Showing papers on "Solvent published in 1990"

BYPASS: an effective method for the refinement of crystal structures containing disordered solvent regions

Abstract: A method is described for the least-squares refinement of the atomic parameters of the ordered part of a crystal structure in the presence of disordered solvent areas. Potential solvent regions are identified automatically. The contribution of the observed contents to the total structure factor is calculated via a discrete Fourier transformation, and incorporated in a further least-squares refinement of the ordered part of the structure. The procedure is iterated a few times to convergence. It is found that this mixed discrete-atom and continuous solvent-area model refinement approach greatly improves the quality of discrete atomic parameters, i.e. the geometry and the e.s.d.'s. An electron count over the solvent region in the final difference electron-density map provides a convenient estimate for the number of solvent molecules present in the unit cell. The application of the method to four structures is described.

Very low temperature casting of controlled release microspheres

Abstract: A process for preparing microspheres using very cold temperatures to freeze polymer-biologically active agent mixtures into polymeric microspheres with very high retention of biological activity and material. Polymer is dissolved in a solvent together with an active agent that can be either dissolved in the solvent or dispersed in the solvent in the form of microparticles. The polymer/active agent mixture is atomized into a vessel containing a liquid non-solvent, alone or frozen and overlayered with a liquified gas, at a temperature below the freezing point of the polymer/active agent solution. The cold liquified gas or liquid immediately freezes the polymer droplets. As the droplets and non-solvent for the polymer is warmed, the solvent in the droplets thaws and is extracted into the non-solvent, resulting in hardened microspheres.

The Gelation Of Proteins

Abstract: Publisher Summary This chapter describes the process of the gelation of proteins Gelation is a phenomenon; therefore, its definition and that of its product, a gel, is dependent on the perspective of the observer and the technique(s) used to observe it For example, gels may be defined by their ability to immobilize a liquid, their macromolecular structure, or their textural or rheological properties The chapter reviews a variety of protein gel systems with a broad perspective stressing similarities between them, yet not without mentioning the important differences that make each unique It describes the multicomponent protein gels The rheological and optical properties of thermally irreversible gels are the outcome of two events First, a change in protein structure is needed that permits protein–protein interactions The subsequent aggregation process, which is highly influenced by the solvent environment, may produce the gel microstructure, which is responsible for the optical and rheological properties The physical integrity of a gel is maintained by counterbalancing forces of attraction and repulsion among polymer molecules and between the polymer network and the surrounding solvent

Lipase catalyzed alcoholysis of sunflower oil

Abstract: Lipase-catalyzed alcoholysis of sunflower oil under anhydrous conditions was examined. Lipases fromPseudomonas fluorescens and 2 immobilized enzymes fromMucor miehei and aCandida sp. gave sufficient conversion with petroleum ether as the solvent, even when methanol and ethanol were used. The overall content of tri-, di- and monoglycerides, as well as the corresponding alkyl esters, was measured. BecausePseudomonas lipase led to almost quantitative esterification, further studies were carried out with that enzyme varying the amounts of enzyme or the alcohols. Acceptable conversions were achieved even without solvent. Reaction rates of alcoholysis with 5 homologous alcohols, with or without the addition of water, were measured, and in all cases the reaction rates increased with higher chain length of the alcohol. In the case of methanol the highest rate was obtained without any addition of water, but a significantly higher rate was observed with 96% ethanol as opposed to absolute ethanol. The main advantages of lipasecatalyzed, nonaqueous alcoholysis as compared to classical procedures are the mild reaction conditions, the isolation of glycerin without further purification and without the formation of chemical waste, and the ability of lipases to catalyze the esterification of free fatty acids.

Preferential interactions determine protein solubility in three-component solutions: the MgCl2 system.

Abstract: The correlation between protein solubility and the preferential interactions of proteins with solvent components was critically examined with aqueous MgCl2 as the solvent system. Preferential interaction and solubility measurements with three proteins, beta-lactoglobulin, bovine serum albumin, and lysozyme, resulted in similar patterns of interaction. At acid pH (pH 2-3) and lower salt concentrations (less than 2 M), the proteins were preferentially hydrated, while at higher salt concentrations, the interaction was either that of preferential salt binding or low salt exclusion. At pH 4.5-5, all three proteins exhibited either very low preferential hydration or preferential binding of MgCl2. These results were analyzed in terms of the balance between salt binding and salt exclusion attributed to the increase in the surface tension of water by salts, which is invariant with conditions. It was shown that the increase in salt binding at high salt concentration is a reflection of mass action, while its decrease at acid pH is due to the electrostatic repulsion between Mg2+ ions and the high net positive charge on the protein. The preferential interaction pattern was paralleled by the variation of protein solubility with solvent conditions. Calculation of the transfer free energies from water to the salt solutions for proteins in solution and in the precipitate showed dependencies on salt concentration. This indicates that the nature of interactions between proteins and solvent components is the same in solution and in the solid state, which implies no change in protein structure during precipitation. Analysis of the transfer free energies and preferential interaction parameter in terms of the salting-in, salting-out, and weak ion binding contributions has led to the conclusions that, when the weak ion binding contribution is small, the predominant protein-salt interaction must be that of preferential salt exclusion most probably caused by the increase of the surface tension of water by addition of the salt. A necessary consequence of this is salting-out of the protein, if the protein structure is to remain unaltered.

Cross-linking reactions during coal conversion

Abstract: During coal conversion, the breakup of the coal macromolecular network and resulting product formation are controlled by the relative rates of bond breaking, cross-linking, and mass transport. THe objective of this work was to systematically study the variations in cross-linking with several parameters (rank, temperature, heating rate, pretreatment, etc.), to identify the factors that control cross-linking rates. This paper describes a study of cross-linking behavior in which chars of a number of coals (including the Argonne premium coal samples) have been pyrolyzed under a variety of temperature histories and analyzed at intermediate extents of pyrolysis for solvent swelling behavior and functionnal group compositions

Preparation of liposome and lipid complex compositions

Abstract: Liposome and lipidic particle formulations of compounds are prepared by dissolving in a solution of liposome-forming lipids in an aprotic solvent such as DMSO, optionally containing a lipid-solubilizing amount of a lower alkanol, and either injecting the resulting solution into an aqueous solution, or the aqueous solution into the resulting solution. The resulting liposome or lipidic particle suspension may then be dialyzed or otherwise concentrated. This method is particularly useful for compounds which are poorly-soluble in aqueous solution, but is generally useful for any compound or combination of compounds which can be dissolved in the aprotic solvent or aprotic solvent/lower alkanol mixture.

Encapsulation of water-soluble drugs by a modified solvent evaporation method. I. Effect of process and formulation variables on drug entrapment.

Abstract: Pseudoephedrine HCl, a highly water-soluble drug, was entrapped within poly (methyl methacrylate) microspheres by a water/oil/water emulsification-solvent evaporation method. An aqueous drug solution was emulsified into a solution of the polymer in methylene chloride, followed by emulsification of this primary emulsion into an external aqueous phase to form a water/oil/water emulsion. The middle organic phase separated the internal drug-containing aqueous phase from the continuous phase. Microspheres were formed after solvent evaporation and polymer precipitation. The drug content of the microspheres increased with increasing theoretical drug loading, increasing amounts of organic solvent, polymer and polymeric stabilizer, and decreased with increasing stirring time, increasing pH of the continuous phase and increased volume of the internal and external aqueous phase.

Method for making uniformly-sized particles from insoluble compounds

Abstract: The invention involves a method for making uniformly sized particles from solid compounds. First, a suitable solid compound is dissolved in a suitable solvent. Then, a precipitating liquid is infused, precipitating non-aggregated particles with substantially uniform mean diameter. The particles are then separated from the solvent. Depending on the solid compound and the desired particle size, the parameters of temperature, ratio of non-solvent to solvent, infusion rate, stir rate, and volume can be varied according to the invention. The precipitating liquid may be aqueous or non-aqueous, depending upon the relative solubility of the compound and the desired suspending vehicle.

Rod-coil copolymers: their aggregation behavior

Abstract: The aggregation behaviour of rod-coil block copolymers in a selective solvent of low molecular weight is considered within the scaling approach. Thermodynamically stable micelles are characterized by extended, starlike coronas. The lamellae are expected to exhibit a tilting, smectic A-smectic A-smectic C, first-order phase transition driven by the competition between surface and deformation free energies. The Petschek-Wiefling design for ferroelectric smectic phases is analyzed in the case of swollen lamellar phases formed by ABC coil-rod-coil copolymers

Weakly charged polyelectrolytes in a poor solvent

Abstract: 2014 We show that a weakly charged polyelectrolyte in a salt free poor solvent undergoes a mesophase separation transition by lowering the temperature. This transition is investigated at low temperature by studying the possibility of micelle formation and at higher temperature by studying the divergence of the peak at finite wave vector of the polymer-polymer correlation function. As salt is added to the solution, the peak in the structure factor disappears and the transition becomes a usual demixing phase separation between polymer and solvent. The multicritical point of these two transitions is a so-called Lifshitz point ; in the vicinity of this point a Ginzburg criterion shows that concentration fluctuations play an important role and that the mean field theory is not sufficient. At higher salt concentration the demixing transition belongs to the Ising universality class. Finally, we discuss the behavior of solutions containing both weakly charged and neutral polymers : here also the peak in the structure factor disappears as the concentration of neutral polymers is increased. J. Phys. France 51 (1990) 545-557 15 MARS 1990, Classification Physics Abstracts 36.20 61.40K 61.10E

Influence of solvent and sorbent characteristics on distribution of pentachlorophenol in octanol-water and soil-water systems.

Abstract: Estimation des facteurs varies et des processus influencant la sorption du pentachlorophenol par des sorbants naturels en utilisant les donnees sur la sorption du pentachlorophenol en solution aqueuse et dans des melanges de solvants par des argiles et des materiaux solides

Partitioning of nonpolar solutes into bilayers and amorphous n-alkanes

Abstract: We have measured the partition coefficients for hexane between an aqueous solvent and phospholipid bilayer membranes as a function of the surface density of the bilayer chains. The surface density was varied by temperature, phospholipid chain length, and the incorporation of cholesterol and was monitored by 2H NMR. We observe that increasing surface density leads to expulsion of the solute: hexane partitioning decreases by a factor of 9 as the surface density of the bilayer chains increases from 50% to 90% of its maximum value, a range readily accessible in biomembranes under physiological conditions. Benzene as solute shows similar behavior; thus, the dependence of partitioning on surface density appears to be a general property of the organization of the bilayer chains. In order to determine the relative contributions to partitioning of (i) chain ordering and (ii) contact interactions, we have also performed reference-state experiments for benzene transfer between water and amorphous n-alkanes of different chain lengths. The widely accepted value of 25 cal/mol per A* of surface area for the free energy of oil/water transfer per methylene group is appropriate only when solute and solvent molecules are of the same size. We find that Flory-Huggins theory satisfactorily corrects for the molecular size differences of solutes and solvents. Comparisons of the bilayer/water and oil/water temperature dependencies of benzene partitioning show that there is an additional entropic expulsion of solute from the bilayer, consistent with recent statistical thermodynamic theory suggesting the importance of the chain ordering in the bilayer.

Butanol recovery from fermentations by liquid-liquid extraction and membrane solvent extraction

Abstract: Extraction can successfully be used for in-situ alcohol recovery in butanol fermentations to increase the substrate conversion. An advantage of extraction over other recovery methods may be the high capacity of the solvent and the high selectivity of the alcohol/water separation. Extraction, however, is a comprehensive operation, and the design of an extraction apparatus can be complex. The aim of this study is to assess the practical applicability of liquid-liquid extraction and membrane solvent extraction in butanol fermentations. In this view various aspects of extraction processes were investigated. Thirty-six chemicals were tested for the distribution coefficient for butanol, the selectivity of alcohol/water separation and the toxicity towards Clostridia. Convenient extractants were found in the group of esters with high molar mass. Liquid-liquid extraction was carried out in a stirred fermenter and a spray column. The formation of emulsions and the fouling of the solvent in a fermentation broth causes problems with the operation of this type of equipment. With membrane solvent extraction, in which the solvent is separated from the broth by a membrane, a dispersion-free extraction is possible, leading to an easy operation of the equipment. In this case the mass transfer in the membrane becomes important. With membrane solvent extraction the development of a process is emphasized in which the extraction characteristics of the solvent are combined with the property of silicone rubber membranes to separate butanol from water. In the case of apolar solvents with a high molar mass, the characteristics of the membrane process are determined completely by the solvent. In the case of polar solvents (e.g. ethylene glycol), the permselectivity of the membrane can profitably be used. This concept leads to a novel type of extraction process in which alcohol is extracted with a water-soluble solvent via a hydrophobic semipermeable membrane. This extraction process has been investigated for the recovery of butanol and ethanol from water. A major drawback in all processes with membrane solvent extraction was the permeation of part of the solvent to the aqueous phase. The extraction processes were coupled to batch, fed batch and continuous butanol fermentations to affirm the applicability of the recovery techniques in the actual process. In the batch and fed batch fermentations a three-fold increase in the substrate consumption could be achieved, in the continuous fermentation about 30% increase.

Annealed and Quenched Polyelectrolytes

Abstract: We study theoretically the behaviour of two types of polyelectrolytes in a poor solvent. If the number of charges and their position along the chain is quenched, the collapse is well described by a blob model introduced by Khokhlov. Weak polyacids or polybases have however an annealed structure where the charges are "mobile" along the chain. We show that these polymers undergo a discontinuous charge instability between a small-charge globular state and a high-charge extended state. This could explain several titration and viscometric data. At higher concentration, the electrostatic interaction is more screened for annealed than for quenched polyelectrolytes.

Mechanisms of reactions of iron(III) porphyrins with hydrogen peroxide and hydroperoxides: solvent and solvent isotope effects

Abstract: The effect of solvent polarity and acidity and the solvent isotope effects on the reaction of iron(III) porphyrins with hydrogen peroxide, tert-butyl hydroperoxide, and peracids have been investigated. Both the solvent isotope effects on the reactions of hydroperoxides with hemins are almost identical with these effects on the reactions of hydroperoxides with dialkyl sulfides and the effects on the reaction of peracids with iron(III) porphyrins. Since both of the latter reactions are known to involve heterolytic cleavage of the O-O bond, this similarity provides strong evidence for heterolytic cleavage of hydroperoxide by reaction with hemins. The strong alcohol catalysis of the studied reactions provides a mechanistic rationale for the O-O bond cleavage in cytochrome P-450 in terms of general-acid catalysis by water in the enzyme site.

Polymer recycling by selective dissolution

Abstract: A method for separating polymers from a physically commingled solid mixture containing a plurality of polymers comprises dissolving a first one of the polymers in a solvent at a first lower temperature to form a first preferably single phase solution and a remaining solid component. The solid component contains additional polymers which are not soluable to the solvent at the first temperature but which may be soluble at higher temperatures. The method includes subsequently heating the solvent to dissolve additional polymer from the solid component to form subsequent solutions. The polymers are then separated from their respective solution either using flash evaporation techniques when more than one polymer has been dissolved at a single temperature, or conventional techniques for extracting a polymer from a solvent in a solution.

Photodynamics of the S1 state of some hydroxy- and amino-substituted naphthoquinones and anthraquinones

Abstract: Photodynamics of the S1 state of the 1,4- and 1,8-disubstituted hydroxyquinones and aminoquinones have been studied in different organic solvents by absorption and fluorescence spectroscopy. The internal conversion process is the major deactivation path for the S1 state. The solvent and temperature dependence and deuterium isotope effect on the fluorescence dynamics suggest that hydrogen stretching vibrations in different intra- and inter-molecular hydrogen bonds are responsible for very fast non-radiative decay processes. Among the intra-molecular hydrogen-bonded molecules studied only the S1 state of 1,8-dihydroxy-9,10-anthraquinone showed the evidence of excited-state intramolecular proton-transfer process. Preliminary observations on the interaction of the S1 state of these quinones with benzene and other aromatic hydrocarbon solvents via the formation of an exciplex have also been reported.

On the n-pi blue shift accompanying solvation

Abstract: It is well-known that compounds containing lone pairs undergo a blue shift (higher energy) in their n-π* electronic transition upon solvation. The reason generally put forth to explain such a blue shift is that the lone pair, because of its spacial extent, interacts with the solvent, either weakly, as in aprotic solvents, or strongly through the direct formation of hydrogen bonds, as with water or alcohols. It is the purpose of this communication to suggest that this explanation is at best incomplete and, in many cases, incorrect

Chromium compounds and uses thereof

Abstract: Novel chromlum-containing compounds, such as, for example, chromium pyrrolides, are prepared by forming a mixture of a chromium salt, a metal amide, and an electron pair donor solvent, such as, for example, an ether. These novel chromium-containing, or chromium pyrrolide, compounds can be used either unsupported or supported on an inorganic oxide support to trimerize and/or polymerize olefins.

Delivery system for agricultural chemicals

Abstract: An emulsifiable concentrate comprising an agriculturally active chemical, a surfactant, an organic diluent and a solvent having first and second components, the first component being capable of solubilizing the agriculturally active chemical and the second component in conjunction with the surfactant being effective to disperse the agriculturally active chemical. The inventive concentrate allows for high concentrations of the active ingredient, exhibits excellent stability and produces highly stable compositions upon dilution with water.