Mixed Solvent Media Research Articles

Electrical Conductances of Tetrabutylammonium Bromide, Sodium Tetraphenylborate, and Sodium Bromide in Acetonitrile (1) + Water (2) Mixtures at (308.15, 313.15, and 318.15)K

The electrical conductances of solutions of tetrabutylammonium bromide (Bu4NBr), sodium tetraphenylborate (NaBPh4), and sodium bromide (NaBr) in acetonitrile (1) + water (2) mixed-solvent media containing 0.20 and 0.40 volume fractions of acetonitrile have been reported at (308.15, 313.15, and 318.15) K. The conductance data have been analyzed by the 1978 Fuoss conductance−concentration equation in terms of the limiting molar conductance (Λ0), the association constant (KA), and the association diameter (R). The ionic contributions to the limiting molar conductance have been estimated using tetrabutylammonium tetraphenylborate (Bu4NBPh4) as the “reference electrolyte”. These three electrolytes are found to exist as free ions in both solvent mixtures in the temperature range of (308.15 to 318.15) K. Besides the relative permittivity and the viscosity of the media, specific interactions of the ions with the solvent media have been found to exhibit a profound influence on their mobilities. Although the association constants of the electrolytes do not vary significantly as a function of temperature, the limiting equivalent conductances of the electrolytes as well as the single-ion conductivity values increase appreciably with temperature.

Controlled grafting of well-defined epoxide polymers on hydrogen-terminated silicon substrates by surface-initiated ATRP at ambient temperature.

Controlled grafting of well-defined epoxide polymer brushes on the hydrogen-terminated Si(100) substrates (Si-H substrates) was carried out via the surface-initiated atom-transfer radical polymerization (ATRP) at room temperature. Thus, glycidyl methacrylate (GMA) polymer brushes were prepared by ATRP from the alpha-bromoester functionalized Si-H surface. Kinetic studies revealed a linear increase in GMA polymer (PGMA) film thickness with reaction time, indicating that chain growth from the surface was a controlled "living" process. The graft polymerization proceeded more rapidly in the dimethylformamide/water (DMF/H(2)O) mixed solvent medium than in DMF, leading to much thicker PGMA growth on the silicon surface in the former medium. The chemical composition of the GMA graft-polymerized silicon (Si-g-PGMA) surfaces were characterized by X-ray photoelectron spectroscopy (XPS). The fact that the epoxide functional groups of the grafted PGMA were preserved quantitatively was revealed in the reaction with ethylenediamine. The "living" character of the PGMA chain end was further ascertained by the subsequent growth of a poly(pentafluorostyrene) (PFS) block from the Si-g-PGMA surface, using the PGMA brushes as the macroinitiators.

Estimation of micellization parameters of sodium dodecyl sulfate in water+1-butanol using the mixed electrolyte model for molar conductance

The mixed electrolyte model of Shanks and Franses has been applied to estimate the critical micelle concentration, aggregation number, and counterion binding constant of sodium dodecyl sulfate in a water + 1-butanol medium from its measured conductivity data at 25 °C. The surface potential of the ionic micelle in this mixed solvent medium was computed by solving the nonlinear Poisson–Boltzmann equation. The standard free energy terms of micellization were also calculated. The present study confirms further the observation made in the previous studies that ionic micelles do not contribute to the ionic strength of a surfactant solution, an inference originally made by McBain and coworkers.

Insights into the alkaline transformation of ferricytochrome c from (1)H NMR studies in 30% acetonitrile-water.

Recently, we found that ferricytochrome c (ferricyt c) undergoes significant structural changes in mixed aqueous-nonaqueous media, resulting in the formation of a mixture of alkaline-like species. The equilibrium composition of this mixture of species is dependent on the dielectric constant of the mixed solvent medium. One-dimensional (1D) and two-dimensional (2D) (1)H nuclear magnetic resonance (NMR) methods have now been used to study these alkaline-like forms in 30% acetonitrile-water solution. A native-like (M80-ligated) III* form, two lysine-ligated forms (IVa* and IVb*), and a hydroxide-ligated form (V*) were observed. Heme proton resonance assignments for these forms were accomplished using 1D (1)H NMR and 2D nuclear Overhauser effect spectroscopy methods at 20 degrees C and 35 degrees C. The chemical exchange between the alkaline forms in 30% acetonitrile solution facilitated heme proton resonance assignments. Based on examination of the heme proton chemical shifts and several highly conserved amino acid residues, the electronic structure, secondary structure, and hydrogen bond network in the vicinity of the heme in the III* form were found to be intact. Similarly, the heme electronic structure of the IVa* form was found to be comparable to that of the IVa form. Differences in the order of the heme methyl resonances in the IVb* form, however, suggest that the heme active site in this form is somewhat different from that observed in aqueous alkaline solution. In addition, resonance assignments for the 8- and 3-methyl heme protons were made for the hydroxide-ligated V* form for the first time. The observation of chemical exchange peaks between all species except IVb* and IVa* or V* was used to propose an exchange pathway between the different forms of ferricyt c in 30% acetonitrile solution. This pathway may be biologically significant because ferricyt c, which resides in the intermembrane space of mitochondria, is exposed to medium of relatively low dielectric constant when it interacts with the mitochondrial membrane.

Dynamic nuclear polarization in suspensions of asphaltene obtained from MC-30 liquid asphalt

Nuclear magnetic resonance and the Overhauser effect type dynamic nuclear polarization were used to study some colloidal suspensions of asphaltene obtained from MC-30 liquid asphalt in a weak magnetic field of 1.437mT. Intermolecular spin–spin interactions occur between diffusing nuclear spins in the solvent medium and the unpaired electron spin in the asphaltene micelles. The samples were prepared in ten pure and six mixed solvent media. The Electron Paramagnetic Resonance spectrum of the asphaltene was obtained and the saturation experiments were applied to the samples. The dipolar interaction is predominant for all suspensions. Solvent molecules can diffuse freely in and out of the asphaltene micelles making dipolar interaction. Some solvent molecules, in the same sample, can attach to the asphaltene particles for an extremely short time, forming complexes and making scalar interaction. The variation of the nuclear-electron coupling parameter with the low frequency relaxation component for each pure solvent medium and the solvent effects of CCl4 and CS2 which do not contain proton (1H nucleus) were investigated using mixed solvent media. The results are compared with some other studies.

Mixed ligand complexes : stabilities and reactivities

The investigations of stabilities and reactivities of coordination complexes have been actively pursued at several research centres in this country. A brief review is presented here in this area with emphasis on our own work. The ligand substitution dynamics at several metal centres both in aqueous and mixed solvent media, isomerisation reactions and the reactions of coordinated ligands form the general theme of this review.

Cytochrome c Structure and Redox Function in Mixed Solvents Are Determined by the Dielectric Constant

Cyclic voltammetry (CV), UV−visible (UV−vis), circular dichroism (CD), and resonance Raman (λex = 406.7, 413.1 nm) spectroscopy have been used to probe the structure and redox function of horse cytochrome c (cyt c) in aqueous mixtures of three water-miscible organic solvents, specifically, acetonitrile (ACN), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) containing 100−60% water. As the concentration of the organic solvent is increased, significant changes are observed in the spectroscopy of ferricytochrome c (ferricyt c) with the greatest structural changes observed for ferricyt c in mixed solvent media with the lowest dielectric constant (30% ACN in this work). In the UV−visible spectrum, the Soret band blue shifts (1 nm) and the intensity of the 695 nm band decreases. UV CD (185−240 nm) suggest that changes in the protein secondary structure on going from aqueous to nonaqueous media are relatively small and that the protein structure remains largely intact in nonaqueous media. In the visible C...

Effects of solvent on the reactions of coordination complexes. part 24. kinetics of base hydrolysis of some (aminomonocarboxylato)(tetraethylene-pentamine)cobalt(III) complexes in methanol+water media: The role of substrate hydrophobicity and solvent structure

The kinetics of base hydrolysis of some (aminomonocarboxylato)(tetraethylenepentamine)cobalt(III) complexes, [(tetren)CoO2CR]2+ (R=NH2CH2, pyridine-2, NH2CH2CH2, NH2CH(CH3) (αβS isomer); R=NH2CH(CH3) (αβR isomer)), have been investigated in methanol–water media (0–80 vol % MeOH) at 15.0≤t°C≤40.0 (0.02 mol dm−3 NaOH). The second-order rate constant at zero ionic strength, k2°, increases nonlinearly with XMeOH. The transfer free energy of the initial state and the transition state of the amido conjugate base ([ΔtG (i)](sw)) for the glycinato- and pyridine-2–carboxylato complexes have been calculated using the solubility data of their picrate salts, pKNH date of their N-protonated forms, and the k2° values in mixed solvent media. The kinetic solvent effects have been interpreted in terms of preferential solvation of the initial state, transition state, and the solvent structure. The activation enthalpies and entropies varied nonlinearly with XMeOH displaying extrema, which is attributable to the solvent structural effects on these thermodynamic parameters. It is also evident that the mutation process, αβRαβS isomer for the α-alaninato complex, where this isomerisation refers to the arrangement of the tetren skeleton around the planar secondary NH is sensitive to the nature of the cosolvent molecules and solvent structure. The mutation process is generally more favorable for the five coordinate amido conjugate bases than the initial state. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 55–64, 1999

NMR, Voltammetric, and Photoelectrochemical Studies on the Dark and Light-Catalyzed Reactions of α-[S2Mo18O62]4- with Aryl- and Alkylphosphines

The polyoxometalate, α-[S2Mo18O62]4- is reduced by phosphines in the mixed-solvent medium CH3CN/H2O (95/5 v/v). Monitoring of reaction pathways by voltammetric and NMR (17O, 31P) techniques shows that the identity of the final products is determined by the basicity of the phosphine. Thus reaction with aryl- (Ph3P, Ph2PCH2PPh2, and Ph2PCH2CH2PPh2) and alkyl- (Et3P and nBu3P) phosphines leads to two- and one-electron reduced polyoxometalate products, respectively, for example: [S2Mo18O62]4- + Ph3P + H2O → [HS2Mo18O62]5- + Ph3PO + H+; 2[S2Mo18O62]4- + 3nBu3P + H2O → 2[S2Mo18O62]5- + nBu3PO + 2nBu3PH+. For each reaction, the primary electron-transfer step is believed to be [S2Mo18O62]4- + R3P → [S2Mo18O62]5- + R3P•+with R3P•+ then reacting with water to generate R3PO and protons. If R3P is relatively basic (R = nBu, Et), a 1:2 mixture of R3PO and R3PH+ is formed due to the protonation of R3P. However, if R3P (R = Ph) is a weak base, H+ preferentially initiates the following disproportionation reaction: 2[S2...

Medium effects on the reactions of coordination complexes: Base hydrolysisof(αβS)(p-hydroxybenzoato)(tetraethylenepentamine) cobalt(III) in iso-propanol–water, tert-butanol–water and dimethyl sulfoxide–water media

The base hydrolysis of (αβS)(p-hydroxybenzoato)-(tetraethylenepentamine)cobalt(III) has been investigated in aqueous–organic solvent media using i-PrOH, t-BuOH and dimethyl sulfoxide (DMSO) as cosolvents at 20.0 ≤ T (°C) ≤ 40.0 (I=0.02 mol dm -3) with 80% (v/v) of cosolvents. Only the base-catalysed path (kobs=kOH[OH-]) is observed. The relative second order rate constant kOHos/kOHow at I=0 increases nonlinearly with increasing mol fraction (xO.S.) of the cosolvents, the rate acceleration in alcoholic cosolvents being greater than in DMSO. The destabilization of -OH in mixed solvent media alone does not explain the observed rate acceleration. The solvent composition dependence, log kOHos = log kOHow + ∑aixosi[i=1,2,kOH0 denotes kOH at I=0 in mixed solvent(s) and water (w)] indicates specific solute–solvent interactions. The values of the relative transfer free-energy data [ΔTG(t.s.) - ΔTGo (i.s.)](s←w)(25 °C)(G′), where t.s. and i.s. denote the transition state and initial state of the substrates respectively, are positive for all substrates at all compositions, indicating a greater destabilizing effect of the mixed solvent on the transition state than on the initial state. The G′ values also correlate with GE(G′ = axO.S. + cGE) for all solvents, supporting the fact that solvent structural effects mediate the rates and energetics of the reaction. However, the solvent effects on the solvation components of ΔH‡ and ΔS‡ are mutually compensating, thus indicating that there is no change in the mechanism.

Conductance of selected alkali metal salts in aqueous binary mixtures of 2-methoxyethanol at 25°C

Precise conductance measurements have been performed for lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, sodium perchlorate, and sodium tetraphenylborate in 2-methoxyethanol–water mixtures at four different mole fractions, i.e., 0.056, 0.136, 0.262, and 0.486 of 2-methoxyethanol (69.73 ≥ D ≥26.55) at 25°C in the concentration range 0.0004–0.0642 mol-dm−3. The limiting molar conductivity Λ°, the association constant KA, and the association distance R for the solvent mixtures have been evaluated from the conductance concentration data using the 1978 Fuoss conductance equation. The single-ion conductances have been estimated using the “reference electrolyte” tetrabutylam-monium tetraphynylborate(Bu4NBPh4). The analysis of the data indicates that for most salts ion association is appreciable in the solvent mixtures with a mole fraction of the cosolvent of 0.262 or higher. The results have been interpreted in terms of ion-solvent interactions and structural changes in the mixed solvent media.

Effect of mixed solvent media on the sorption and separation of uranium and plutonium on macroporous resins

Ion-exchange studies on uranium and plutonium using macroporous (MP) anion-exchange resins from an aqueous-organic solvent mixed media were carried out to develop a separation method. Out of the several water miscible organic solvents tried methanol and acetone were found to be best suited. Distribution data were obtained for U(VI) and Pu(IV) using three macroporous resins under various parameters. Based on these data, separation factors for Pu/U were calculated. Column experiments using Tulsion A-27(MP) were also carried out. The method has the advantage of loading plutonium from as low as 1M nitric acid in the presence of methanol or acetone and could be used satisfactorily for its recovery from solutions containing plutonium and uranium.

Stereoselectivities and reaction rates in Diels-Alder reactions promoted by non-aqueous solvents and their aqueous mixtures: Correlations with non-adjustable parameters

Stereoselectivities and rates of Diels-Alder reactions in different solvents were correlated with a parameter, Q, defined based on internal pressure, which is normalized with respect to a polarity scale. The polarity scale or index, although purely empirical in nature, can be calculated from the refractive index of the solvent. These correlations were linear for both stereoselectivities and reaction rates. Using expressions derived in this study, it is possible to represent the kinetic profiles of several Diels-Alder reactions in about 75 pure and mixed solvent media. The activation volume of a reaction can be directly estimated from the regression of data on rates and Q in various solvents using the proposed correlations.

Production of no-carrier-added 199Au for gold cluster-labelled antibodies

The production of no-carrier-added (NCA) 199Au by neutron activation of 198Pt was developed and the radiochemical separation of 199Au from Pt targets was investigated. Three different procedures, based on anion exchange in mixed solvent media, solvent extraction and extraction chromatography, are reported. The extraction chromatography method was superior in terms of yield (>95%), Pt separation factor (⩽10 −5), and separation time (≈ 1 hr). This scheme has been used to successfully process large amounts (>30 GBq) of 199Au, with an average specific activity of 7.59 GBq/μg (205 mCi/μg).

Oxygenation reactions of saturated and unsaturated substrates by molecular O 2 catalyzed by the versatile catalyst K[Ru(EDTAH)Cl]·2H 2O: Rate and activation studies

Oxygenation of olefinic and saturated substrates, including triphenylphosphine, with molecular oxygen is catalyzed by (ethylene-diaminetetraacetato)aquoruthenate(III) ion, [Ru III(EDTA)(H 2O)] − 1, in a mixed solvent medium to yield a variety of products. The wide range of products formed includes oxides, epoxides, alcohols, ketones and aldehydes. The kinetics of these oxidation reactions have been investigated by oxygen absorption methods and product analysis at pH 3.0 (μ = 0.1 M NaClO 4) in the temperature range 288–318 K in water—dioxane medium (1:1). In order to eliminate any radical chemical pathway with dioxane:H 2O, the oxygenation of a few selected substrates, cyclohexane and 1-hexane was conducted in water—DMF (1:1) medium. General mechanisms are proposed for both saturated and unsaturated substrates on the basis of the kinetic data and are discussed in terms of reactivity. In all cases, a μ-peroxoruthenium(IV)—substrate complex was suggested as the active intermediate. The activation parameters corresponding to rate constants at different temperatures were computed. The activation enthalpies are much more favourable for the oxidation of PPh 3 and olefins, as compared to saturated substrates.

Candida cylindracea Lipase-Catalyzed Hydrolysis of Methyl Palmitate in Detergentless Microemulsion and Paraffin/Water Biphasic Media

The activity of lipase from Candida cylindracea has been investigated in the detergentless microemulsion media of n-hexane/propan-2-ol/water and in paraffin/water biphasic mixtures. Analysis of the product of hydrolysis, palmitic acid, was by a titrimetric method developed for non-specific assay of free fatty acids in these mixed-solvent media. The enzyme-catalysed hydrolysis of methyl palmitate was very slow in a complete non-aqueous system, but addition of small amounts of water led to an increased rate of reaction at atmospheric pressure and 37°C. The ability of the enzyme to work in these mixed-solvent systems under very favourable experimental conditions makes it particularly amenable to practical and cost-effective transformations.

Ion exchange of uranium(IV) in hydrochloric acid—Organic solvent mixed media

In this paper, the distribution of U(IV) between a solution and a strong basic anion exchange resin 201×7 in mixed media HCl-methanol, HCl-ethanol, HCl-n-propanol and HCl-acetone has been investigated. The distribution ratio of U(IV) D in various conditions has been determined, the species of U(IV) sorbed on resin has been studied by visible and infrared spectroscopy and in mixed media by visible spectroscopy. The results show that the distribution ratio of U(IV) obviously increases in the presence of organic solvents, but the species of U(IV) sorbed on the resin are not influenced.

Epoxidation of cyclohexene, methylcyclohexene and cis-cyclooctene by molecular oxygen using ruthenium(III) aquo ion as catalyst: a kinetic study

Kinetics of the epoxidation of cyclohexene, methylcyclohexene and cis-cyclooctene by molecular oxygen catalysed by Ru(III) ion, [RuCl 2H( 2O) 4] + at pH 2.0 in a mixed water-dioxane solvent medium are reported. The reactions were studied manometrically as a function of catalyst, substrate and dioxygen concentrations. For all the substrates studied, the rate of oxidation is first order with respect to catalyst and substrate concentrations and one-half order with respect to dioxygen concentration. The oxidations yielded epoxide as the only product in all cases; these were identified and analysed by GLC technique. Based on the kinetics, a mechanism involving homolytic cleavage of the O-O (peroxo) bond with concerted transfer of the oxygen atom to the substrate was proposed. The presence of methyl group on the cyclohexene ring (reaction 2) and an increase in the number of carbon atoms in the ring for cyclooctene (reaction 3) cause a decrease in the rates of epoxidation. The rate of epoxide formation and the corresponding yields for reactions ( 1), ( 2) and ( 3) decreases in the order: cyclohexene > methylcyclohexene > cyclooctene.

Effect of solvent on the reactions of coordination complexes. Part 10. kinetics of solvolysis of cis(chloro)(1‐amino‐propan‐2‐ol)bis‐(ethylenediamine)cobalt(III) in methanol + water, propan‐2‐ol + water, and t‐butanol + water media

AbstractThe solvolysis of cis(chloro)(1‐amino‐propan‐2‐ol)bis(ethylenediamine)cobalt(III) in aqueous alcoholic media using methanol, propan‐2‐ol, t‐butanol as cosolvents, resulted in the formation of the (N,O) chelated productcis[Co(en)2(NH2CH2CHOHCH3)]3+. The pseudo first order rate constant decreased with increasing molfraction (Xorg) of alcohols, the decrease being less marked as the bulkiness of hydrophobic moiety of alcohol increased. The plots of log 𝓀obs vs. reciprocal of the bulk dielectric constant of the solvents, log 𝓀obs vs. Grunwald‐Winstein solvent parameter and log 𝓀obs, vs. Xobs under isodielectric condition (Ds = 50, at 50°C) for CH3OH/H2O, C2H5OH/H2O, (CH3)2CHOH/H2O, (CH3)3COH/H2O, (CH2)2(OH)2/H2O, and (CH3)2 C  O/H2O in water rich media indicated that both solvent structural effects and presumably the hydrophobic interaction appreciably mediate the reaction. The calculated values of the relative transfer free energy at 25°C[ΔGt(C3−)‐ΔGt(i.s)(s←w)] where C3+ and i.s. denote the dissociative transition state {cis[Co(en)2(NH2CH2CHOHCH3)]3+}* and the initial state, respectively, indicated that the tripositive transition state is more effectively solvated by the mixed solvent media, than the dipositive initial state, the effect appeared to be more significant with increasing Xorg. The plots of activation enthalpy and entropy against Xorg exhibited maxima and minima indicating that enthalpy and entropy changes associated with the solvent shell reorganization of the reactant contribute to the overall activation parameters.

Anionic penta-coordinated complexes of iron(II) containing pyridine- N-oxide, γ-picoline- N-oxide, ascorbic acid and pseudohalides

A series of novel complexes of the type M 2 [FeX 4L] where M +=K + or Na +; X=cyanate, thiocyanate or azide; L=pyridine- N-oxide, γ-picoline- N-oxide or L- ascorbic acid; have been synthesized by the reaction of the metal salt and ligands in the appropriate ratio in mixed solvent (water-ethanol) medium. The complexes were characterized on the basis of their analyses, molar conductance, magnetic susceptibility, infrared, electronic spectral data and molecular weight measurements. The compounds are found to be 1:2 electrolytes and penta-coordinated monomers with presumably square pyramidal geometry.