Methanol and acetonitrile associates in aqueous and chloroform solutions according to 1H NMR spectroscopic data

Association of methanol and acetonitrile in a nonpolar (CDCl₃) and polar (H₂O and D₂O) solvents was studied by ¹H NMR spectroscopy and quantum chemistry. The results were compared with the data obtained by decomposition of the spectral curves in the range 800–1100 nm by the independent component analysis (ICA) technique. The content of homoassociates consisting of four and two or three molecules in the case of methanol and acetonitrile, respectively, gradually increased with the amount of the organic solvent in solution. The aqueous solutions under study consisted of few associates of compositions 1: 1, 1: 2, and 1: 4.5 for acetonitrile and 1: 1 and 1: 3 for methanol (water: organic solvent). The quantum-chemical calculation of the NMR spectra of the particles existing in solution confirmed our conclusions about the structure of the solutions.     

Enthalpy characteristics of L-proline dissolution in certain water–organic mixtures at 298.15 K

A thermochemical study of the processes of L-proline dissolution in aqueous solutions of acetonitrile, 1,4-dioxane, acetone, dimethyl sulfoxide, nitromethane and tetrahydrofuran at Т = 298.15 K in the range of organic solvent concentrations x₂ = 0–0.25 mole fractions is performed. Standard values of the enthalpies of solution and transfer of L-proline from water to mixed solvent, and the enthalpy coefficients of pairwise interactions between L-proline and molecules of organic solvents, are calculated. The effect the composition of a water–organic mixture and the structure of organic solvents have on the enthalpy characteristics of L-proline dissolution and transfer is examined. The effect the energy properties of intermolecular interactions between components of a mixed solvent has on the intermolecular interactions between L-proline and molecules of cosolvent is estimated. The correlation between the enthalpy characteristics of L-proline dissolution and electron-donor properties of organic cosolvent in aqueous solutions is determined.     

Thermodynamic linkage of ion binding and hydration in myofibrillar protein solutions determined from multinuclear spin relaxation studies

The mechanisms for the anionic and cationic interactions with myofibrillar proteins in aqueous solutions were investigated by nuclear magnetic resonance over a wide range of salt concentration. Markedly nonlinear dependeces of the ¹⁷O and ²³Na NMR transverse relaxation rates on salt concentration were analyzed with a thermodynamic linkage model of salt-dependent solubility and hydration (ligand-induced association model), according to Wyman's theory of linked functions. Nonlinear regression analysis of both ¹⁷O and ²³Na NMR data suggested cooperative, reversible binding of hydrated ions to myofibrillar proteins. Both ions and water were found to exchange fast, on the NMR timescale, between the binding sites of the myofibrillar proteins and the aqueous solution. At sodium chloride concentrations higher than about 0.1 grams salt/gram water, ion activities have marked effects upon the NMR relaxation rates of both ions and water. A salt activity model allowed quantitative fitting of the NMR data at high salt concentrations. The effect of neglecting the ion activity in solutions of myofibrillar proteins was also estimated and compared with the ligand-induced, cooperative association model for myofibrillar proteins. The comparison between the ¹⁷O and ²³Na results strongly suggests that water is exchanged as the hydrated ion species between the myofibrillar protein binding sites and the bulk, aqueous solution.     

The solubility of cellulose in liquid ammonia/salt solutions

The dissolution of cellulose in solutions of liquid ammonia and ammonium thiocyanate is discussed. Viscosity measurements on dilute solutions of cellulose in this solvent over a range of shear rates and shear stresses are reported. A four-bulb Ubbelohde suspended level viscometer was used for the measurements. Plots of log [η] versus log M gave Mark-Houwink coefficients of a = 0.95 and K = 6.686 × 10^?5 at 25°C for [η] as dl/g. The Bloomfield equation was used to calculate effective bond lengths (b) from limiting viscosity numbers of cellulose in solutions of ammonia/ammonium thiocyanate and Cuene, respectively. Results indicate that cellulose may have similar configurations in both solvents and also that the ammonia solutions are true cellulose solutions. Miscibility of the cell- ulose/ammonia/ammonium thiocyanate solutions with organic solvents, such as glycerol, is also reported. Further, a few interesting characteristics of the liquid ammonia/ammonium salt solutions, discussed briefly, are the convenient boiling point, the rheological behavior, and the relatively high concentration of cellulose obtainable.     

Rate constants of the thermal cis‐trans isomerization of azobenzene dyes in solvents,acetone/water mixtures,and in microheterogeneous surfactant solutions

Rate constants k_iso of the thermal cis‐trans isomerization of four 4,4’‐nitro‐aminoazobenzenes with different amino groups have been determined in homogeneous aprotic solvents and polyglykol oligomers, primarily by means of conventional flash photolysis. The rate constants have been correlated with polarity (according to λ_max from UV/Vis absorption spectra of the trans isomers) and bulk viscosity of the solvents. Qualitative conclusions about the influence of varying concentrations of water with respect to polarity and hydrogen bonding on k_iso‐ and λ_max‐values in acetone/water mixtures were derived. Based on these results the data from microheterogeneous solutions have been interpreted. In microheterogeneous water/surfactant solutions k_iso‐values of selected azo dyes were strongly dependent on the concentrations of SDS, Triton^®X‐100, C₁₂EO₈ in water, and varied with the composition of bicontinuous microemulsions of Igepal^® CA‐520/ heptane/water. The large spread of isomerization rate constants is in part due to varying microviscosity. Replacement of H₂O by D₂O in aqueous surfactant solutions produced surprisingly large kinetic solvent isotope effects. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 337–350, 1999     

Counterion mixing effects on the conformational transitions of polyelectrolytes. I. Volume phase transition and coil‐globule transition of alkali metal poly(acrylate)s

Counterion mixing effects on the volume phase transition and the coil‐globule transition of alkali metal poly(acrylate)s (PAAM) in aqueous organic solvents were investigated by observing the swelling behavior of PAAM gel and by measuring the solution viscosity and the conductivity as a function of the counterion mixing ratio. Marked transitions to the collapsed states were induced only for Li⁺/Cs⁺ system in most solvent systems; namely, PAA gel significantly collapsed in the presence of Li⁺ and Cs⁺ counterions irrespective of the solvent species employed, while only a slight deswelling was observed for Li⁺/K⁺ system in some aqueous organic solvents. Corresponding specific decrements in the solution viscosity and conductivity were also confirmed for the combination of Li⁺ and Cs⁺ in aqueous dimethyl sulfoxide (DMSO) system. A simple analysis of the conductivity decrement observed for Li⁺/Cs⁺ system in 60 vol % DMSO suggested that only Cs⁺ is tightly bound upon addition of Li⁺ while no restriction is induced for Li⁺ upon mixing with Cs⁺. A working hypothesis is proposed for the apparently intriguing behaviors of the counterions in the mixed system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2122–2131, 2009     

Effect of mixtures of water and organic solvents on the acidities of 5‐membered heteroaromatic carboxylic acids

pKa values of various 5‐membered heterocyclic aromatic carboxylic acids (pKa^s) were determined in solutions of 20.3, 35.2, 50.1, 65.1, and 79.9 weight percent of organic solvents in water. The pKa^s values show good linearity when they are ploted as a function of the dielectric constants of the mixed solvents methanol, ethanol, isopropyl alcohol, and tert‐butyl alcohol. On the other hand, the pKa^s values show a poor correlation with the dielectric constants in aqueous acetonitrile, N,N‐dimethylformamide, dimethyl sulfoxide, and dioxane. The pKa values in pure water and in pure organic solvent could be calculated by extrapolation of the plot of the pKa^s versus percentage of organic solvent. The pKa^s values of 4‐ and 5‐sub‐stituted 2‐thiophenecarboxylic acids were also determined, and the ρ values are calculated in the same series of the solvents.     

Solvent effects upon the proton NMR linewidth in dimethylformamide

The NMR linewidth of the formyl proton in dimethylformamide has been studied in the neat liquid and twelve organic solvents. It varies by more than one order of magnitude which is referred to solvent viscosity changes and to the protonation of dimethylformamide. The data do not allow the distinction between N- and O-protonation without further knowledge of the solvent dependence of either the nitrogen-14 spin lattice relaxation time or the spin-spin coupling ²J(NH).     

Extraction of rhenium(VII) by phosphorylated podands

The interphase distribution of ReO ₄ ^- between aqueous H₂SO₄ solutions and solutions of phosphorylated podands in organic solvents is studied. The stoichiometry of extracted complexes is determined. The rhenium extraction efficiency is considered as a function of the structure of the extractant and the nature of the organic solvent.     

Kinetics of 1,3‐dipolar cycloaddition reaction between C,N‐diphenylnitrone and dimethyl fumarate in various solvents and aqueous solutions

Second‐order rate constants and activation parameters of 1,3‐dipolar cycloaddition reaction between C,N‐diphenylnitrone and dimethyl fumarate were obtained in various solvents and aqueous solutions at 65°C. Second‐order rate constants of the reaction in water and ethylene glycol are approximately 33 and 8 times faster than those expected from solvent polarity, respectively. Increase of the reaction rate in aqueous solutions of ethanol is higher than that of propan‐1‐ol. A multiparameter correlation of log k₂ vs Sp and E_T^N in various solvents and aqueous solutions of ethanol shows that solvophobicity and solvent polarity parameter are important factors in occurrence of the reaction. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 431–434, 2000     

Formation of ordered phases of (hydroxypropyl)cellulose in miscible and immiscible isobutyric acid-water mixtures

(Hydroxypropyl)cellulose (HPC) is known to form birefringent liquid-crystalline phases at elevated polymer concentrations in either water or isobutyric acid (IBA). The HPC concentration at which the polymeric phase exhibits birefringence decreases as the IBA content in mixed H₂O/IBA solvents decreases, even though the concentration ?ⁱ_c for the formation of an ordered phase of HPC in water is greater than that in IBA. Water is a spectator component and apparently does not participate in the formation of a birefringent phase when IBA is present. A birefringent phase forms once the concentration of HPC in the solution omitting the H₂O equals the ?ⁱ_c of binary HPC/IBA solutions for temperatures from 23 to 95°C. The strong preferential affinity of HPC for IBA is visually evident as an HPC coagulate separates from dilute solution when the solvent mixture contains as little as 5% IBA. The coagulate dissolves to give a monophasic isotropic solution as the IBA content in the solvent is increased. A heterogeneous system in which a clear supernatant fluid covers a pearly white polymeric phase forms when the solvent mixture is immiscible and the HPC content is less than 50%. At high HPC content, the classical appearance associated with concentrated HPC solutions is seen. The optical and rheological properties of the heterogeneous systems are compared with those of homogeneous solutions at several HPC concentrations.     

Increased paraoxon detection by acetylcholinesterase inactivation with ionic liquid additives

This is the first study using ionic liquids (ILs) as additive in the aqueous solvent medium for detection of paraoxon by acetylcholinesterase inhibition method. A systematic comparison of various ILs with organic solvents has been made. The aqueous buffer solution containing ionic liquid ethylpyridinium hexafluorophosphate [EtPy]⁺[PF₆]⁻ has been found to give the best results. The inhibition kinetic follows the first order model. Ionic liquids modified aqueous solutions show the potential to provide a promising and effective medium in detection of paraoxon with acetylcholinesterase.     

NMR Investigations of Temperature-Induced Phase Transition in Aqueous Polymer Solutions

The different dynamics of polymer segments forming phase-separated globular structures in aqueous (D₂O) solutions affects both the shape of NMR spectra and NMR relaxation times of polymer and solvent. Two types of the approach are discussed. The first one is based on the reduction of integrated intensities of polymer NMR lines in high-resolution NMR spectra in the system undergoing the coil-globule phase transition. The fraction p of phase-separated units (units with significantly reduced mobility) and subsequently, e.g., thermodynamic parameters ΔH and ΔS characterizing the coil-globule phase transition can be determined. The second approach is based on measurements of ¹H NMR relaxation times of water (HDO) which provide information on behaviour of water during phase transition. The power of both approaches is demonstrated on results obtained with solutions of several thermoresponsive homopolymers and copolymers.     

Laser induced photoacoustic spectroscopy in liquid samples: temperature and solvent effects

Summary Laser induced photoacoustic spectroscopy (LIPAS) has been confirmed as an analytical tool, suitable for the determination of inorganic and organic species in aqueous and organic solvents, reaching detection limits two orders of magnitude lower than those obtained with conventional spectrophotometry.A significant influence of the solvent on the enhancement of the magnitude of the LIPAS response waveform of the piezoelectric detector has been found experimentally for inorganic species (neodymium ions) and organic compounds (e.g. -carotene and the cytochrome c), when water was replaced by organic solvents. This is particularly true, when a laser excitation wavelength above 500 nm was used, where the absorption coefficient for H₂O increases drastically.Furthermore, the temperature dependence of the magnitude of the LIPAS signal has been measured, in the range between 20–70°C in samples containing Nd³⁺,-carotene and the macromolecule cytochrome c in various aqueous and organic (polar and non-polar) solvents (H₂O, HClO₄, acetone, ethanol and petrol ether). A very strong enhancement of the LIPAS response was observed with increasing temperature (a factor of two in an interval between 20 and 40°C) for all inorganic and organic substances in aqueous solution. On the other hand, in polar and non-polar organic solvents only a slight decline of the signals of about 20% was found over the same temperature range.     

Self-diffusion and electron spin exchange of hydrophobic probes in dilute solution

Electron spin exchange rate constants have been measured by ESR spectroscopy for a nitroxide spin probe in a number of solvents, including water. The apparent collision rate constants (k _c ^′ ) calculated from the spin exchange rate constants showed marked deviations from the Smoluchowsky equation (k _c ^′ η=const), which were greatest in solvents of lowest viscosity. These effects are attributed to inefficiency of the spin exchange process. Self-diffusion coefficients (D) were measured for diamagnetic analogs of the nitroxide spin probe in similar solvent systems by pulsed field gradient NMR spectroscopy. TheD values gave reasonable agreement when corrected for viscosity (Dη=const). Collision rate constants calculated fromD were in good agreement with those measured by ESR in solvents of high viscosity. Thek _c ^′ value for the spin probe in water was significantly lower than that in isoviscous organic solvents. This effect is discussed in terms of a hydrophobic hydration shell for the spin probe which acts as an additional barrier to collision.     

A comparison of redox processes for polypyrrole/dodecylsulfate films in aqueous and non-aqueous media

Polypyrrole/dodecylsulfate (PPy/DDS) films were synthesized in aqueous and ethanolic solutions and investigated in aqueous, ethanolic, methanolic and acetonitrile solutions by cyclic voltammetry (CV). The amounts of anions and cations in the films before and after electrochemical treatment were determined by electron probe microanalysis (EPMA); the film morphology was studied by scanning electron microscopy (SEM). The results prove that the mobility of bulky DDS^– ions in PPy increases in the order: water<acetonitrile<ethanol<methanol. It was found that dopant DDS^– ions can be easily removed from PPy matrix swollen in alcohols or acetonitrile by electrochemical reduction or by soaking in electrolyte solutions of these solvents. The influence of electrochemical treatment on the change of doping level in aqueous solution is essentially less and depends on the cations in the test solution. Although the electroneutrality of PPy/DDS films during redox cycling is realized mainly by movement of the cations in aqueous solution and by movement of the anions in organic solvents, nevertheless the participation of anions in aqueous and cations in organic solvents is also established. The redox properties of PPy/DDS are more dependent on the solvent of the test solutions than of the synthesis solutions. Electronic Publication     

Magnesium-25 NMR studies of magnesium salts and complexes in nonaqueous solvents

Magnesium-25 NMR measurements were carried out on aqueous and non-aqueous solutions of magnesium salts. In the former case the²⁵Mg resonance frequency was independent of the concentration or of the counterion. In nonaqueous solvents, however, the resonance frequency was dependent on the solvent, the concentration, and on the nature of the counterion, indicating some cation-anion interactions. Measurements on Mg²⁺—phosphonoacetic acid mixtures in aqueous solutions gave strong indications of complexation. Only inconclusive evidence was obtained on the complexation of Mg²⁺ by macro-bicyclic cryptand C211 in methanol solutions, and no evidence of complexation was obtained with macrocycle 12-crown-4 in dimethylformamide solutions.     

Solvation of Aniline in Mixtures of Water with N,N-Dimethylformamide and Acetonitrile

Thermal effects of aniline solution in water-N,N-dimethylformamide (DMF) and water-acetonitrile mixtures were measured at 25°C. In almost the whole range of compositions of the mixed solvents, the thermal effects are more positive in aqueous acetonitrile than in aqueous DMF. Particular attention was given to binary solvents with a very low content of the organic cosolvent. In the mixture with the mole fraction of DMF of 10^{- 3}, the enthalpy of aniline solution is higher than in water by 5%, and in the mixture with the mole fraction of acetonitrile of 4 × 10^{- 4}, even by 15%. Features of specific solvation of aniline and an aliphatic amine (n-BuNH₂) in the water-DMF mixture were discussed taking into account the acid-base properties of the mixtures. The coefficients of pair interactions aniline-organic solvent in water and aniline-water in the organic solvent were calculated using the McMillan-Mayer theory. These coefficients correlate with the enthalpies of hydration of aprotic solvent molecules.     

Solvent effect on the rate of metal deposition reactions: Investigations of the kinetics of Ni(II) discharge at the mercury in mixed water methanol solvents

The kinetics of Ni²⁺ deposition at the mercury electrode, from non-complexing electrolytes in mixed water methanol solvents has been investigated. The rate of this reaction, in the whole composition range, at large overpotentials is controlled by a limiting “chemical” process. The rate constants of the limiting process when plotted as a function of the solvent composition display a relationship similar to those observed for the large organic, bidendate ligands substitution reactions, believed to the controlled by the Ni(II)-solvent bond rupture. Hence, the limiting “chemical” process in Ni²⁺ discharge can be identified as the slow solvent dissociation step. These results confirm the mechanism of Ni²⁺ dischrage already proposed by Gierst and Dandoy on the basis of investigations performed in aqueous solutions.     

Determination of99Tc in reactor loop decontamination liquid waste using extraction chromatography

A new analytical method has been developed to determine⁹⁹Tc in samples of aqueous radioactive waste containing high levels of⁶⁰Co (10³–10⁶ Bq/l) in mixed forms (simple cationic⁶⁰Co²⁺ and coordinated complex forms). The method consists of selective extraction of the⁹⁹Tc with a combination of cation exchange and solid-phase extraction resins under strong acid conditions. Quantification of the isolated⁹⁹Tc is accomplished using liquid scintillation counting. The method provides high recoveries (>98%), is simpler than classical solvent extraction methods, eliminates the use of chlorinated organic solvents and the time of analysis is reduced by a factor of 2 when compared to solvent extraction methods.