Showing papers on "Solvent published in 1993"

Solubility of fullerene (C60) in a variety of solvents

Abstract: The room temperature solubility of pure C[sub 60] has been determined in 47 solvents. The solubilities cover a wide range, from 0.01 mg/mL in methanol to 50 mg/mL in 1-chloronaphthalene. The solubilities in CS[sub 2], toluene, and hexane, three of the commonly employed solvents, are 7.9, 2.8, and 0.04 mg/mL, respectively. An examination of the solubilities of C[sub 60] as a function of the solvent properties such as index of refraction, dielectric constant, molecular size, Hildebrand solubility parameter, and H-bonding strength reaffirms the century-old principle like dissolves like. No single solvent parameter can uniformly predict the solubility of C[sub 60], but a composite picture of solvents with high solubility for C[sub 60] emerges: large index of refraction, dielectric constant around 4, large molecular volume, Hildebrand solubility parameter equal to 10 cal[sup 1/2] cm[sup [minus]3/2], and tendency to act as a moderate strength nucleophile. 18 refs., 4 figs., 1 tab.

Frozen density functional approach for ab initio calculations of solvated molecules

Abstract: A new density functional method for treatment of chemical processes in solution is presented. The method is based on freezing the electron density of the solvent molecules, while solving the eigenvalue problem for the solute Hamiltonian, which includes the effective potential of the solvent molecules. The method is developed and examined in the simple case of one solvent and one solute molecule. The results are encouraging and the deviation between the unfrozen and frozen treatments can be attributed to the kinetic energy functional used. The method can be implemented in ab initio calculations of solvation free energies, following a recent pseudopotential approach [Vaidehi et al., 19921.

Structure of grafted polymeric brushes in solvents of varying quality: a molecular dynamics study

Abstract: A molecular dynamics simulation of polymeric brushes in solvents of varying quality is presented. The resulting equilibrium structures are compared with those predicted by self-consistent field (SCF) analysis. The scaling of the brush height is found to agree with the SCF predictions at temperatures T>T θ , where T θ is the θ temperature of the solvent. The agreement at T

Dynamic Solvent Effects on Electron-Transfer Reactions

Abstract: Electron-transfer processes in solution are among the most important reactions in chemistry and biology. The huge number of redox reactions of transition metal ions and complexes, many preparatively important oxidations and reductions of organic compounds, photosynthesis, and metabolism are only a few examples where electron-transfer reactions play a pivotal role. This ubiquity, as well as their relative simplicity, makes them excellent models for the study on a molecular level of chemical reactions in solution. A particularly important question in chemical reaction dynamics in solution is the influence of the solvent on the reaction rate. In this context one distinguishes between static and dynamic solvent effects. Static effects refer to the stabilization of reactants, transition state, and products, that is, how the solvent affects the free energies of these species and the energy of activation. This interpretation of solvent effects on all kinds of chemical reactions is well established. A more recent development is the investigation of the influence of solvent dynamics on the rate of a reaction. The transfer of an electron is usually thought to be triggered by a fluctuation of the dielectric polarization in the surrounding solvent. The dynamics of such fluctuations is determined by the finite response time of the orientational polarization of the solvent. Under certain conditions this dielectric response time can become the rate-determining factor of the reaction. In this article I intend to give a review of these modern developments in the theory and experimental study of electron-transfer processes. We shall see that solvent dynamics may lead to a whole plethora of phenomena in reaction dynamics. The concepts needed for their description are not limited to electron transfer but bear relevance to many other chemical reactions in solution.

Three-state analysis of sperm whale apomyoglobin folding.

Abstract: We give a quantitative description of the urea- and acid-induced transitions of apomyoglobin at 0 OC and 2 mM sodium citrate. Our data consist of two series of unfolding curves: (1) acid-induced unfolding carried out in the presence of various concentrations of urea and (2) urea-induced unfolding at various pH values. A three-state equation is derived which relates the stability of three different conformations of apomyoglobin (native (N), unfolded (U), and intermediate (I)) as a function of urea and of pH. This equation fits our data reasonably well. The parameters which give the best fit have both thermodynamic and structural implications for N, I, and U. Specifically, I is closer in Gibbs energy to U than to N, indicating that side-chain packing results in much of the stability of native protein structure. The equilibria between N and I and between I and U are equally sensitive to urea, suggesting that much of the surface of I is inaccessible to solvent. The acid-induced transition in which N unfolds can be described as the result of titration of approximately two histidines with low pK,s in N. Under physiological conditions (neutral pH, no urea) I is the most stable non-native conformation.

Toxicity of homologous series of organic solvents for the gram-positive bacteria Arthrobacter and Nocardia Sp. and the gram-negative bacteria Acinetobacter and Pseudomonas Sp.

Abstract: The toxicity of homologous series of organic solvents has been investigated for the gram-positive bacteria, Arthrobacter sp. and Nocardia sp., and the gram-negative bacteria, Acinetobacter sp. and Pseudomonas sp. The hydrophobicity of the solvent, expressed by its logP(octanol), proves to be a good measure for the toxicity of solvents in a two-phase system. The transition from toxic to nontoxic solvents occurs between logP(octanol) 3 and 5 and depends on the homologous series. No correlation has been found between the hydrophobicity of the substituent on the alkyl backbone of the solvent and the location of the transition point in toxicity. The logP(octanol), above which all solvents are nontoxic, is used to express the solvent tolerance of the bacteria. In general, the solvent tolerance of gram-negative bacteria is found to be slightly higher than that of gram-positive bacteria, but this does not hold for all homologous series of organic solvents investigated.Because the toxicity effects of organic solvents in a two-phase system can be ascribed to molecular as well as phase toxicity effects, molecular toxicity effects were investigated separately in a one-phase system with subsaturating amounts of organic solvent. The solvent concentration in the aqueous phase, at which 50% of the metabolic activity of the bacteria is lost, is used to express solvent toxicity. This concentration is found to be similar for the gram-positive Arthrobacter and the gram-negative Acinetobacter. Assuming the critical membrane concentration theory (G. J. Osborne et al. Enzyme Microb. Technol. 1990, 12: 281-291) to be valid, it can be concluded that differences in solvent tolerance between these two bacteria, cannot be ascribed to differences in response to molecular toxicity. Prediction of the toxicity of any solvent, using the critical membrane theory, appears to be possible in the case of alkanols or alkyl acetates. However, prediction of the toxicity of ethers appears to be impossible.

Solvent-dependent conformation and hydrogen-bonding capacity of cyclosporin A: Evidence from partition coefficients and molecular dynamics simulations

Abstract: The partition coefficient of cyclosporin A (CsA) was measured in octanol/water and heptane/water by centrifugal partition chromatography. By comparison with results from model compounds, it was deduced that the hydrogen-bonding capacity of CsA changed dramatically from an apolar solvent (where it is internally H-bonded) to polar solvents (where it exposes its H-bonding groups to the solvent). Molecular dynamics simulations in water and CCl4 support the suggestion that CsA undergoes a solvent-dependent conformational changes and that the interconversion process is slow on the molecular dynamics time scale.

The role of hydrogen bonding in carbohydrates: molecular dynamics simulations of maltose in aqueous solution

Abstract: Molecular dynamics simulations of maltose in two different conformations in vacuum and aqueous (TIP3P) solution have been used to examine the types of hydrogen bonds made by this carbohydrate molecule. Maltose was found to be extensively hydrogen bonded to solvent molecules in aqueous solution, and these hydrogen bonds were found to have potential conformational consequences. The exchange of an intramolecular O2-O3' hydrogen bond found in the crystal structure for hydrogen bonds to solvent was observed to produce significant changes in the solvation of the sugar molecule

Anomalous solubility behaviour of C60

Abstract: SOLUBILITIES of solids in liquids exhibit a temperature dependence that is correlated with the energetics of dissolution. For organic solutes in organic solvents the common experience is that solubility increases on warming (that is, dissolution is normally endothermic), forming the basis for purification by crystallization by slow cooling of hot solutions1. Interactions between inorganic compounds and water are often more energetic, and may lead to complicated temperature-dependent solubilities, often associated with the formation of hydrated solid phases2. We have investigated the temperature-dependent solubility of C60 in hexane, toluene and CS2. We observe a solubility maximum near room temperature (around 280 K) for all three solvents. Although the solubility of C60 in these three solvents differs by several orders of magnitude, the temperature dependence of the relative solubilities is much the same in each case. We conclude that dissolution is endothermic below room temperature and exothermic above. We interpret this change as being due to a phase change in solid C60 presumably the phase change observed previously in the absence of a solvent3–5, modified by solvent wetting. A solubility maximum (or minimum) for organic compounds in non-electrolytes is highly unusual, and may be unprecedented. The effect may have consequences for solvent-extraction techniques of fullerene purification.

Relative viscosities and quasi-thermodynamics of solutions of tert-butyl alcohol in the methanol–water system: a different view of the alkyl–water interaction

Abstract: Viscosity B coefficients for tert-butyl alcohol (TBA) in water, methanol and mixtures of the two have been obtained at 25 and 35 °C. Hence solute contributions to the activation parameters for viscous flow of the solutions have been calculated, as well as the Gibbs energy, enthalpy and entropy of transfer from the ground-state solvent to the hypothetical viscous ‘transition-state solvent’. The enthalpies of mixing of the two transition-state solvents and their relative partial molar enthalpies L′1 and L′2 are also given.In water, B is large and positive and dB/dT large and negative; in methanol B is small and dB/dT near zero. The classical explanation of these observations, which is solely in terms of water–water (1,1) interactions in an ‘iceberg’ around the alkyl group, is inconsistent with the Compensation Principle because B is proportional to a Gibbs energy function, and this requires that solute–solvent (3, 1) interactions must determine B.It is proposed that in water the alkyl group occupies a ‘cage’ in which the strength of the bonding with respect to the pure solvent is of minor importance. The ‘caging’ of the alkyl group enhances its van der Waals interactions with its nearest neighbours. The expected dependence of such interactions on distance leads to energy–reaction coordinate diagrams in the form required to explain the inversion in sign of the solvation energies from ground-state solvent to transition-state solvent in highly aqueous media.These results are consistent with, though independent of, recent structural studies by neutron diffraction with isotopic substitution.

Crystallization of polymorphs : the effect of solvent

Abstract: The effect of solvent in crystallization of polymorphs has been studied using the drug sulphathiazole as a model compound. The solubilities of the four polymorphic forms of sulphathiazole were determined as a function of temperature in various solvents. Within the temperature ranges studied, the rank order of solubility of the polymorphs was the same in all solvent systems. On the basis of this knowledge of the temperature dependence of the solubilities, recrystallization experiments, in which the supersaturation was systematically varied, were carried out in an endeavour to isolate each of the polymorphic forms from each solvent system. These recrystallization experiments reveal that not all of the know polymorphic forms can be crystallized from any given solvent by varying the supersaturation. Indeed some solvents selectively favour the crystallization of a particular form of forms. The authors conclude that thermodynamic effects are not responsible for the selective behaviour of a solvent. A kinetic mechanism is proposed. It is considered that the solvent acts by selective adsorption to certain faces of some of the polymorphs, and thereby either inhibits their nucleation or retards their growth to the advantage of others.

Donor numbers of anions in solution: the use of solvatochromic Lewis acid–base indicators

Abstract: The shift of the visible d–d absorption bands of [Cu(acac)(tmen)]+(acac = acetylacetonate, tmen =N,N,N′,N′-tetramethylethylenediamine) in various solvents resulting from the co-ordination of solvent molecules and anionic species X has been investigated. The results have been used to establish donor numbers (DNX, dce) of anions dissolved in 1,2-dichloroethane (dce) on the same scale as that already established for solvents. Apparent donor numbers for anions dissolved in various solvents other than dce (DNX, solv) are also given and used to show that the donor properties of anions are directly related to the acceptor number (ANsolv) of the solvent. This arises from competition between solvent and acceptor ion for the anion leading to a steady decrease in the effective donor number of the anion as ANsolv increases, culminating in an effective donor number of zero for all anions in the strong acceptor solvent water (ion pairing is at a minimum in water). The values of the anion donor numbers are related to some experimental results such as spectral, equilibrium, kinetic and thermodynamic data for selected systems. This method of establishing donor numbers for anions is shown to be reliable and easy to apply.

Aggregation and denaturation of apomyoglobin in aqueous urea solutions.

Abstract: The effects of urea on apomyoglobin solubility have been investigated. Apomyoglobin precipitation was found to be a thermodynamically reversible process independent of the pathway of aggregation. A liquid-solid phase diagram was constructed for the precipitation of apomyoglobin as a function of urea and protein concentration. Apomyoglobin solubility decreases by an order of magnitude between 0 and 1.5 M urea, reaching a minimum near 2.4 M urea and increasing at higher urea concentrations (the denaturation midpoint is at approximately 2.6 M urea). This decrease in protein solubility is opposite to that expected based on amino acid solubilities, since both polar and nonpolar molecules become more soluble with increasing urea concentration. Solubility minima for proteins have been rationalized in terms of folding intermediates. However, our structural studies show no evidence for folding intermediates in apomyoglobin under the experimental conditions, apart from small predenaturation changes. Our data are consistent with an alternative hypothesis, namely, that the primary aggregating species are denatured protein molecules, rather than intermediate states. Consistent with recent thermodynamic and statistical mechanical models, the solubility minimum may be described as the result of two competing effects of urea: (1) urea denatures the protein, and (2) urea makes the solvent more favorable for the native and any denatured state. At low urea concentration, solubility decreases with increasing urea concentration due to the domination of the solubility behavior by the increase in the population of aggregation-competent (denatured) protein molecules. However, at high urea concentration, the increasingly favorable nature of the solvent dominates, resulting in increasing solubility with urea concentration. The phase diagram provides guidance for the best experimental conditions (pathway) to use to avoid aggregation during the refolding of denaturant-unfolded protein.

Proton and carbon-13 NMR investigations of lead zirconate titanate (Pb(Zr,Ti)O3) thin-film precursor solutions

Abstract: Solvent reactions, ligand substitutions, and the oligomer/polymer backbone structure are important factors in the solution preparation of ceramic films. In this study the authors have used H and C NMR spectroscopy to characterize solvent and ligand effects in precursor solutions used for the deposition of ferroelectric PZT (lead zirconate titanate) thin films. Solutions were prepared by a sequential precursor addition method from carboxylate and alkoxide precursors of the three cations, and the solvent, acetic acid, methanol, and water. The results indicate that acetic acid was a key component in the solution preparation process. As observed previously for single metallic component systems, its presence resulted in esterification reactions, leading in the present case to the formation of methyl, isopropyl, and n-butyl acetates. Second, acetic acid functioned as a chemical modifier, or chelating agent, replacing essentially all of the alkoxy ligands of the original precursors. Since alkoxy replacement appeared to be complete, we may describe the PZT species formed in solution as oxo acetate in nature. Finally, the solvent and ligand behavior of a solution prepared by an inverted mixing order was compared to the behavior of the solution prepared by a sequential precursor addition. The spectra for the two solutions weremore » similar, and only differences in the relative intensities of the ester and alcoholic resonances were observed. 29 refs., 5 figs., 3 tabs.« less

Lateral instabilities in a grafted layer in a poor solvent

Abstract: The effect of solvent quality on a layer of end-grafted polymers is determined using the random phase approximation combined with a numerical mean field analysis. For sufficiently poor solvents, the laterally homogeneous grafted layer is linearly unstable to fluctuations tangential to the grafting plane. In the unstable regime, the grafted layer forms a «dimpled» surface in which the depth and separation of the dimples depend on chain length, solvent quality, and grafting density

Polymeric electrolytic cell separator membrane

Abstract: A flexible polymeric film useful as an interelectrode separator or electrolyte member in electrolytic devices, such as rechargeable batteries, comprises a copolymer of vinylidene fluoride with 8 to 25% hexafluoropropylene. The film may be cast or formed as a self-supporting layer retaining about 20% to 70% of a high-boiling solvent or solvent mixture comprising such solvents as ethylene carbonate, propylene carbonate, dimethyl carbonate, and dibutyl phthalate. The film may be used in such form or after leaching of the retained solvent with a film-inert low-boiling solvent to provide a separator member into which a solution of electrolytic salt is subsequently imbibed to displace retained solvent or replace solvent previously leached from the polymeric matrix.

Influence of chemical modifiers on the solubility of o- and m-hydroxybenzoic acid in supercritical carbon dioxide

Abstract: The equilibrium solubilities of o- and m-hydroxybenzoic acid in 3.5 mol% acetone-CO 2 and 3.5 mol% methanol-CO 2 binary mixtures were measured at temperatures between 318 and 328 K and pressures in the range of 90-200 bar using a continuous flow apparatus. The solubility isotherms in these ternary systems exhibit a retrograde vaporization phenomenon and a common upper crossover pressure for each ternary system. Moreover, the results demonstrate that the addition of small amounts of cosolvents to CO 2 increase significantly the solubility of o- and m-hydroxybenzoic acid. A molecular thermodynamic model was developed based on the Scatchard-Hildebrand regular solution and Flory-Huggins theory using the concept of component solubility parameters to obtain the unlike-pair molecular interactions between the solute and cosolvent. An effective volume fraction term that reflects the preferential solvation of the solute by the cosolvent molecules was included in the model development to account for differences between the bulk solvent environment and that in the immediate vicinity of the solute molecule

Quantitative determination of monoglycerides and diglycerides by high-performance liquid chromatography and evaporative light-scattering detection

Abstract: Neutral lipid classes were separated with normal-phase high-performance liquid chromatography, and mono- and diglycerides were determined with an evaporative light-scattering detector (ELSD). The 1,3-diacylglycerols were resolved from the 1,2-diacylglycerol positional isomers, although some 1,3-diacylglycerols of low molecular weight interfered with the 1,2-diacylglycerols of high molecular weight. For monoglycerides, the separations between 1-(and 3-)acyl and 2-acylglycerols were optimized only between those pairs with identical fatty acyl groups. Samples were dissolved in a solvent mixture and analyzed without derivatization. The results (monoglyceride) obtained from this method agreed well with those derived from gas chromatographic and supercritical fluid chromatographic methods. The universal nature of the ELSD makes this method applicable to oils and emulsifiers containing both saturated and unsaturated fatty acyl moieties.

Dependence of the Solvent Diffusion Coefficient on Concentration in Polymer Solutions

Abstract: Self-diffusion coefficients of several different solvents in polystyrene as well as methyl methacrylate in poly(methyl methacrylate) have been determined at polymer concentrations from 0 to 50 wt% at 25 o C. The solvents used for the polystyrene solutions were toluene, ethylbenzene, cumene, tert-butyl acetate, chloroform, and methyl ethyl ketone. The diffusion data were used to evaluate several theories for the concentration dependence of the solvent diffusion coefficients, and in most cases the reduced diffusion coefficients, of solvents in polymer solvent systems. In addition to the experimental data presented here, literature data for several systems were also used to test the models

Protein dynamics and solvation in aqueous and nonaqueous environments

Abstract: Herein we report the results of molecular dynamics simulations of a protein in an organic solution. We present a detailed analysis of the dynamics and structure of both protein and solvent. These results are compared and contrasted with corresponding results from simulations of protein in water. We find that placement of protein in an organic medium results in a dramatically increased hydrogen bond network. We propose that it is this increase in intramolecular hydrogen bond interactions that is responsible for the experimentally observed properties of proteins in organic solvents. Furthermore, on the basis of our results we propose that creation of salt bridges will significantly enhance the thermostability of enzymes in nonaqueous solvents