高分子溶剂的选择——薄层色谱法表征溶剂对高分子的溶解能力

本文讨论了用薄层色谱法(下称 TLC)测得的流动比率R_f和溶剂对高分子溶解能力之间的关系,提出了用R_f表征溶剂对高分子的溶解能力。根据此方法选择了两组氯丁胶粘剂的低毒溶剂。     

Environmental effects on the aggregation of some xanthene dyes used in lasers

The effect of solvents, halo substituents and surfactants on the aggregation of xanthene dyes (fluorescein and eosin Y) has been investigated. It has been found that polar protic solvents promote the aggregation of both the dyes while polar aprotic solvents hinder the aggregation process. Apolar solvents can disintegrate the aggregates previously formed in aqueous medium. The nature of the driving forces for aggregation in the two dyes is different. Surfactants bearing charge opposite to the dye molecules facilitate aggregation. These observations bear direct relevance to the possible use of fluorescein and eosin Y as quantum yield standards, photosensitizers, laser dyes and biological labels.     

Thermal racemization of spiropyrans: implication of substituent and solvent effects revealed by computational study

The thermal racemization of spiropyrans has been investigated by computational study at the density functional theory and MP2 levels. Two possible reaction mechanisms for the thermal racemization of spiropyrans were found. One reaction mechanism is a three-step reaction, the other represents a four-step reaction. The competition between these two mechanisms is dependent on the strengths of the substituents, as well as the polarities of the solvents. Different substituents and solvents change the rate-determining step and lead to various pathways. The solvent effect is found more pronounced on the electron-withdrawing substituted spiropyrans. In addition, bond length alternation (BLA), a structural parameter, has been employed in this study. It has been proven that BLA is a useful parameter to correlate the effect of substituents and solvents with the racemization mechanism of spiropyrans. The relationship between BLA and activation energies clearly explains the origin of the competition between the two reaction mechanisms. The results from this work would be valuable for selecting substituents and solvents for development of desired optical properties of spiropyrans.     

Dual Solvatochromism of Neutral Red

Abstract— The effect of solvent polarity on the electronic absorption and fluorescence properties of neutral red (NR), a phenazine-based dye of biological importance has been investigated in several neat and mixed solvents. An unusual dual solvatochromic behavior has been observed that reveals the existence of two closely spaced electronic excited states in NR. In low-polarity solvents the fluorescence of the NR is mainly emitted from the localized excited state, whereas in high-polarity solvents the emission from the charge transfer state dominates. The dipole moments of the localized and charge transfer states of NR have been estimated from the solvatochromic shifts. The dipole moment of the localized excited state (4.8 D) is only slightly higher than that of the ground state (2.0 D), while that of the charge transfer state is drastically higher (17.5 D). Fluorescence quantum yields and the life-times of NR have been determined in different solvents and correlated with the solvatochromic shifts.     

A thermogravimetric study of acidic and basic solutes in two molten alkali metal nitrate eutectics

The reaction of ten oxyanions and oxides, acting as Lux—Flood acids have been studied in both lithium nitrate—potassium nitrate and sodium nitrate—potassium nitrate eutectics and the stoichiometries of the reactions established. A series of acids arranged in order of decreasing strength has been drawn up, which is very similar for both eutetic solvents.Five Lux—Flood bases have also been studied in these two solvents. Sodium carbonate proved to be very stable to 500°C. Sodium peroxide revealed significant differences in stability and solubility in the two solvents, while lithium, sodium and potassium hydroxides showed differences in the two melts and also considerable differences between each other, suggesting that little cation exchange takes place in these solutions.     

Spectroscopic study of association of a hemicyanine dye in mixed aqueous binary solvents

Electronic absorption and steady state emission properties of a hemicyanine dye [4-[4-(dimethylamino)styryl]-1-docosylpyridinium bromide], have been studied in several pure solvents and two mixed binary solvents (water+ethanol and water+acetonitrile). In pure solvents the band-width of the absorption spectrum correlates well with the Stoke's shift. In mixed aqueous solvents two different molecular forms of the solute, viz. the monomer and the dimer of the solute exists in equilibrium. Thermodynamic parameters (e.g. the Delta G degrees and Delta H degrees ) characterizing the equilibrium have been determined. While the value of Delta G degrees changes very slightly with the composition of the binary mixture, the value of Delta H degrees has been observed to depend significantly with solvent composition.     

Use of green alternative solvents in dispersive liquid-liquid microextraction: A review

Dispersive liquid-liquid microextraction is one of the most widely used microextraction techniques currently in the analytical chemistry field, mainly due to its simplicity and rapidity. The operational mode of this approach has been constantly changing since its introduction, adapting to new trends and applications. Most of these changes are related to the nature of the solvent employed for the microextraction. From the classical halogenated solvents (e.g., chloroform or dichloromethane), different alternatives have been proposed in order to obtain safer and non-pollutants microextraction applications. In this sense, low-density solvents, such as alkanols, switchable hydrophobicity solvents, and ionic liquids were the first and most popular replacements for halogenated solvents, which provided similar or better results than these classical dispersive liquid-liquid microextraction solvents. However, despite the good performances obtained with low-density solvents and ionic liquids, researchers have continued investigating in order to obtain even greener solvents for dispersive liquid-liquid microextraction. For that reason, in this review, the evolution over the last five years of the three types of solvents already mentioned and two of the most promising solvent alternatives (i.e., deep eutectic solvents and supramolecular solvents), have been studied in detail with the purpose of discussing which one provides the greenest alternative.     

The use of two nematic solvents in the determination of molecular structure of systems providing deceptively simple spectra

The use of two liquid crystals as solvents in the determination of molecular structure has been demonstrated for systems which do not provide structural information from studies in a single solvent owing to the fact that the spectra are deceptively simple, with the result that all the spectral parameters cannot be derived with reasonable precision. The specific system studied was 2-(p-bromophenyl)-4,6-dichloropyrimidine, for which relative inter-proton discances have been determined from the proton NMR spectra in two nematic solvents.     

N-C bond cleavage in the acid-catalyzed decomposition of some dithio- and diselenocarbamates : A kinetic investigation of dithiocarbamate decomposition in benzene and methanol

The acidic decomposition of some dithiocarbamic internal salts in MeOH and in benzene has been carried out by UV spectrophotometric measurements under pseudo-first order conditions. In both solvents the reaction is a first order with respect to the substrate and the HCl concentration. A mechanism with two consecutive irreversible steps is proposed in benzene and the results are compared with the diselenocarbamates under the same conditions. In methanol a reversible first stage has been proposed. A general conclusion on the kinetic behaviour of dithio- and diselenocarbamates in solvents at different dielectric constants has been suggested.     

Spectrophotometry of iron chelates with 2-hydroxy-5-methylpropiophenone oxime

2-Hydroxy-5-methylpropiophenone oxime (HMP) forms two complexes with ferric iron, the formation of which is pH dependent. Blue colored complex is formed at lower pH values. This complex cannot be extracted into water-immiscible organic solvents. Another complex, brown-red in color, is formed at higher pH values which is extracted into organic solvents. The molar composition of two complexes has been ascertained by using logarithmic and Asmus methods. In the case of blue chelate, the composition is 1:1 (Fe³⁺: oxime) and the composition of brown-red extracted complex is 1:3. The two complexes adhere to Beer's law over a fairly wide range of iron concentration and the sensitivity in terms of Sandell's definition has been calculated. Effect of some foreign ions has been investigated and tentative structures have been assigned to the two complexes.     

Studies on solute solvent interaction. Charge transfer band maxima of N-alkyl pyridinium iodides in solution — A correlation with a dielectric parameter

The longest wavelength band of n-alkyl pyridinium iodides (NAPI) in solution, which is due to charge transfer processes within a contact ion pair species, serves as an empirical measure of solute-solvent interaction for a polar solute in polar solvents. An attempt has been made to correlate the energy of the transition with the chemical potential of the dipolar ion pair in a solvent. The latter quantity has been calculated using a dielectrically saturable Block-Walker reaction field model. It has been found that, for protic solvents there is a good linear correlation between the two parameters enabling the calculation of the transition energy in water. An alternative correlation involving the individual molecule dipole reaction field is also discussed. For dipolar aprotic solvents both correlations yield poorer results indicating a single parameter correlation is not sufficient. In alcoholic binary systems the solute-solvent interaction is a linear function of the bulk reaction fields of the component solvents. But in the case of aprotic-alcoholic solvents, where specificities of interaction differ, the solute sees an environment the composition of which differs from that of the bulk.     

Ellipsometry: The effect of atomic roughness on the optical properties of gold electrodes covered with a monolayer of thallium

The polarographic behavior of iodine in aprotic solvents has been investigated. Iodine gave two well-defined cathodic waves in aprotic solvents at either a platinum or a mercury electrode. At the dropping mercury electrode, the reduction wave of iodine corresponded to the reduction of mercuric iodide, formed at the electrode surface by the reaction of iodine with mercury. It was found that the difference in the half-wave potential between the first and second waves depends upon the stability of an intermediate, triiodomercurate ion in the solvents.In some aprotic solvents and water-mixed solvents, the solvent effects on the reduction potential of iodine and the formation of the complex ion, triiodomercurate ion, were discussed with the dropping mercury electrode.     

Dilute solution behaviour of amylose tributyrate. Light scattering,viscosity and osmotic pressure measurements in non-ideal solvents

Amylose tributyrate has been prepared and fractionated; the dilute solution behaviour of the polymer has been studied in four solvents, ethyl acetate, methyl ethyl ketone, carbon tetrachloride and tetrahydrofuran. The Mark-Houwink relations have been established for each solvent and the exponent γ varies between 0·72 and 0·86, for different solvents. Perturbed dimensions have been measured for the polymer dissolved in two solvents and an estimate of the unperturbed dimensions has been made; the latter appear to be solvent dependent. Heterogeneity corrections were applied using data established by gel permeation chromatography measurements. Values of the effective bond length b of between 17 and 19·5 × 10^?8 cm and the Kuhn statistical segment A_m of 70 to 180 x 10^?8 cm, indicate that amylose tributyrate behaves like a moderately stiff coil in good solvents, with a chain stiffness intermediate between the vinyl and cellulosic polymers.     

Thermodynamics of mixing of polymers

A method of calculating free energy of mixing of two polymers has been proposed. From a study of the sorption of organic solvents vapour by different polymers and their mixtures, Gibbs free energy of mixing for polymer-solvent systems are calculated. The free energy of mixing of two polymers has been calculated by the method proposed. Heats of mixing of polymers have been measured with a Calvet type microcalorimeter. Entropy of mixing has been calculated. Stabilities of polymer-polymer systems have been discussed.     

Solvation of tetraphenylphosphonium dicyanamide

NMR and IR spectroscopy have been used to study the solvation of the dicyanamide ion in solvents with different chemical natures. The occurrence of two kinds of interaction of the N(CN)₂ ^–ion with the solvents has been established: by means of hydrogen bonds (alcohols, chloroform) and by ion-dipole interaction (aprotic solvents).Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 3, pp. 361–366, May–June, 1988.     

Prediction of maxima and minima in the curve of total sorption parameter in ternary polymer systems. Influence of ternary interaction parameter

Flory-Huggins theory modified by Pouchly has been applied to predict maxima and minima in the curve of total sorption in ternary polymer systems formed by a polymer and two liquids. In this work, different diagrams based on experimental magnitudes easily obtained such as the difference in affinities of liquids, solvents and non solvents, and the solvent molar volume ratio. Total sorption parameter has been considered to be the decisive magnitude to define extrema conditions in both cosolvent and cononsolvent ternary polymer systems. The theoretical prediction is not altered by the inclusion of ternary interactions. Different examples of ternary systems dealing with vinyl polymers and polydimethyl siloxane have been used to test the above formalism.     

Prediction of solvents suitable for crystallization of small organic molecules

Selection of suitable solvent is essential for crystallization of pharmaceuticals. Based on chemical structures of 6397 compounds and 15 single solvents that were used to obtain their single crystals, correlations between the molecular characteristics and the solvents have been investigated by cheminformatics methods. Decision-tree and Bayesian-probability methods have been applied to make classification models. These two models are complementary in character in the present case. It has been proven that the prediction of the solvent rankings for particular compounds by use of the classification models is satisfactory from the practical point of view. The present study has demonstrated that cheminformatics methods could greatly help rational crystallization of small organic molecules such as pharmaceuticals.     

Low operational stability of enzymes in dry organic solvents: changes in the active site might affect catalysis

The potential of enzyme catalysis in organic solvents for synthetic applications has been overshadowed by the fact that their catalytic properties are affected by organic solvents. In addition, it has recently been shown that an enzyme's initial activity diminishes considerably after prolonged exposure to organic media. Studies geared towards understanding this last drawback have yielded unclear results. In the present work we decided to use electron paramagnetic resonance spectroscopy (EPR) to study the motion of an active site spin label (a nitroxide free radical) during 96 h of exposure of the serine protease subtilisin Carlsberg to four different organic solvents. Our EPR data shows a typical two component spectra that was quantified by the ratio of the anisotropic and isotropic signals. The isotropic component, associated with a mobile nitroxide free radical, increases during prolonged exposure to all solvents used in the study. The maximum increase (of 43%) was observed in 1,4-dioxane. Based on these and previous studies we suggest that prolonged exposure of the enzyme to these solvents provokes a cascade of events that could induce substrates to adopt different binding conformations. This is the first EPR study of the motion of an active-site spin label during prolonged exposure of an enzyme to organic solvents ever reported.     

Effect of solvent nature on intrinsic viscosity of an ethylene-propylene-dicyclopentadiene copolymer

The hydrodynamic behavior of solutions of the ethyleneópropylene-dicyclopentadiene copolymer in hydrocarbon solvents has been studied. It has been shown that the effects of temperature on the thermodynamic affinities of aromatic and aliphatic solvents for copolymer macromolecules are substantially different. The thermodynamic quality of aromatic solvents relative to the copolymer and the hydrodynamic volume of macromolecular coils increase with temperature; the aliphatic solvents show quite a reverse behavior.     

A study of the ionic liquid mediated microwave heating of organic solvents

The use of ionic liquids as aids for microwave heating of nonpolar solvents has been investigated. We show that hexane and toluene together with solvents such as THF and dioxane can be heated way above their boiling point in sealed vessels using a small quantity of an ionic liquid, thereby allowing them to be used as media for microwave-assisted chemistry. Using the appropriate ionic liquid, the heating can be performed with no contamination of the solvent. To show the applicability of the system, two test reactions have been successfully performed.