed the sorption of Cu(II) and Zn(II) with a competing efficiency followed the order: Mg(II) > Mn(II) > Co(II).The magnetic composite was rapidly recovered from aqueous solution by an external magnetic field, and effectively regenerated using 0.1 mol·L−1 HCl and 0.1 mol·L−1 FeCl3as eluents. Sorption of Cu(II) and Zn(II) onto the surface of AC/NiF composite occurred via a spontaneous reaction. And thermodynamically favorable process had ΔHo values of 30.9 kJ·mol−1 and 19.7 kJ·mol−1, respectively. The results confirm that the magnetic composite can be viewed as a promising novel composite opens new opportunities for the attainment of required adsorption and operative magnetic separation.

Local Electrocatalytic Activity of Electrodeposited Ni-Fe Particles

NiFe electrocatalysts have received increasing attention for the electrocatalysis of the oxygen evolution reaction.[1] Recently, using bipolar electrochemistry for the electrodeposition of nickel, we have demonstrated the formation of nickel particle gradients, which vary in size and particle density as a function of electrode position.[2] In this work, low ionic strength aqueous solutions containing Ni(II) and Fe(II) salts, without supporting electrolyte, were used for the electrodeposition of NiFe particles onto a conducting fluorine doped tin oxide substrate. In this talk, I will present our recent results exploring the effect of composition, size and morphology of NiFe particles on the local electrocatalytic activity towards the oxygen evolution reaction, measured using scanning electrochemical cell microscopy.[3][1] Song, F., Bai, L., Moysiadou, A., Lee, S., Hu, C., Liardet, L., Hu, X. Transition Metal Oxides as Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Solutions: An Application-Inspired Renaissance (2018) Journal of the American Chemical Society, 140 (25), pp. 7748-7759.[2] Beugré, R., Dorval, A., Lavallée, L.L., Jafari, M., Byers, J.C. Local electrochemistry of nickel (oxy)hydroxide material gradients prepared using bipolar electrodeposition (2019) Electrochimica Acta, 319, pp. 331-338.[3] Bentley, C.L., Kang, M., Unwin, P.R. Nanoscale Surface Structure-Activity in Electrochemistry and Electrocatalysis (2019) Journal of the American Chemical Society, 141 (6). pp. 2179-2193

Solubility and Activity Coefficients of Three Triazine-Type Compounds in Various Low Ionic Strength Aqueous Solutions

In this work, three triazine-type compounds were studied for their solubilities in aqueous and saline environments. Using potentiometric titration and gravimetric methods, the solubility of cyanuric acid (CA), 1,3,5-triacryloylhexahydro-1,3,5-triazine (TR), and 1,3,5-tris(2-hydroxyethyl) cyanuric acid (IC) were studied in aqueous NaCl solutions (0 to 0.226 mol·kg–1). From the solubility results, Setschenow coefficients, octanol/water partition coefficients, activity coefficients, and infinite dilution activity coefficients were determined. The trend of the solubility in pure water was IC ≫ TR > CA. The activity coefficient for CA increased continuously throughout the studied salt range. TR and IC, on the other hand, revealed a salting-in and salting-out effect based on the salt composition.

Magnetic ZIF-8 as a stable support for biomolecules via adsorption in aqueous buffered solutions at pH = 7

Metal-organic frameworks (MOFs) have been recently proposed as supports for biomolecules immobilization, particularly for enzymes. However, their stability under the high ionic strength aqueous solutions required for practical applications of these biocatalysts have not been evaluated. Here, it is shown that surface modifications and a careful analysis and selection of the buffer components must be accomplished to ensure the resistance of the MOF to hydrolysis, even for the previously assumed robust Zeolitic Imidazole Frameworks (ZIFs). By taking into account these considerations, and introducing polyvinylpyrrolidone (PVP) as a surfactant in the synthetic process, a resistant and stable magnetic MOF (Fe3O4@ZIF-8/PVP) was obtained. Further, the loading capacity of the stabilized synthetized structures for the immobilization of a model protein, such as bovine serum albumin (BSA), was analyzed. It was found that physical interaction provides an efficient and suitable method to immobilize this biomolecule without evidence of leaching.

Mechanisms of Methylparaben Adsorption onto Activated Carbons: Removal Tests Supported by a Calorimetric Study of the Adsorbent⁻Adsorbate Interactions.

In this study, the mechanisms of methylparaben adsorption onto activated carbon (AC) are elucidated starting from equilibrium and thermodynamic data. Adsorption tests are carried out on three ACs with different surface chemistry, in different pH and ionic strength aqueous solutions. Experimental results show that the methylparaben adsorption capacity is slightly affected by pH changes, while it is significantly reduced in the presence of high ionic strength. In particular, methylparaben adsorption is directly dependent on the micropore volume of the ACs and the π- stacking interactions, the latter representing the main interaction mechanism of methylparaben adsorption from liquid phase. The equilibrium adsorption data are complemented with novel calorimetric data that allow calculation of the enthalpy change associated with the interactions between solvent-adsorbent, adsorbent-adsorbate and the contribution of the ester functional group (in the methylparaben structure) to the adsorbate–adsorbent interactions, in different pH and ionic strength conditions. It was determined that the interaction enthalpy of methylparaben-AC in water increases (absolute value) slightly with the basicity of the activated carbons, due to the formation of interactions with π- electrons and basic functional groups of ACs. The contribution of the ester group to the adsorbate-adsorbent interactions occurs only in the presence of phenol groups on AC by the formation of Brønsted–Lowry acid–base interactions.

Dyes Adsorption onto Fe3O4‐Bis(trimethoxysilylpropyl)amine Composite Particles: Effects of pH and Ionic Strength on Electrostatic Interactions

AbstractHydrophobic, electrostatic interactions, and hydrogen bonding play important roles in dyes adsorption. Solution pH and ionic strength of aqueous solutions are important factors affecting dye adsorption by influencing the aforementioned interactions. In order to understand adsorption mechanism of dye on the mesoporous particles, Fe3O4‐bis(trimethoxysilylpropyl)amine (Fe3O4‐TSPA) composite particles were used to adsorb methyl orange (MO), methyl blue (MB), and neutral red (NR) from aqueous solutions. Fe3O4‐TSPA particles were found to be effective in removing acidic dyes at pH < 8.5. The adsorption amount of MB was nearly 0.047 mmol/g at pH 2. The adsorptions of MO and MB were found to be significantly influenced by electrostatic interactions. Based on the effect of NaCl on the adsorption, it was concluded that the electrostatic attraction was suppressed by high ionic strength, but suppression was less significant when electrostatic attraction was strong.

In Situ Nanoscale Investigation of Step Retreat on Fluoranthene Crystal Surfaces.

Fluoranthene, a polycyclic aromatic hydrocarbon, has been detected on Earth as well as in asteroids and meteorites and may have played a role in the formation of life. Increasing the ionic strength of aqueous solutions has been observed to lower the fluoranthene solubility, but it is unclear how solution composition controls the release rate of fluoranthene to an aqueous solution. To elucidate this, we performed in situ atomic force microscopy experiments in which we characterized the sublimation and dissolution behavior of fluoranthene crystal surfaces. From this, we quantify the step retreat rate upon exposure to air, deionized water, and a 0.4 M NaCl or 0.1 M MgSO4 solution. Surface roughness is the main factor that determines the dissolution or sublimation rate. The results imply that during fluoranthene remediation or breakdown in meteorites and asteroids, ionic strength will be more important than chemical composition for controlling fluoranthene release into solution.

Stability of Polydopamine Coatings on Gold Substrates Inspected by Surface Plasmon Resonance Imaging.

Polydopamine (PDA)-based surface modification has been used in a variety of fields. However, a vague impression on the stability of PDA still exists due to a lack of systematic studies. To ascertain the issue and make better use of this surface modification method, a technique of surface plasmon resonance imaging (SPRi) was exploited to study the stability of PDA coated on gold surface. The results showed that PDA-coating stability was largely dependent on the pH of aqueous solutions, giving detachment ratios up to 66% and 80% at pH 1.0 and pH 14.0, respectively. However, increasing the ionic strength of aqueous solutions could reduce the detachment of PDA in strong acid and strong alkali conditions. Besides, organic solvents also made a difference on the PDA-coating stability. Among the tested 10 kinds of organic solvents, including n-hexane, toluene, ethyl ether, tetrahydrofuran, ethyl acetate, isopropanol, acetone, acetonitrile, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), DMSO caused the most serious detachment of PDA, up to 56%, followed by DMF with a detachment ratio of 31%. Ultrasonication caused less than 10% detachment of the coated PDA. It should be mentioned that the PDA coatings deposited on gold surface were not detached completely in all the test conditions, even at pH 14.0 (ca. 20% PDA retained). In alkaline conditions, detachment competes with further polymerization, which gave a slight increase of the SPRi signals at pH 9.0-11.0. Based on the obtained information about PDA-coating stability, thickness-controllable and alkali-resistant PDA coatings were prepared. Moreover, the alkali-resistant PDA coatings remained reactive to biomolecules, supporting further functionalization of PDA coatings.

Determination of the ‘apparent pKa’ of selected food hydrocolloids using ortho-toluidine blue

Charged food hydrocolloids provide structure and texture in manufactured foods by their organization and electrostatic interactions with other species and small ions. These electrostatic interactions depend on their actual charge density which cannot be predicted from pKa values of weakly charged monomers. In practice, this is circumvented by the use of an ‘apparent pKa’ (pKa (app)) which depends on solution ionic strength and type of counter-ions. pKa (app) values for food hydrocolloids are poorly documented, especially in low ionic strength solvents. Titration of a metachromatic alginate-ortho-toluidine blue complex revealed a sigmoidal, pH-dependent, dye metachromasy response with its centre located at pH = pKa (app). To test the suitability of this approach for the determination of pKa (app), five food hydrocolloids and a synthetic polyanion are analysed and the results are compared to classical methods such as potentiometric titration. Metachromasy is shown to occur when the polymer inter-charge distance is 4.5 Å; the complex stoichiometry depending on inter-charge distance. pKa (app) of stoichiometric complexes at low ionic strength (1 mM phosphate buffer) were successfully determined (alginate 4.5, sodium carboxymethylcellulose 4.6, κ-carrageenan 1.2, polyphosphate 2.1 and polyacrylic acid 5.8) and are comparable to values obtained by potentiometric titration. Further, it is shown that pKa (app) determination by metachromatic dye titration is feasible for stoichiometric polymer-dye complexes at low concentrations (20–200 μM) in low ionic strength (<10 mM) aqueous solutions. Unlike classical methods, the metachromatic dye titration approach is suitable for low polymer concentrations in aqueous solutions at low ionic strength.

Interfacial and Confined Colloidal Rod Diffusion.

Optical microscopy is used to measure translational and rotational diffusion of colloidal rods near a single wall, confined between parallel walls, and within quasi-2D porous media as a function of rod aspect ratio and aqueous solution ionic strength. Translational and rotational diffusivities are obtained as rod particles experience positions closer to boundaries and for larger aspect ratios. Models based on position dependent hydrodynamic interactions quantitatively capture diffusivities in all geometries and indicate particle-wall separations in agreement with independent estimates based on electrostatic interactions. Short-time translational diffusion in quasi-2D porous media is insensitive to porous media area fraction, which appears to arise from a balance of hydrodynamic hindrance and enhanced translation due to parallel alignment along surfaces. Findings in this work provide a basis to interpret and predict interfacial and confined colloidal rod transport relevant to biological, environmental, and synthetic material systems.

Molecular Dynamics Simulations of the Interfacial Region between Boehmite and Gibbsite Basal Surfaces and High Ionic Strength Aqueous Solutions

Classical molecular dynamics (MD) simulations were used to study the interactions of up to 2 M NaCl and NaNO3 aqueous solutions with the presumed inert boehmite (010) and gibbsite (001) surfaces. The force field parameters used in these simulations were validated against density functional theory calculations of Na+ and Cl– hydrated complexes adsorbed at the boehmite (010) surface. In all the classical MD simulations and regardless of the ionic strength or the nature of the anion, Na+ ions were found to preferably form inner-sphere complexes over outer-sphere complexes at the aluminum (oxy)hydroxide surfaces, adsorbing closer to the surface than both water molecules and anions. In contrast, Cl– ions were predicted to distribute preferably in outer-sphere positions. The resulting asymmetry in adsorption strengths offers molecular-scale evidence for the observed isoelectric point (IEP) shift to higher pH at high ionic strength for aluminum (oxy)hydroxides. As such, the MD simulations also provided clear evidence against the assumption that the basal surfaces of boehmite and gibbsite are inert to background electrolytes. Finally, the MD simulations indicated that the different affinities of NO3– and Cl– for the surfaces might have macroscopic consequences, such as difference in the sensitivity of the IEP to the electrolyte concentration.

Ionic strength-response hyperbranched polyglycerol/polyacrylic acid hydrogel for the reversible immobilization of enzyme and the synthesis of biodiesel

Hyperbranched polyglycerol and acrylic acid were polymerized into smart hydrogel for enzyme immobilization. Lipase from Aspergillus oryzae was selected as model enzyme. Highest enzyme loading amount of 78.93mg/g and relative enzyme activity of 67.02% were observed. The immobilized enzyme was further used for the synthesis of biodiesel, and the synthesis parameters were optimized with response surface methodology. The yield of fat acid methyl esters (FAMES) under the optimum synthesis condition was determined to be 95.98%. The immobilized enzyme showed good reusability for five consecutive operations without significant loss of FAMES yield. By means of regulating the ionic strength of aqueous solution, the immobilized enzyme was successfully released from the hydrogel. After the enzyme reload, FAMES yield of fresh immobilized enzyme still remained 90% approximately for five consecutive operations. The results demonstrated that the hyperbranched polyglycerol/polyacrylic acid hydrogels could be effectively applied to the enzyme carrier and biodiesel synthesis.