Showing papers on "Solvent published in 1982"

Calculation of liquid-liquid phase separation in a ternary system of a polymer in a mixture of a solvent and a nonsolvent

Abstract: A numerical method for the calculation of the binodal of liquid-liquid phase separation in a ternary system is described. The Flory-Huggins theory for three-component systems is used. Binodals are calculated for polymer/solvent/nonsolvent systems which are used in the preparation of asymmetric ultrafiltration or reverse osmosis membranes: cellulose acetate/solvent/water and polysulfone/solvent/water. The values for the binary interaction parameters are taken from literature sources. The effect of a concentration-dependent solvent/nonsolvent interaction parameter is discussed. Although knowledge of the interaction parameters for all compositions in the ternary system is rather poor, fairly good agreement has been found between calculated and experimentally found miscibility gaps when the solvent/nonsolvent parameter is taken to be concentration dependent and the other parameters, the polymer/solvent and the polymer/nonsolvent interaction parameter, are kept constant.

Dynamical polar solvent effects on solution reactions: A simple continuum model

Abstract: The influence of polar solvent dynamics in solution reactions is investigated for a simple model. In this model, a charge is subject to a chemical free energy barrier, the successful crossing of which constitutes reaction. The retarding influence of the time dependent solvent polarization is described at the continuum Debye relaxation level. The reaction rate constant k is determined as a function of the barrier curvature, the charge‐solvent interaction strength, and the solvent polarization relaxation time. The reduction of k with increasingly slower solvent relaxation is found to depend sensitively on the magnitude of charge‐solvent interactions. When the latter are weak, the behavior of k differs qualitatively from a standard Kramers prediction. The analogous reaction problem for a dipole is discussed briefly. Directions for a more realistic treatment of polar solvent effects on reaction rates are described.

Thermal oil recovery with solvent recirculation

Abstract: A process for the production of heavy oil from a subterranean oil reservoir by the injection of a hot aqueous fluid into the reservoir and the injection of a diluent solvent down the production well to produce a blend of solvent and oil having a decreased viscosity. The reservoir oil has a density greater than the density of water. The diluent solvent has a density such that the density of the resulting blend recovered from the production well also has a density greater than the density of the water. The water produced from the production well is separated from the blend and the blend then fractionated to recover a solvent fraction of the requisite density. This solvent fraction is then returned to the production well to produce additional blend within the well in a continuation of the process.

The elucidation of solvent exchange mechanisms by high-pressure NMR studies

Abstract: Abstract

The nature of dilute solutions of sodium ion in water, methanol, and tetrahydrofuran

Abstract: Monte Carlo statistical mechanics simulations have been carried out for dilute solutions of Na+ in water and tetrahydrofuran (THF) at 25 °C and 1 atm. The intermolecular interactions were described by Lennard‐Jones and Coulomb terms in the TIPS format including the TIPS2 parameters for water–water interactions. In conjunction with previous simulation results for Na+ in methanol, the present study provides detailed insights into the nature of ionic solvation by dipolar protic and aprotic solvents. In agreement with x‐ray data, the coordination number of Na+ in water is six, identical to the value obtained for Na+ in methanol. However, the coordination number of Na+ in THF fluctuates between five and six. Consistently, the ion–solvent interaction is less exothermic in THF compared to the other two solvents. However, the heat of solution which is the sum of two large opposing contributions, viz., the ion–solvent energy and the solvent reorganization energy, does not differ much for the three solvents. In all...

High‐pressure nmr study of dynamical effects on conformational isomerization of cyclohexane

Abstract: The effect of temperature and pressure on the conformational inversion of cyclohexane in solution has been investigated with use of /sup 1/H FT NMR spectroscopy. The solvents used in this study are methylcyclohexane-d/sub 14/, carbon disulfide, and acetone-d/sub 6/. The activation parameters as obtained from the temperature dependence of the rate constant are independent of solvent. However, the coalescence temperature in the methylcyclohexane-d/sub 14/ solvent is 1.5/sup 0/C higher than in the other two solvents. It is observed that the ring inversion in cyclohexane is accelerated when pressure is increased, and this pressure dependence of the rate constant is nonlinear. In the lower viscosity solvents, acetone-d/sub 6/ and carbon disulfide, the rate constant shows a larger pressure dependence than in the more viscous methylcyclohexane-d/sub 14/ solvent. The experimental data are interpreted with use of results of the stochastic models for isomerization reactions. In these models it is proposed that there are dynamical effects on isomerization because the reaction coordinate is coupled to the surrounding medium. We find that the observed activation volume is strongly pressure and solvent dependent as is the collisional contribution to the activation volume. Since the collision frequency reflecting the coupling of the reaction coordinate to themore » solvent mediums is proportional to solvent viscosity, the observed activation volume and the transmission coefficient kappa correlate well with the solvent viscosity. In addition, the experimental data indicate a non-monotonic transition between the inertial (weak coupling) and diffusive (strong coupling) regimes of isomerization reactions. Our results represent the first experimental proof of the predictions of stochastic models for isomerization reactions in condensed phases. 9 figures, 1 table.« less

Solvent effects on chemical reactivity

Abstract: Typical examples for different types of solvent effects on chemical reactivity are given, e. g. solvent effects on reaction rates, on the position of chemical equilibria, on competitive reaction mechanisms, on dichotomic reaction paths, on chemoselectivity, and on stereoselectivity. According to their solvent sensitivity, most organic reactions can be classified into dipolar transition-state reactions, isopolar transition-state reactions, and free-radical transition-state reactions, typical examples of which are given. Attempts to describe the solvation capability by virtue of empirical parameters of solvent polarity are mentioned. Particular attention is merited by the ET(30)-parameter, an empirical parameter derived from a negatively solvatochromic pyridinium-N-phenolate betaine dye as reference compound. Some examples of the application of this ET(30)-scale to solventsensitive chemical reactions are given.

FLUORESCENCE LIFETIMES OF CHLOROPHYLL a: SOLVENT, CONCENTRATION AND OXYGEN DEPENDENCE

Abstract: Fluorescence lifetimes (τf) of chlorophyll a (Chi a) have been measured by the single-photon-counting technique over a wide range of concentrations (˜10-7˜10-4M) in deoxygenated pyridine, diethyl ether, toluene and methanol. At pigment concentrations ˜1 μM, reabsorption of fluorescence induces significant artifacts on measured values of τf which are dependent on detection wavelength and the specific geometry of the experiment. There is a clear dependence of τf on the nature and degree of solvation, including both coordination of the central magnesium and hydrogen-bonding of the solvent (viz. alcohols) to the macrocycle. Quenching of the excited singlet state by molecular oxygen was measured quantitatively in ether, and a bimolecular rate constant markedly slower than the diffusion-controlled limit was obtained.

High efficiency organosolv saccharification process

Abstract: Comminuted cellulosic materials which may or may not contain lignin are partially or totally hydrolyzed or saccharified by an improved organosolv process using an aqueous acetone solvent mixture containing a small amount of an acidic compound and containing at least about 70 percent by volume of acetone and up to virtually anhydrous acetone. The process is performed at elevated reaction temperatures, preferably at 145° C. to 230° C., for a limited period of time and then with cooling such that the resultant dissolved sugars from the hydrolysis are not degraded into non-sugars. In particular the reaction is conducted such that the cellulosic material is dissolved and such that at least ninety percent or more of available sugars in the cellulosic material are recovered. Unexpectedly it has been found that acetone at high concentration forms stable complexes with the sugars which prevents their degradation and also facilitates separation of the sugars. Lignin and sugars derived are commercially useful chemical compounds.

Solvent effects on the tautomeric equilibrium of 2,4-pentanedione

Abstract: The influence of solvent on the equilibrium position of the tautomeric forms of 2,4-pentanedione was studied by calorimetric and nmr spectroscopic techniques. For solvents such as CCl4 and cyclohex...

Percutaneous absorption of organic solvents. III. On the penetration rates of hydrophobic solvents through the excised rat skin.

Abstract: The penetration rates of 10 hydrophobic solvents through the excised abdominal rat skin were quantitatively determined by using the in vitro method reported previously. The hydrophobic solvents studied were benzene, toluene, ethyl-benzene, o-xylene, styrene, n-pentane, 2-methylpentane, n-hexane, n-heptane and n-octane. There was a great diversity in the ability of the hydrophobic solvents to penetrate through the excised rat skin, but a good correlation between the penetration rates and the solubility of the solvents in water was observed. The log-log plot of the penetration rate versus the solubility in water showed a linear relationship ; log y=1.41 log x-0.297, r =0.981, where y is the penetration rate (n moles/min/cm2 of skin) and x is the solubility of the solvent in water (mM at 25°C). The solubility in water was found to be a reliable index of the skin penetration rate for the hydrophobic solvents.

SYNTHESIS OF METHYL-t-BUTYL ETHER FROM METHANOL AND ISOBUTENE USING A CLAY CATALYST

Abstract: The acid-catalyzed reaction between methanol and isobutene to give methyl-t-butyl ether may be carried out using a cation-exchanged smectite as the catalyst. In 1,4-dioxan solvent at 60°C smectites exchanged with Al3+, Fe3+, or Cr3+ give yields of ∼60% after 4 hr, whereas smectites exchanged with Cu2+, Pb2+, Ni2+, Co2+, Ca2+, and Na+ give less than ∼8% yield. The reaction is efficient only when certain solvents are used, e.g., with Al3+-smectite the yield is ∼5% when using 1,2-dimethoxyethane, diethyleneglycol diethylether, n-pentane, tetrahydropyran, N-methylmorpholine, or tetrahydrofuran solvents compared with ∼60% using 1,4-dioxan solvent (4 hr). Moreover, the effective solvents depend somewhat on the clay interlayer cation. The use of tetrahydrofuran and tetrahydropyran gives ∼35% yields at 60°C (4 hr) with Fe3+- or Cr3+-smectites but ∼4% yield with Al3+-smectite.

Soluble polysilastyrene and method for preparation

Abstract: The preparation of soluble polysilastyrene comprising reacting phenylmethyldichlorosilane and dimethyldichlorosilane in the mole ratio of 1 to 0.25-2 in the solvent solution in the presence of sodium.

Photosensitized production of hydrogen by hydrogenase in reversed micelles.

Abstract: Hydrogenase (hydrogen:ferricytochrome c3 oxidoreductase, EC 1.12.2.1) from Desulfovibrio vulgaris was encapsulated in reversed micelles with cetyltrimethylammonium bromide as surfactant and a chloroform/octane mixture as solvent. Reducing equivalents for hydrogenase-catalyzed hydrogen production were provided by vectorial photosensitized electron transfer from a donor (thiophenol) in the organic phase through a surfactant-Ru2+ sensitizer located in the interphase to methyl viologen concentrated in the aqueous core of the reversed micelle. The results show that reversed micelles provide a microenvironment that (i) stabilizes hydrogenase against inactivation and (ii) allows an efficient vectorial photosensitized electron and proton flow from the organic phase to hydrogenase in the aqueous phase.

Removal of Refractory Organics from Water by Aeration. II. Solvent Sublation of Methylene Blue and Methyl Orange

Abstract: The solvent sublations of methylene blue-sodium tetradecylsulfate and of methyl orange-hexadecyltrimethylammonium bromide were carried out in lab-scale batch apparatus Removal was from water to 2-octanol Three possible mechanisms were examined in detail; the colored species removed in both cases is believed to consist of one ion of dye complexed with two of surfactant Mass transfer rate effects in the vicinity of the bubble-water interface were examined theoretically, and the empirical time constant for mass transfer related to the lowest eigenvalue of a suitably selected diffusion problem