Gel-like Precipitate Research Articles

Spontaneous Liquid Droplet-to-Gel Transition of Citrulline Polypeptide Complexed with Nucleic Acids.

Inside cells, proteins complex with nucleic acids to form liquid droplets resulting from liquid-liquid phase separation. The presence of mutated proteins can change the state of these liquid droplets to solids or gels, triggering neurodegenerative diseases. The mechanism of the liquid to solid or gel transition is still unclear. Solutions of poly(l-ornithine-co-l-citrulline) (PLOC) copolymers, which exhibit upper critical solution temperature-type behavior, change state upon cooling. In this study, we evaluated the effect of nucleic acids complexed with PLOC on phase changes. In the presence of nucleic acids, such as polyC and polyU, PLOC formed liquid droplets at low temperatures. The droplets dissolved at temperatures above the phase separation temperature. The phase separation temperature depended on the chemical structure of the nucleobase, implying that electrostatic and hydrogen bonding interactions between the nucleic acid and PLOC influenced phase separation. Furthermore, the liquid droplets spontaneously changed to gel-like precipitates due to spontaneous release of nucleic acids from the complex. The rate of the liquid droplet-to-gel transition depended on the magnitude of electrostatic and hydrogen bonding interactions between PLOC and nucleic acid. PLOC complexed with mRNA also underwent a liquid droplet-to-gel transition upon the release of mRNA. This work provides insights into the mechanism of pathogenic transitions of the cellular droplets.

Can Differently Stabilized Silver Nanoparticles Modify Calcium Phosphate Precipitation?

Calcium phosphates (CaPs) composites with silver nanoparticles (AgNPs) attract attention as a possible alternative to conventional approaches to combating orthopedic implant-associated infections. Although precipitation of calcium phosphates at room temperatures was pointed out as an advantageous method for the preparation of various CaP-based biomaterials, to the best of our knowledge, no such study exists for the preparation of CaPs/AgNP composites. Motivated by this lack of data in this study we investigated the influence of AgNPs stabilized with citrate (cit-AgNPs), poly(vinylpyrrolidone) (PVP-AgNPs), and sodium bis(2-ethylhexyl) sulfosuccinate (AOT-AgNPs) in the concentration range 5-25 mg dm-3 on the precipitation of CaPs. The first solid phase to precipitate in the investigated precipitation system was amorphous calcium phosphate (ACP). The effect of AgNPs on ACP stability was significant only in the presence of the highest concentration of AOT-AgNPs. However, in all precipitation systems containing AgNPs, the morphology of ACP was affected, as gel-like precipitates formed in addition to the typical chain-like aggregates of spherical particles. The exact effect depended on the type of AgNPs. After 60 min of reaction time, a mixture of calcium-deficient hydroxyapatite (CaDHA) and a smaller amount of octacalcium phosphate (OCP) formed. PXRD and EPR data point out that the amount of formed OCP decreases with increasing AgNPs concentration. The obtained results showed that AgNPs can modify the precipitation of CaPs and that CaPs properties can be fine-tuned by the choice of stabilizing agent. Furthermore, it was shown that precipitation can be used as a simple and fast method for CaP/AgNPs composites preparation which is of special interest for biomaterials preparation.

Neoformation of exotic copper minerals from gel-like precursors at the Exótica deposit, Chuquicamata, Chile

At the Exótica deposit, south of the giant porphyry copper deposit of Chuquicamata (Atacama Desert, Chile), Cu-rich groundwater seeped out at several locations in the Exótica open pit (Mina Sur) during sampling in 2009–2011. At their outflows, these solutions formed blueish and greenish copper-bearing gel-like precipitates. These gels contained atacamite and copper sulfate hydroxides such as devilline, spangolite, posnjakite, schulenbergite, and brochantite, which were identified by XRD, SEM, ESEM, and FTIR. The formation of the gel materials was studied under humid and dry conditions during and after maturation and water evaporation. Atacamite was found associated to outflowing saline solutions with pH 5.7, SO4/Cl weight ratios of 0.42–0.48, SO4/NO3 ratios of 0.48–0.50. These solutions are seen as an expression of the the lower aquifer of the Calama basin. Most copper sulfate hydroxides (spangolite, posnjakite, schulenbergite) were associated with slightly acidic freshwaters (pH 6.0 to 6.5, SO4/Cl ratios of 3.08–4.99, SO4/NO3 ratios of 2.52–3.13). In contrast, devilline formed in gels with near neutral to slightly alkaline water (pH 7.2 to 7.8, SO4/Cl ratio of 8.34, and SO4/NO3 ratio of 6.05). Non-copper-bearing precipitates formed by evaporation of the supernatant solutions from the gel. Gypsum precipitated first, then blödite (sodium-magnesium sulfate), and finally halite. Slightly negative sulfur isotope values suggest that the sulfur source in the neoformed gels is primarily the oxidation of sulfides rather than sulfate of sedimentary origin. The studied copper-gel seeps suggest that they might represent a modern precursor of the latest atacamite-brochantite-gypsum mineralization event at Exótica. These data support that the atacamite-brochantite-gypsum mineralization at Exótica is linked to the inflow of Cl-SO4-dominated groundwater from the lower saline aquifer of the Calama basin into the Chuquicamata-Exótica-Radomiro Tomic complex.

Effect of xanthan gum-based food thickeners on the dissolution profile of fluoroquinolones oral formulations

BackgroundXanthan gum-based food thickeners (XG-FTs) are often ingested by patients with dysphagia to prevent aspiration during drug treatment. Reportedly, XG-FTs affect tablet disintegration, drug dissolution rates, and reduce the efficacy of postprandial antihyperglycemic agents. The absorption rate and quantity of fluoroquinolone antimicrobial agents correlate with drug efficacy, raising concern about the impact of XG-FTs. Previously, we reported that film-coated tablets were less susceptible to the effects of XG-FT than conventional and orally disintegrating tablets. Here, we compare the effect of XG-FTs on dissolution profiles of three oral fluoroquinolone-based film-coated tablets by evaluating the dissolution of crushed products, fine granules, and film-coated fine granules.MethodsWe examined formulations of tosufloxacin tosylate monohydrate (TFLX), levofloxacin hemihydrate (LVFX), and ciprofloxacin hydrochloride hydrate (CPFX). The formulations were immersed in 20 mL of 1.5% (w/v) XG-FT aqueous solution for 2.5 min followed by a dissolution test using the paddle method according to the Japanese Pharmacopoeia (dissolution test solution pH 1.2; volume 900 mL; temperature 37 ± 0.5 °C). The dissolution profile was evaluated according to the dissolution quantity indicated in product specifications and guidelines for bioequivalence testing of generic drugs. The 15-min mean dissolution rate was determined for a formulation immersed in 1.5% (w/v) XG-FT aqueous solution and compared with that for a non-immersed formulation (control). Fluoroquinolone film-coated tablets were mixed with starch-based FTs, guar gum-based FTs, or XG-FTs to observe their appearances.ResultsThe dissolution profile of LVFX film-coated tablets was not affected by XG-FTs, but the dissolution of TFLX and CPFX was delayed. For crushed film-coated tablets, the 15-min mean dissolution rate was significantly delayed for all three fluoroquinolones when compared with that of uncrushed products. The dissolution profile of TFLX film-coated fine granules was unchanged by XG-FTs. CPFX film-coated tablets and crushed products produced a gel-like precipitate when mixed with XG-FTs and failed to meet product-dissolution specifications. A gel-like precipitate was also observed with guar gum-based FTs.ConclusionThe effect of XG-FTs on the dissolution profile of film-coated fluoroquinolone formulations varied depending on the formulation. The CPFX formulation formed a gel-like precipitate when immersed in XG-FTs resulting in a significantly delayed dissolution.

Experimental Fundamentals of the Colloid Hydrogeochemistry of Continental Runoff

The authors have previously developed the separation principles of filtering water samples rich in colloids into size fractions by the method of cascade filtration and by studying the migration of trace elements with them. Data acquired by studying the migration of a great number (forty to sixty) elements and organic matter (OM) of riverine runoff show that the method is highly promising for the separation of size fractions in search for stable geochemical associations of trace elements variably adsorbed on suspensions. Filtration with the application of seven to ten filter nominals, with the systematically decreasing pore diameter, was proven to be an efficient method for studying trace-element complexation with dispersed organic matter (DOM) in water bodies. However, the application of membrane filters made of organic materials of various nature (Nylon 6, lavsan, modified nitrocellulose, etc.) places certain constraints, which are caused not so much by the adsorption activity of the membranes but mostly by the rapid clogging of pores in the membranes with the gel-like amorphous precipitate. The paper is devoted to analysis of the newly suggested method of continuous gel-filtration.

The Effect of Humic Acid Adsorption on the Coagulation Stability of Soil Suspensions

Humic acid adsorption and its effect on the coagulation stability of soil suspensions were studied. The humic acid (Aldrich Chemie) adsorption from its dilute aqueous solutions was examined for gray forest soil (Greyzemic Phaeozem (Loamic)), its components (clay fraction and parent rock), and chernozem (Chernozem (Pachic)). Physicochemical methods proved that the sorption of humic acid in these objects is determined by the time of interaction, concentration of the acid, and the properties and concentration of the solid phase; it may be accompanied by ion exchange. The effect of humic acid adsorption on the coagulation stability of soil suspensions was judged from changes in the volume of gel-like precipitate released from the suspensions and the time required for the suspension brightening. It was shown that the stability of suspensions increases with an increase in the concentration of humic acid and depends on the type of soil.

Application of sol–gel technique for preparation of nanosilica from coal powered thermal power plant fly ash

Fly ash, a thermal power plant by-product is increasing and it is expected that this by-product will increase with increase in energy demand, especially in developing countries. Considering this, current study presents a novel process for the utilization of thermal power plant waste i.e., (fly ash) by fractionating it into silica particles and aluminum enriched residual ash. Fractionation of fly ash helped to extract industrial grade pure form of nanosilica. As a first step fly ash was added into 2N Sodium hydroxide solution, and resulting suspension was autoclaved for 120 min duration at 100 °C. After filtration, supernatant was collected and 1N HCl was added drop-wise, allowing precipitation of silica in the solution. A white cloudy gel-like precipitate of silica was formed which was filtered and washed several times to remove the impurities of Na, K, and chloride salts. This procedure yielded the nanosilica a final product, which was characterized for crystalline index and composition by using X-ray diffraction and fluorescence technique. The result of X-ray diffraction shows a broad hump in the region of 20°–22° indicating amorphous nature of the product was studied using field emission scanning electron microscopic and transmission electron microscopic analytical technique was used to study about internal structure, morphological features and size of the molecules which depicts spherical globular silica particles in 10–40 nm range. Further, chemical bonds were studied by infrared analysis (fourier transform infrared) and results indicated the presence of silica oxygen bonding (–Si–O–Si–) at 785 cm−1 and Si–OH bonds 3000–3500 cm−1. Energy dispersive X-ray analysis shows the presence of silicon and oxygen elements, which forms 99.9% of prepared nanosilica particles by weight.

New Dissolved Nitrogen Predispersed Solvent Extraction Method, 2: Selectivity

Dissolved Nitrogen Predispersed Solvent Extraction (DNPDSE) is a new method in which the two-phase mixture operation is based on bubble dispersion of the organic phase, instead of its drop dispersion into the aqueous phase. This substitution is intended to increase the two-phase contact area (to improve recovery of metal ions from the dilute solutions) and also to enhance the buoyancy force of organic phase (to improve separation of two phases). However, the performance mode of this method can increase the recovery of interfering elements, because of increased surface contact, entrainment, and precipitate flotation, and thus have an unfavorable effect on the selectivity of this method. Therefore, selectivity of this innovative method was compared with the conventional solvent extraction (SX) method. The results of experiments conducted on the dilute and dense copper, zinc, and iron synthetic solutions within the copper extraction pH values (1.2–2.1) showed a lack of zinc recovery (i.e., no effect on total selectivity) in both methods and increased iron recovery (i.e., effect on partial selectivity) only in the DNPDSE method. The results also showed that increased iron recovery was not just dependent on increased contact area of the two phases, and gel-like precipitates observed on the surface of organic phase indicated precipitation of Fe ions and their transfer onto the surface by bubble entrainment and precipitate flotation.

Analytical Characterization of Sediment Formed in Soy Oil-Based B20 Biodiesel

Abstract A thick, gel-like precipitate, which separated from a B20 blend, has been characterized using a variety of analytical techniques. The sediment formed in a blend of soy-based fatty acid methyl ester (FAME) and ultra low sulfur diesel (ULSD), which had been stored under ASTM D4625 conditions without the presence of antioxidant. Analysis indicates that the majority of the sediment consists of a wide variety of polar oxygenated compounds. These vary considerably in molecular weight. The lower molecular weight species arise due to oxidative cleavage of the methyl esters. Also present are free fatty acids formed due to hydrolysis of FAME. The highest molecular weight compounds are oxidized FAME that have not undergone cleavage and oligomers of the oxidized FAME. Monoglycerides and steryl glucosides are not significant components in this sediment. The sediment does contain some diesel fuel, intact FAME, and impurities, which are inherent to the FAME. A proposed scenario for the formation of this type of sediment is as follows: As free radical formation, oxidation, and hydrolysis occur in unsaturated methyl esters, there are numerous polar organic compounds formed. These vary widely in molecular weight and include carboxylic acids, alcohols, aldehydes, and ketones, as well as compounds containing combinations of these functional groups. All of these reaction products undergo aggregation due to their polarity differences with the bulk matrix. Included in these aggregates are the trace components present as impurities in the FAME, such as water, glycerol, glycerides, plant sterols, and steryl glucosides. Eventually, a level of thermodynamic instability is reached and phase separation occurs. While the aggregates and sediment contain significant polar functionality, they still maintain a very high degree of alkyl and olefinic character. As a result, intact FAME and diesel fuel components have a certain degree of solubility in the aggregated mixture and make up part of the precipitated sediment.

Fractionation and physicochemical characterization of psyllium gum

Psyllium gum has been used in the landscape industry as an environmental friendly natural binder. This research is focused on the basic chemical and some physical properties of psyllium gum. Psyllium husk was extracted with hot water (80 °C), and with 0.5 M NaOH, 1.2 M NaOH and 2.0 M NaOH solutions (at room temperature), respectively, to produce a series of psyllium gum fractions labeled as water extractable (WE), 0.5 M alkali extractable (AES 0.5), 1.2 M alkali extractable (AES 1.2), and 2.0 M alkali extractable (AES 2.0). The alkaline extracted solutions were neutralized, during which, a gel-like precipitate was observed in the 0.5 M NaOH extracted solution, which was then separated by centrifugation to give a soluble fraction (AES 0.5) and a gel fraction (AEG 0.5). The yields of fractions WE, AES 0.5 and AEG 0.5 were 18.6%, 8.7%, and 61.4%, respectively, based on the dry weight of husk. The combined yields from the 1.2 and 2.0 M NaOH extracted fractions were less than 1.5%. Monosaccharide analysis revealed that WE, AES and AEG contained mainly xylose and arabinose. Substantial amounts (∼15%) of uronic acids were found in the WE and AES 0.5 fractions compared to AEG 0.5, which only contains neutral sugars. Methylation analysis proved that WE and AEG 0.5 basically contain 1 → 4) and 1 → 3) linked β- d-xylopyranosyl residues in the main chain. The side chains are primarily composed of terminal arabinose and xylose connected to the main chain by O-3 and/ or O-2 linkage. In contrast, the backbone chain of AES 0.5 fraction was mainly composed of 1 → 4 linked β- d-xylopyranosyl residues and the side chains only contained arabinosyl residues where were linked only through the O-3 position of the 1 → 4 linked β- d-xylopyranosyl residues. Further study was carried out to evaluate the particle size of the molecules in solutions by dynamic light scattering (DLS). It was found that these fractions were very soluble in 1 M NaOH solutions, and the fraction AES 0.5 and AEG 0.5 were stable in such alkaline conditions, but slight degradation occurred for WE with time. Fraction WE also dissolved well in water, however, aggregation formed over storage. The apparent hydrodynamic diameters ( d h) determined by DSL were over 100 nm for all three fractions. Theses gums are of high molecular weight and can be expected to be good viscosity enhancers. The current study provided new structural information on psyllium gum, which will be helpful for explaining its unique functional properties.

Structural Characteristics of Xyloglucan – Congo Red Aggregates as Observed by Small Angle X-ray Scattering

Xyloglucan is a type of hemicellulose with a cellulose backbone containing (1→6)-α-xylose or (1→2)-β-galactoxylose as a side chain. It is soluble in water. Its aqueous solution forms a gel or gel-like precipitate by addition of Congo red. Xyloglucan gel structures with various concentrations of Congo red were observed by small angle X-ray scattering (SAXS) at the nano-level. SAXS results indicated that the xyloglucan chains interacted with Congo red, and that an increase of concentration of Congo red induced a characteristic cross-linking domain, which consisted of a flat structure containing stacked xyloglucan chain assemblies. The Congo red molecules are inserted between the xyloglucan chains.

Sol Gel Synthesis of Li1+αV3O8. 1. From Precursors to Xerogel

The nature of Li1+αV3O8·nH2O gel-like precipitate and corresponding xerogels has been studied. Unlike V2O5 gel, the Li1+αV3O8 gels are diphasic materials made of intimately mixed solid and liquid phases having very close Li/V stoichiometry. At room temperature, the solid component contains presumably an hewettite type structure, whereas the liquid part crystallizes into lithiated decavanadic acids. Upon drying at 90 °C, both components lead to hydrated Li1+αV3O8·nH2O hewettite-like compounds with different interlayer spacing but with the same water content. By coupling pH measurements, X-ray diffraction, and liquid and magic-angle spinning 51V NMR, a new mechanism of formation of the Li1+αV3O8 gel-like precipitate via solubilized vanadic species is proposed.

Coprecipitation of Th(4+) and the purified extracellular polysaccharide produced by bacterium Bradyrhizobium (Chamaecytisus) BGA-1.

The soil bacterium Bradyrhizobium (Chamaecytisus) strain BGA-1 produces an extracellular polymeric substance (EPS) that, in the presence of Fe(3+), Al(3+) or Th(4+) solutions, forms a gel-like precipitate composed of polysaccharide, protein, lipopolysaccharide and the metal. Precipitation of the main component of the EPS, the extracellular polysaccharide, and thorium was studied. The precipitate was stable, but redissolved at pH values below 3.0 or in the presence of 10 mM EDTA. In the precipitate, the ratio thorium/basic repeating unit of the polysaccharide ranged from 0.4 to 0.8 mol/mol. Soluble polysaccharide-thorium complexes were not found, and larger polysaccharide molecules were precipitated in preference to smaller ones. Kinetic studies showed a non-linear dependence of the precipitate on the concentrations of both thorium and polysaccharide. The behaviors of the purified polysaccharide and of whole EPS with the thorium-containing precipitate were compared. The results suggested that EPS components other than polysaccharide are able to modify the precipitating ability of the polysaccharide. Thus, whole EPS is a better substrate than the purified polysaccharide for the removal of thorium from its solutions.

Effect of thorium on the growth and capsule morphology of Bradyrhizobium.

The thorium effect on Bradyrhizobium growth was assayed in liquid media. Th4+ inhibited the growth of Bradyrhizobium (Chamaecytisus) BGA-1, but this effect decreased in the presence of suspensions of live or dead bacterial cells. Th4+ induced the formation of a gel-like precipitate when added to a dense suspension of B. (Chamaecytisus) BGA-1 cells. Viable Bradyrhizobium cells remained in suspension after precipitate formation. Thorium was recovered in the precipitate, in which polysaccharide, lipopolysaccharide and proteins were also found. After Th4+ addition, the morphology of B. (Chamaecytisus) BGA-1 or Bradyrhizobium japonicum USDA 110 sedimented cells studied by scanning electron microscopy changed from an entangled network of capsulated bacteria to uncapsulated individual cells and an amorphous precipitate. Energy-dispersive X-ray spectroscopy showed that thorium was mainly in the amorphous fraction. Precipitate was also formed between B. (Chamaecytisus) BGA-1 and Al3+, which was also toxic to this bacterium. Precipitate induced by Th4+ or Al3+ was found in all Bradyrhizobium and Sinorhizobium strains tested, but not in Rhizobium, Salmonella typhimurium, Aerobacter aerogenes or Escherichia coli. These results suggest a specific defence mechanism based on metal precipitation by extracellular polymers.

Fabrication of 3D chitosan–hydroxyapatite scaffolds using a robotic dispensing system

A new robotic desktop rapid prototyping (RP) system was designed to fabricate scaffolds for tissue engineering applications. The experimental setup consists of a computer-guided desktop robot and a one-component pneumatic dispenser. The dispensing material (chitosan and chitosan–hydroxyapatite (HA) dissolved in acetic acid) was stored in a 30-ml barrel and forced out through a small Teflon-lined nozzle into a dispensing medium (sodium hydroxide–ethanol in ratio of 7:3). Layer-by-layer, the chitosan was fabricated with a preprogramed lay-down pattern. Neutralization of the chitosan forms a gel-like precipitate, and the hydrostatic pressure in the sodium hydroxide (NaOH) solution keeps the cuboid scaffold in shape. Comparison of the freeze-dried scaffold to the wet one showed linear and volumetric shrinkage of about 31% and 62%, respectively. A good attachment between layers allowed the chitosan matrix to form a fully interconnected channel architecture. Results of in vitro cell culture studies revealed the scaffold biocompatibility. The results of this preliminary study using the rapid prototyping robotic dispensing (RPBOD) system demonstrated its potential in fabricating three-dimensional (3D) scaffolds with regular and reproducible macropore architecture.

Experimental investigations on iron corrosion products formed in bicarbonate/carbonate-containing solutions at 90°C

The influence of initial oxygen level and bicarbonate content on iron corrosion was examined during the early stages of a storage at 90°C. The nature of corrosion products formed on iron surface was investigated through FTIR, Raman spectrometry and scanning electron microscopy. It has been shown that high bicarbonate content (1 M) promotes the formation of siderite whatever the initial oxygen level. Decreasing the bicarbonate content (from 0.1 to 0.001 M) will favour the formation of Fe II(OH −) x (HCO 3 −/CO 3 2−) y species with increasing x values and decreasing y values. Fe II(OH −) x (HCO 3 −/CO 3 2−) y species may suffer a series of transformations into green rust GR1(CO 3) (intermediate species), Fe II(OH −) n (HCO 3 −/CO 3 2−) m gel-like precipitate, magnetite or lepidocrocite depending on the initial oxygen level.

Crystal Structure of Tetramethylformamidinium Disulfide μ-Oxo-bis-[trichloroferrate(III)

reaction products depending on the substituents of the thioureas, the type of the oxidizing agent, the polarity of the reaction medium and other reaction conditions.1 We have shown that oxidation of 1,1-diphenylthiourea and N-methylthiourea with iron(III) leads to 1,2,4-thiadiazoline derivatives.2,3 Kinoshita et al. studied the mechanism of oxidation reaction of substituted thioureas with BPO and suggested that 1,2,4-thiazolidine heterocyles are formed through abstraction of thiourea hydrogen and sulfur atoms.4 In this respect, it is interesting to examine oxidation products of thioureas having no urea hydrogen atoms. In this work, we report on the crystal structure of the oxidation product of tetramethylthiourea with iron(III) chloride. Tetramethylthiourea (1.32 g: 0.01 mol) and anhydrous iron(III) chloride (1.62 g: 0.01 mol) dissolved in absolute ethanol (30 ml) were reacted at room temperature for 1 h. A dark-red gel-like precipitate was formed, which was subsequently separated from the mother liquor by decantation. The residue was re-dissolved in anhydrous acetone dried with a molecular sieve (60 ml). The solution was kept at 4 C in a refrigerator for a few days. Precipitated crystals were filtered off and dried in a desiccator. Two different types of crystals were obtained from the same acetone solution: diamond shaped petal-like crystals (Form I) and rhomboid platelet-like crystals (Form II). X-ray analyses were carried out on these two crystal modifications. The detailed measurement conditions and crystal data are listed in Table 1. The atomic scattering factors and anomalous dispersion terms were taken from International Tables for X-ray Crystallography, Vol. IV.5 All calculations were performed using the program TEXSAN crystallographic software package.6 The final atomic parameters are listed in Table 2. The molecular structure is shown in Fig. 1, together with the atomic labeling scheme. Selected bond distances and bond angles are listed in Tables 3 and 4, respectively. Forms I and II turn out to be polymorphic forms of formamidinium salts of μ-oxo-bis-[trichloroferrate(III)] ions where two tetramethylthiourea molecules are linked by a disulfide bond. Similar formamidinium disulfide salts were also 1003 ANALYTICAL SCIENCES SEPTEMBER 2000, VOL. 16 2000 © The Japan Society for Analytical Chemistry

Preparation and preliminary characterization of a poly(4-vinylpyridine) complex of a water-soluble polyaniline

A polymer complex between a sulfonated water-soluble polyaniline (SPANI) and poly(4-vinylpyridine) (PVP) is formed by mixing an aqueous solution of the protonated (hydrogen chloride) form of PVP, a cationic polyelectrolyte, with an aqueous solution of SPANI. A gel-like precipitate forms which has limited solubility in many common solvents. Thus, the approach represents a possible route to the aqueous solution processing of polyaniline. The nitrogen-to-sulfur ratio of the complex indicates approximately a 1:1 stoichiometry between PVP and SPANI monomer units. The complex exhibits modest conductivity (3.3 × 10 −5 S/cm) and is electroactive when immobilized on carbon or platinum electrodes. The swellability of the gel form of the complex is characterized by a solvent content of 16 grams per gram of dry material. Thermal analysis of the dry complex indicates stability to 225 °C.

Synthesis of Nanosize Silica in Aerosol OT Reverse Microemulsions

Silica nanoparticles were synthesized in sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/decane/ammonium hydroxide reverse microemulsions via the base-catalyzed hydrolysis of tetraethoxysilane (TEOS). The fluorescence spectra of 1,3,6,8-pyrenetetrasulfonic acid (PTS) trapped in the water pools indicate that free water molecules do not become available until the water-to-surfactant molar ratio ( R) exceeds about 10. At R values below about 4, no particles are produced. Under these conditions, the surfactant molecules are closely packed at the interface and the water molecules are strongly bound to the surfactant polar groups and the sodium counterions; the net result is to inhibit TEOS hydrolysis. As R increases from 5 to 9.5, the diameter of the resulting particles increases, and the size distribution decreases. These trends are rationalized in terms of changes in the local concentration of hydrolyzed TEOS species in the reversed micellar pseudophase, coupled with the protective action of the surfactant film. The fact that the final particle sizes exceed the diameters of the corresponding water pools indicates a role for intermicellar matter exchange in the particle growth process. The dispersions are not stable, and gel-like precipitates eventually form. The low dispersion stability is attributed to the presence of sodium ions and to the possible decrease in pH caused by the hydrolysis of the AOT surfactant molecules.

Salt bridge induced changes in the secondary structure of ionic polypeptides.

The carboxylate-containing homopolypeptides poly(L-glutamate) [poly(Glu)] and poly(L-aspartate) [poly(Asp)] were found to form different types of ordered structures in the presence of poly(L-lysine) [poly(Lys)]. Mixing poly(Glu) with poly(Lys) in aqueous solution at neutral pH results in the instantaneous formation of a gel-like precipitate. The secondary structure of the gel precipitate can be best described as intermolecular antiparallel beta-strands, involving the backbone amide groups, as evidenced by the presence of characteristic amide I bands in the ir spectrum at 1684 and 1612 cm-1. Mixing poly(Asp) with poly(Lys) under identical conditions results in the formation of a fine precipitate with a different morphology. Examination of the ir spectrum of the precipitate revealed that unlike poly(Glu), poly(Asp) did not yield any discrete secondary structure upon precipitation with poly(Lys). Addition of solutions containing Ca2+ or Mg2+ to the poly(Glu)/poly(Lys) aggregates resulted in complete dissolution of the gel, with the disappearance of the ir bands characteristic of the intermolecular hydrogen-bonded network. The results demonstrate the importance of salt bridges in establishing strong hydrogen bonds between the backbone amide groups. Reaggregation occurred upon heating the poly(Glu)/poly(Lys) mixture in the presence of Ca2+, but not in the presence of Mg2+ ions. In the presence of Ca2+ ions, aggregation and formation of an extended hydrogen-bonded network occurred upon heating. The aggregates formed upon heating poly(Glu)/poly(Lys) in the presence of Ca2+ were attributed solely to complexation of Ca2+ to the carboxylate groups of poly(Glu) with poly(Lys) remaining free in solution. Dissolution of the aggregate could be accomplished through addition of Mg2+ at room temperature.