Solvation and coordination interactions of tetrapyridylporphyrin in aqueous solutions. Thermal stability

Russian Journal of General Chemistry - The results from application of calorimetric method for studying the energetics of the interparticle interaction in aqueous solutions of tetrapyridylporphyrin...     

Formation and stability of peptide enolates in aqueous solution

Second-order rate constants k(DO) (M(-1) s(-1)) were determined in D(2)O for deprotonation of the N-terminal alpha-amino carbon of glycylglycine and glycylglycylglycine zwitterions, the internal alpha-amino carbon of the glycylglycylglycine anion, and the acetyl methyl group and the alpha-amino carbon of the N-acetylglycine anion and N-acetylglycinamide by deuterioxide ion. The data were used to estimate values of k(HO) (M(-1) s(-1)) for proton transfer from these carbon acids to hydroxide ion in H(2)O. Values of the pK(a) for these carbon acids ranging from 23.9 to 30.8 were obtained by interpolation or extrapolation of good linear correlations between log k(HO) and carbon acid pK(a) established in earlier work for deprotonation of related neutral and cationic alpha-carbonyl carbon acids. The alpha-amino carbon at a N-protonated N-terminus of a peptide or protein is estimated to undergo deprotonation about 130-fold faster than the alpha-amino carbon at the corresponding internal amino acid residue. The value of k(HO) for deprotonation of the N-terminal alpha-amino carbon of the glycylglycylglycine zwitterion (pK(a) = 25.1) is similar to that for deprotonation of the more acidic ketone acetone (pK(a) = 19.3), as a result of a lower Marcus intrinsic barrier to deprotonation of cationic alpha-carbonyl carbon acids. The cationic NH(3)(+) group is generally more strongly electron-withdrawing than the neutral NHAc group, but the alpha-NH(3)(+) and the alpha-NHAc substituents result in very similar decreases in the pK(a) of several alpha-carbonyl carbon acids.     

The removal of thorium from aqueous solutions using zeolites

The ability of zeolites to remove thorium-232 from naturally radioactive thorium nitrate solutions was studied by ion exchange, adsorption and surface phenomena. The best zeolites were a clinoptilolite from Mudhills, California, USA and a mordenite rich tuff from Eastgate, Nevada, USA. As the concentration of thorium in solution was increased the mechanism of thorium removal switches from that of partial ion-exchange to a surface precipitation. This was confirmed by electron probe microanalysis (EPMA) in conjunction with XRD and Energy Dispersive X-ray analysis (EDX). A Langmuir-type of adsorption occurred with the thorium-232 daughter isotopes at the same time as partial exchange/precipitation of the thorium parent which is in regular equilibrium with its natural daughters. Some experiments with²³⁴Th were carried out to confirm this hypothesis as well as column experiments to confirm the usefulness of the Th removal.     

Thermal stability of solid and aqueous solutions of humic acid

The effects of temperature on the stability of a soil humic acid were studied in the present work. Solid samples of Gohy-573 humic acid (HA) and dissolved ones in aqueous solution (pH 6.0, 0.1 mol L⁻¹ NaClO₄) were investigated in order to understand the impact of temperature on the chemical properties of the material. The methods applied to solid samples in the present investigation were thermogravimetric analysis (TGA), temperature-programmed desorption coupled with mass spectrometry (TPD-MS), and in situ diffuse reflectance infrared Fourier transformed spectroscopy (in situ DRIFTS). Humic acid samples were studied in the 25-800 °C range, with focus on thermal/chemical processes up to 250 °C. The reversibility of the changes observed was investigated by cyclic changes to specified temperature ranges (40-110 °C). All measurements were conducted under inert-gas atmosphere in order to avoid samples combustion at increased temperatures. Aqueous solutions were analyzed by UV-vis absorption spectroscopy after storage at temperatures up to 95 °C, and storage times up to 1 week. For temperatures below 100 °C experiments on solid and aqueous samples have shown results which were consistent to each other. The amount of water desorbed is temperature dependent and up to 70 °C this process was totally reversible. Above 70 °C an irreversible loss of water was also observed, which according to UV-vis spectroscopy corresponds to water produced by condensation leading to more condensed polyaromatic structures. The water released up to 110 °C was about 7 wt% of the total mass of the dried humic acid, where less than 50% corresponded to reversibly adsorbed water. At higher temperatures (>110 °C), gradual decomposition resulting in the formation of carbon dioxide (110-240 °C), and carbon monoxide (140-240 °C) takes place. Hence, thermal treatment of Gohy-573 humic acid above 70 °C results in irreversible structural changes, that could affect chemical properties (e.g., complex formation) of the material.     

Acid-base properties and stability of sulfoxylic acid in aqueous solutions

A comparative study of decomposition of sulfoxylic acid S(OH) ₂ and sulfoxylate anion and their reactions with formaldehyde in aqueous solutions with different acidity was carried out. Using the obtained results, the pK ₁ value of S(OH) ₂ was determined for the first time.     

Solubility and hydrolytic stability of indomethacin in aqueous micellar solutions

Russian Chemical Bulletin - The solubility of the indomethacin drug in aqueous micellar solutions of a series of dicationic surfactants was characterized by spectrophotometry. The use of these...     

Conductivity and electrochemical stability of concentrated aqueous choline chloride solutions

Russian Journal of Physical Chemistry A - The conductivity and electrochemical stability of choline chloride (ChCl) solutions with water contents ranging from 20 to 39 wt % are studied. Exposing...     

Long term stability of organic selenium species in aqueous solutions

Long term stability of organic selenium compounds (selenocystine, selenomethionine, trimethylselenonium ion) has been studied over a one year period for 2 analyte concentrations: 25 and 150 μg/L Se, at pH 4.5 in the dark, under different storage conditions: temperature of –20°C, 4°C, 20°C, 40°C; in Pyrex, Teflon, or polyethylene containers; in an aqueous matrix or in the presence of a chromatographic counter ion (pentyl sulfonate at 10^–4 mol/L concentration). Light effects have also been tested. The stability of the selenium species was monitored by HPLC-ICP/MS. Storage conditions can drastically alter the stability of organic selenium species. Organoselenium compounds were shown to be stable in the dark over a one year period in an aqueous matrix at pH 4.5 in Pyrex containers at both 4°C and 20°C. Pyrex vials exposed to natural sunlight at room temperature resulted in a steady decrease of the selenoamino acid concentration. Teflon containers caused losses of less than 25% at both 4° C and 20° C in the dark. However, polyethylene vials presented, at all temperatures tested, a rapid decrease of the TMSe⁺ concentration. The stability of the Se species studied did not show significant differences between 4° C and 20° C in any container material used. Storage of solutions at 40° C led to slight differences between the Pyrex and Teflon containers. However, polyethylene presented a drastic decrease of the three species over time at this higher temperature. Solutions frozen at –20° C in polyethylene vials did not stabilize the TMSe⁺ signal. Finally, concentrations and matrices of the samples did not significantly affect the stability of the species. Received: 15 July 1996 / Revised: 14 July 1997 / Accepted: 18 July 1997     

Formation of surfactant and polyelectrolyte gel particles in aqueous solutions

Mixing of oppositely charged surfactants and polyelectrolytes in aqueous solutions can lead to associative phase separation, where the concentrated phase assumes the form of a viscous liquid, gel, or precipitate. This phenomenon can lead to the formation of gel-like particles whose size and polydispersity can be controlled. Here we present phase behavior and structural studies of gel-like particles formed by mixing drops of N,N,N-trimethylammonium derivatized hydroxyethyl cellulose (JR-400) polyelectrolyte solution with oppositely charged anionic and catanionic surfactant solutions composed of sodium perfluorooctanoate (FC₇) and cetyltrimethylammonium bromide (CTAB). Gel formation apparently occurs due to the collapse of the polyelectrolyte chains upon the adsorption of surfactant. This process results in the release of simple ions and water, and yields dense gel-like beads. The diameter of these beads ranges approximately from 200 to 4000 μm. Both the effects of solution composition and the method of preparation are studied by optical and confocal microscopy, and are linked to the structure and stability of the bead. Our observations suggest that the structure of the resulting particles is governed by the solution composition and the method of preparation, while the particle stability is governed by phase behavior alone.     

Formation and stability of proton-amine-inorganic anion complexes in aqueous solution

Interaction of some amines with inorganic ligands was studied potentiometrically in aqueous solutions at 25°C at different ionic strengths. The systems taken into account were: meta-PP, meta-TPP, en-PP, en-TPP, en-HPO ₄ ^2– , dap-SO ₄ ^2– , tetren-HPO ₄ ^2– , 6da-PP and 8da-TPP (meta=methylamine, en=ethylenediamine, tetren=tetraethylenepentamine, dap=1,2-diaminopropane, 6da=1,6-hexanediamine, 8da=1,8-octanediamine, PP=pyrophosphate, TPP=tripolyphosphate). Several ALH_r species are formed for all the studied mixed systems (A=amine, L =inorganic ligand), and in some cases A_pL_qH_r species were also found. The stability of mixed proton-ligand-ligand species is proportional to the charges involved in the complexation reaction and, to some extent, it depends on steric characteristic of the ligands. A simple relationship has been found between formation constants of mixed ligand species and charges involved in the complexation reaction. The ionic strength dependence of formation constants has been taken into account, and it was found that, also for these types of mixed complexes, a general equation independent of the nature of the reactants can be used.     

The structure of aqueous solutions

A series of neutron diffraction experiments has been carried out on solutions of NiCl₂, NaCl and BaCl₂ in heavy water. Both the concentration of the solute and the degree of isotopic enrichment were varied in order to investigate whether the multiple-pattern method, which has been used previously to determine the partial structure factors for simple liquids, can be applied to aqueous solutions. It is concluded that the multiple-pattern method is feasible. Some general comments on the structural information contained in the single-pattern data are made.     

Formation of glucose in irradiation of aqueous solutions of Β-benzyl-D-glucoside

Conclusions On the example of-benzyl-D-glucoside (I) it was shown by the isotope exchange method that the radiation cleavage of the glycoside linkage does not include the step of the intermediate formation of the glucose carbonium ion.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 722–724, March, 1975.