Homogeneous Aqueous Solution Research Articles

Homogenous Synthesis of Hydroxyethylcellulose in NaOH/Urea Aqueous Solution

Hydroxyethylcellulose (HEC) was synthesized by a fully homogenous method from cellulose in 7.5 wt.-% NaOH/11 wt.-% urea aqueous solutions under mild conditions. HEC samples were characterized with NMR, SEC-LLS, solubility, and viscosity measurements. The MS and DS values of the obtained HEC samples are in the range from 0.54 to 1.44 and 0.45 to 1.14, respectively, and the relative DS values at C-2 and C-6 hydroxyl groups are slightly higher than those at C-3 hydroxyl groups. HEC samples are soluble in water starting from a MS of 0.57 and DS of 0.49, which display high viscosity in aqueous solutions. Moreover, a NaOH/urea aqueous solution is a stable system for cellulose etherification. In this way, we could provide a simple, pollution-free, and homogeneous aqueous solution system for synthesizing cellulose ethers.

Cyclodextrin in polymer chemistry: Kinetic studies on the free-radical polymerization of cyclodextrin-complexed styrene from homogeneous aqueous solution

AbstractStyrene was encapsulated by an equimolar amount of randomly methylated β-cyclodextrin (CD) in water resulting in a water-soluble CD complex. Polymerization of this complex was carried out at 50°C using different amounts of the water-soluble azo-initiator 2,2’-azobis[N,N’-dimethyleneisobutyramidine] dihydrochloride. Unthreading of CD during polymerization led to water-insoluble polystyrene. Analysis of the polymers obtained showed that the polymerization from homogenous aqueous solution can be described by an equation similar to the classical polymerization in solution (number-average degree of polymerization vs. initiator concentration: Pn -1 ~ [I]0.5).

Luminescence quenching of Ru(bpy) 32+ by nitrophenols in silicate thin films

The luminescence quenching of ruthenium(II)–tris-1,2-bipyridine ion (Ru(bpy) 3 2+) included in methyltriethoxysilane-derived films by 2-, 3- and 4-nitrophenol was investigated. Sol–gel technology was used to prepare the “sol”. Coating thin films were obtained from the “sol” by spin-coating on glass slide. A blue shift in the Ru(bpy) 3 2+ emission spectrum included in the films with respect to the aqueous solution was observed. Ru(bpy) 3 2+ luminescence quenching experiments were carried out with the quencher in aerated aqueous solution at pH 2 in contact with the film. The quenching plots obtained from either steady-state or time-resolved luminescence intensity measurements showed a downward curvature. Both plots were fitted satisfactorily by a sum of two terms of Stern–Volmer with quenching constants K SV1 and K SV2 associated to two different binding sites of the ruthenium complex, which indicates the presence of a matrix microheterogeneity in the films. The K SV1 and K SV2 values and the corresponding fractions of the total emission f 1 0 and f 2 0 for both sites suggest that only a low percentage of the probe is accessible to the quencher and that the probe is efficiently quenched in one of the sites, assigned to site 1. The value of its respective constant K SV1 was higher than the value of the constant K SV in homogeneous aqueous solution. A dynamic process of electron transfer was suggested for the luminescence quenching of Ru(bpy) 3 2+ in the film by nitrophenols in aqueous solution.

Transformation of Carbon Tetrachloride by Thiol Reductants in the Presence of Quinone Compounds

Quinones are present in trace amounts in natural organic matter. The addition of thiol compounds to quinones produces reactive electron-transfer species that may be important for the transformation of chlorinated hydrocarbons under sulfate-reducing conditions. This study systematically investigated the transformation of carbon tetrachloride (CCl4) in homogeneous aqueous solutions containing quinones as electron-transfer mediators and thiol compounds as bulk reductants. The thiol compounds, including sodium hydrosulfide (NaHS) and cysteine, were found to effectively transform CCl4. The transformation of CCl4 followed pseudo-first-order kinetics, and the pseudo-first-order rate constants (kobs) were (3.24 +/- 0.46) x 10(-7) and 1.04 x 10(-7) s(-1), respectively, when solutions contained NaHS and cysteine alone. Addition of quinone compounds, including anthraquinone-2,6-disulfonate (AQDS), benzoquinone (BQ), juglone (JQ), naphthoquinone (NQ), lawsone (LQ), and menadione (MQ), increased the transformation rate and efficiency of CCl4. The kobs values for CCl4 transformation in the presence of quinones were 2.6-71 times higher than those for the thiol compounds alone. The enhancement efficiency followed the order JQ > NQ > BQ >> AQDS > LQ > MQ. Spectroscopic studies indicated that the quinone compounds generated various active electron-transfer mediators to transfer electrons from the bulk reductants to CCl4. BQ and NQ produced mercaptoquinones as active redox mediators that significantly enhanced the transformation rate of CCl4 in the presence of NaHS. The addition of thiol reductants produced large amounts of AQDS semiquinone radical as the electron shuttle. In addition, MQ and LQ were reduced by NaHS to give hydroquinone, which slightly enhanced the transformation efficiency of CCl4. These results clearly indicate that the enhanced efficiency of quinones for the transformation of chlorinated hydrocarbons is specifically related to the produced reactive species. Mercaptoquinone is a more active mediator than either semiquinone or hydroquinone for transferring electrons in a reducing environment containing thiol reductants.

Sodium chloride enhanced oligomerization of L‐arginine

NaCl significantly enhanced the longer oligoarginine formation in the oligomerization of L-arginine activated by N,N'-carbonyldiimidazole (CDI) in homogeneous aqueous solution. The optimal concentration of NaCl for the highest yield of longer oligoarginine formation is around 1M. It is suggested that the weak interactions of Cl(-) with the positive-charged guanidinium group of the oligoarginines formed in the oligomerization of L-arginine are responsible for the enhancement by NaCl.

Reversible Colorimetric Probes for Mercury Sensing

The selectivity and sensitivity of two colorimetric sensors based on the ruthenium complexes N719 [bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) bis(tetrabutylammonium) bis(thiocyanate)] and N749 [(2,2':6',2' '-terpyridine-4,4',4' '-tricarboxylate)ruthenium(II) tris(tetrabutylammonium) tris(isothiocyanate)] are described. It was found that mercury ions coordinate reversibly to the sulfur atom of the dyes' NCS groups. This interaction induces a color change in the dyes at submicromolar concentrations of mercury. Furthermore, the color change of these dyes is selective for mercury(II) when compared with other ions such as lead(II), cadmium(II), zinc(II), or iron(II). The detection limit for mercury(II) ions--using UV-vis spectroscopy--in homogeneous aqueous solutions is estimated to be approximately 20 ppb for N719 and approximately 150 ppb for N749. Moreover, the sensor molecules can be adsorbed onto high-surface-area mesoporous metal oxide films, allowing reversible heterogeneous sensing of mercury ions in aqueous solution. The results shown herein have important implications in the development of new reversible colorimetric sensors for the fast, easy, and selective detection and monitoring of mercuric ions in aqueous solutions.

Is There Stereoselectivity in Spin Trapping Superoxide by 5-tert-Butoxycarbonyl-5-methyl-1-pyrrolineN-Oxide?

[reaction: see text] Ester-containing nitrones, including 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide 5, have been reported to be robust spin traps for superoxide (O2*-). Using a chiral column, we have been able to isolate the two enantiomers of nitrone 5. With enantiomerically pure nitrone 5a and 5b we explored whether one of these isomers was solely responsible for the EPR spectrum of aminoxyl 6. Data obtained demonstrate that the spin trapping of O2*- by nitrone 5a and nitrone 5b affords the identical EPR spectra and lifetimes in homogeneous aqueous solution and exhibits the same ratio of cis and trans isomers. Quantum chemical modeling in vacuo also finds no difference, aside from the expected optical activity, arising from the difference in stereochemistry.

Sodium chloride enhanced oligomerization of l-glutamic acid in aqueous solution

The presence of NaCl was found to significantly enhance the formation of longer peptides in N,N'-carbonyldiimidazole induced oligomerization of L-glutamic acid in homogeneous aqueous solution. The enhancement was detected in the presence of as low as 0.01-M NaCl and the highest yield of longer oligomers was achieved in the presence of 1-M NaCl. The possible prebiotic relevance is discussed.

Facile One‐Pot Synthesis of 2,3,5‐Substituted 1,2,4‐Oxadiazolines from Nitriles in Aqueous Solution.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Mesoporous Silica-Supported Uranyl: Synthesis and Photoreactivity

A mesoporous silica-supported uranyl material (U(aq)O(2)(2+)-silica) was prepared by a co-condensation method. Our approach involves an I(-)M(+)S(-) scheme, where the electrostatic interaction between the anionic inorganic precursor (I(-)), surfactant (S(-)), and cationic mediator (M(+)) provides the basis for the stability of the composite material. The synthesis was carried out under acidic conditions, where the anionic sodium dodecyl sulfate provided the template for the uranyl cation and silicate to condense. Excitation with visible or near-UV light of aqueous suspensions of U(aq)O(2)(2+)-silica generates an excited state that decays with k(0) = 1.5 x 10(4) s(-1). The reaction of the excited state with aliphatic alcohols exhibits kinetic saturation and concentration-dependent kinetic isotope effects. For 2-propanol, the value of k(C)3(H)7(OH)/k(C)()3(D)7(OH) decreases from 2.0 at low alcohol concentrations to 1.0 in the saturation regime at high alcohol concentrations. Taken together, the data describe a kinetic system controlled by chemical reaction at one extreme and diffusion at the other. At low [alcohol], the second-order rate constants for the reaction of silica-U(aq)O(2)(2+) with methanol, 2-propanol, 2-butanol, and 2-pentanol are comparable to the rate constants obtained for these alcohols in homogeneous aqueous solutions containing H(3)PO(4). Under slow steady-state photolysis in O(2)-saturated suspensions, U(aq)O(2)(2+)-silica acts as a photocatalyst for the oxidation of alcohols with O(2).

Phenol Chlorination and Photochlorination in the Presence of Chloride Ions in Homogeneous Aqueous Solution

Phenol chlorination was studied in the presence of dissolved Fe(III) and chloride under irradiation and of hydrogen peroxide and chloride in dark acidic solutions. In the former case phenol photochlorination is most likely due to the formation of Cl2*- as a consequence of Fe(III) irradiation in the presence of chloride. The most efficient pathway is the photolysis of FeOH2+ producing hydroxyl, which oxidizes chloride to Cl*. The latter finally yields Cl2*- upon further reaction with chloride. The importance of the pathway involving FeOH2+ is higher at higher pH and moderately low chloride concentration. At pH 2.0 and [Cl-] > 0.03 M chlorophenol generation rate decreases with increasing [Cl-], due to the formation of the much less photoactive species FeCl2+/FeCl2+. The photolysis of FeCl2+/ FeCl2+ yielding Cl* is likely to play an important role at pH 0.5 and high chloride, but under such conditions chlorophenol formation rates are about an order of magnitude lower than at pH 2.0. Due to pH and kinetic constraints, under most environmental conditions the photochemistry of FeCl2+/FeCl2+ can be expected to play a minor role toward chlorination when compared with the one of FeOH2+, which leads to hydroxyl-mediated chloride oxidation. Hydrogen peroxide and chloride react in dark acidic solutions to yield HClO, involved in electrophilic chlorination processes. Chlorophenol formation rates under such conditions are directly proportional to [H+]. The described chlorination and photochlorination processes can take place in acidic aerosols of marine origin, naturally rich in chloride and Fe(III). Antarctic aerosol is also rich of hydrogen peroxide and often strongly acidic due to the presence of sulfuric acid of biogenic origin.

“On Water”: Unique Reactivity of Organic Compounds in Aqueous Suspension

“On Water”: Unique Reactivity of Organic Compounds in Aqueous Suspension

Fluorescence quenching of β‐carboline alkaloids in micellar media. A study to select the adequate surfactant to use in analytical techniques

The study of fluorescence quenching of the fluorophores allows the localization of the alkaloids (harmane and harmine) in the micelles (SDS, CTAB, Brij-35) to be established. In aqueous micellar solutions (SDS and Brij-35) at pH 13.0, emission corresponding to the neutral or zwitterionic forms can be observed. In the presence of CTAB (pH = 13.0) it was possible to observe the emission of anionic form. These species are not present in buffered aqueous solutions at these pH values. Bromide ion was added to the different surfactant solutions and the quenching effect was studied according to the Stern-Volmer equation. In the presence of SDS the quenching effect is considerably reduced compared to the aqueous solutions without surfactants, while for Brij-35 micelles were similar to those observed in homogeneous aqueous solution. For CTAB micelles a notable fluorescence quenching was observed for the different pH values studied. The fluorescence quenching studies show that the neutral species are associated inside the micelles, instead of the ionic species (cationic, zwitterionic or anionic) remaining on the surface of the micelles. The anionic surface of SDS micelles prevents the quenching effect by anionic quenchers for both neutral and charged species.

Role of O3 and OH· Radicals in Ozonated Aqueous Solution for the Photoresist Removal of Semiconductor Fabrication

The role of direct oxidation by aqueous O3 and advanced oxidation by OH· in the removal of photoresist was studied by chemical kinetic simulation and experiments of O3 reactivity and decomposition in homogeneous aqueous solutions. O3 is the main species responsible for the removal of conventional photoresist in the ozonated water cleaning process, and the timing of initiator addition to ozonated water is important to maintain high O3 concentration. Simulation using t-butanol implies that maintenance of a high OH· concentration is required to remove highly implanted photoresists that O3 itself cannot easily remove.

Oxidation of Sb(III) to Sb(V) by O 2 and H 2O 2 in aqueous solutions

The rates of Sb(III) oxidation by O 2 and H 2O 2 were determined in homogeneous aqueous solutions. Above pH 10, the oxidation reaction of Sb(III) with O 2 was first order with respect to the Sb(III) concentration and inversely proportional to the H + concentrations at a constant O 2 content of 0.22 × 10 −3 M. Pseudo-first-order rate coefficients, k obs, ranged from 3.5 × 10 −8 s −1 to 2.5 × 10 −6 s −1 at pH values between 10.9 and 12.9. The relationship between k obs and pH was: log ⁡ k o b s = 1.0 ( ± 0.3 ) p H − 18.5 ( ± 3.4 ) R 2 = 0.89 No significant Sb(III) oxidation by O 2 was observed between pH 3.6 and 9.8 within 200 days. Oxidation of Sb(III) by H 2O 2 was found to be first order with respect to the total Sb(III), H 2O 2 and inversely proportional to the H + concentrations in a pH range of 8.1 to 11.7. Above pH 11.7 no pH dependence was observed. The rate law for the pH range 8.1 to 12.9 was determined to be: − d [ S b ( I I I ) ] tot d t = k × [ S b ( I I I ) ] tot × [ H 2 O 2 ] tot and k = k 3 × K 1 K a 1 + [ H + ] = k 4 × K 2 K a 2 + [ H + ] where K a1 and K a2 are the deprotonation constants of Sb(OH) 3 and H 2O 2, respectively, and k 3 (=365 M −1 s −1) and k 4 (=342 M −1 s −1) are the specific second-order rate coefficients. The results indicate that the rate-limiting step below pH 11.7 involves one deprotonated species, either Sb(OH) 4 − or HO 2 −. Since the pK a-values of Sb(OH) 3 and H 2O 2 are very close (11.8 and 11.6, respectively), it was not possible to determine which species is involved. Varying the ionic strength between 0.001 and 1.0 M at pH 10 and between 0.01 and 1.0 M at pH 12 resulted in a less than twofold increase in k obs. In both cases, the increase was attributed to a shift in pK a-values as a function of the ionic strength. It could be concluded that O 2 was unlikely to be a significant oxidant in homogenous solution, but that H 2O 2 might be responsible for the oxidation of Sb(III) in natural waters since half-lives could vary from 32 yr to 117 days at pH 8 with H 2O 2 concentrations between 10 −8 M and 10 −6 M, respectively.

Facile One-Pot Synthesis of 2,3,5-Substituted 1,2,4-Oxadiazolines from Nitriles in Aqueous Solution

[reaction: see text] Alkyl/aryl amidoximes, prepared from the corresponding nitriles and N-alkylhydroxylamines, have readily undergone consecutive Michael additions to electron-deficient alkynes and provided highly substituted 1,2,4-oxadiazolines in good yields in homogeneous aqueous solution.

Synchrotron Radiation Effects on Catalytic Systems As Probed with a Combined In-Situ UV−Vis/XAFS Spectroscopic Setup

UV-vis spectroscopy was used in a combined in-situ UV-vis/XAFS spectroscopic setup to study the synchrotron radiation effect on aqueous homogeneous copper solutions. Two different systems were studied. In the first study, the focus was on a copper bipyridine-catalyzed oxidation of benzyl alcohol to benzaldehyde with 2,2,6,6- tetramethylpiperidinyl-1-oxy and base as cocatalysts. It was found that when the reaction mixture is exposed to the X-ray beam, the features present in the in-situ UV-vis spectrum develop differently compared to the situation when the reaction mixture is not exposed to the X-ray beam. Besides a temperature effect of the X-ray beam, both the UV-vis analysis and the XAFS analysis showed a reducing influence of the X-ray beam on the sample. To investigate this in more detail, we studied a series of dilute aqueous copper solutions from different precursor salts, viz., Cu(NO3)2.3H2O, CuSO4.5H2O, CuCl2, and CuBr2. It was found that the different aqueous copper solutions have different stabilities under the influence of the X-ray beam. Especially the solution from the CuCl2 precursor salt was found to be unstable and to be subjected to reduction. These examples illustrate the need for a second technique, such as in-situ UV-vis spectroscopy, to evaluate the effect of synchrotron radiation used to measure in-situ XAFS on catalytic systems.

Kinetics and Modeling of the Fe(III)/H2O2 System in the Presence of Sulfate in Acidic Aqueous Solutions

This work examined the effect of sulfate ions on the rate of decomposition of H2O2 by Fe(III) in homogeneous aqueous solutions. Experiments were carried out at 25 degrees C, pH < or = 3 and the concentrations of sulfate ranged from 0 to 200 mM ([Fe(III)]0 = 0.2 or 1 mM, [H2O2]0 = 10 or 50 mM). The spectrophometric study shows that addition of sulfate decreased the formation of iron(III)-peroxo complexes and that H2O2 does not form complexes with iron(III)-sulfato complexes. The rates of decomposition of H2O2 markedly decreased in the presence of sulfate. The measured rates were accurately predicted by a kinetic model based on reactions previously validated in NaClO4/HClO4 solutions and on additional reactions involving sulfate ions and sulfate radicals. At a fixed pH, the pseudo-first-order rate constants were found to decrease linearly with the molar fraction of Fe(II) complexed with sulfate. The model was also able to predict the rate of oxidation of a probe compound (atrazine) by Fe(III)/H2O2. Computer simulations indicate that the decrease of the rate of oxidation of organic solutes by Fe(III)/H2O2 can be mainly attributed to the complexation of Fe(III) by sulfate ions, while sulfate radicals play a minor role on the overall reaction rates.

Deactivation of Formate Dehydrogenase (FDH) in Solution and at Gas‐Liquid Interfaces

Enzymes, increasingly important in the synthesis of fine chemicals and pharmaceutical intermediates, are often insufficiently stable under reacting conditions. We have investigated the stability, in homogeneous aqueous solution and at gas-liquid interfaces, of formate dehydrogenase (FDH), important for cofactor regeneration, from Candida boidinii and overexpressed in E. coli. When exposed to mechanical stress, residual activity, [E](t)/[E](0), and residual protein were found to scale proportionally with gas-liquid surface area in the bubble column, verifying a surface-driven process, and with time and total throughput in a gear pump, but did not seem to be influenced much by shear in a Couette viscometer. All FDH variants are deactivated by chaotropes but not kosmotropes: the first-order deactivation constant k(d) correlates well with the Jones-Dole coefficient B but not well with the surface tension increment deltasigma of various concentrated ammonium salt solutions. This finding might provide guidance for focusing the search for quantitative theories of Hofmeister effects.

Amyloid-β Protofibrils Differ from Amyloid-β Aggregates Induced in Dilute Hexafluoroisopropanol in Stability and Morphology

The brains of Alzheimer's disease (AD) patients contain large numbers of amyloid plaques that are rich in fibrils composed of 40- and 42-residue amyloid-beta (Abeta) peptides. Several lines of evidence indicate that fibrillar Abeta and especially soluble Abeta aggregates are important in the etiology of AD. Recent reports also stress that amyloid aggregates are polymorphic and that a single polypeptide can fold into multiple amyloid conformations. Here we demonstrate that Abeta-(1-40) can form soluble aggregates with predominant beta-structures that differ in stability and morphology. One class of aggregates involved soluble Abeta protofibrils, prepared by vigorous overnight agitation of monomeric Abeta-(1-40) at low ionic strength. Dilution of these aggregation reactions induced disaggregation to monomers as measured by size exclusion chromatography. Protofibril concentrations monitored by thioflavin T fluorescence decreased in at least two kinetic phases, with initial disaggregation (rate constant approximately 1 h(-1)) followed by a much slower secondary phase. Incubation of the reactions without agitation resulted in less disaggregation at slower rates, indicating that the protofibrils became progressively more stable over time. In fact, protofibrils isolated by size exclusion chromatography were completely stable and gave no disaggregation. A second class of soluble Abeta aggregates was generated rapidly (<10 min) in buffered 2% hexafluoroisopropanol (HFIP). These aggregates showed increased thioflavin T fluorescence and were rich in beta-structure by circular dichroism. Electron microscopy and atomic force microscopy revealed initial globular clusters that progressed over several days to soluble fibrous aggregates. When diluted out of HFIP, these aggregates initially were very unstable and disaggregated completely within 2 min. However, their stability increased as they progressed to fibers. Relative to Abeta protofibrils, the HFIP-induced aggregates seeded elongation by Abeta monomer deposition very poorly. The techniques used to distinguish these two classes of soluble Abeta aggregates may be useful in characterizing Abeta aggregates formed in vivo.