Application of scaling concepts to sedimentation phenomena in semidilute macromolecular solutions

Sedimentation velocity data on polystyrene in a good solvent (toluene) and in a theta solvent (cyclopentane), over a large concentration range are reported. Under good-solvent conditions the exponent β in the apparent scaling law describing the concentration dependence of the sedimentation coefficient (s ∝ c^β) in the semidiiute region is found to be concentration dependent. However, a power law fit to data for the highest molecular weight (M = 20.6 × 10⁶) in the concentration region (c < 2 kg m^?3) yields a value β = ?0.59, somewhat smaller than that (?0.54) predicted theoretically. This discrepancy and the observed curvature in logs vs. logc at higher concentrations are discussed. Under theta-solvent conditions, on the other hand, the concentration dependence of s in the semidilute regime can be represented by a simple power law, with β = ?1.0, in excellent agreement with the theoretical prediction. The crossover concentration c^*, separating the dilute and semidilute concentration regimes, was found to be well defined and located at c = 1/[η]. c^* varies with molecular weight as M^?0.73 and M^?0.50 under good-solvent and theta-solvent conditions, respectively.     

Laser flash photolysis study of the decay kinetics of carbocations of 1,2,2,4-tetramethyl-1,2-dihydroquinoline in the porous glass in the presence of methanol

The efficiency of formation and the decay kinetics of carbocations formed under the photolysis of 1,2,2,4-tetramethyl-1,2-dihydroquinoline in methanol and in a porous glass filled with methanol or dried in air or in vacuo were studied by the laser flash photolysis techniques. In MeOH, the carbocations recombine via the second-order law in the reaction with the MeO⁻ anion formed in an equimolar amount and decay via the first-order law in the reaction with the solvent with rate constants of 3·10⁸ L mol⁻¹ s^{− 1} and 1.4·10³ s⁻¹, respectively. When the solution is placed into the porous glass, no recombination of the carbocations with MeO⁻ is observed, and the reaction with the solvent is somewhat inhibited (rate constant 8·10² s⁻¹). More than tenfold inhibition of the reaction of the carbocations with methanol is observed on going to a monolayer of MeOH on the surface. The main route of carbocation decay in the porous glass dried in vacuo is the geminate recombination with the SiO groups. The corresponding kinetics is described in terms of the model of freely diffusing reactants. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2239–2243, October, 2005.     

Carbon-13 NMR measurments of polybutadiene local dynamics in dilute solution: Further evidence for non-Kramers' behavior

The local segmental dynamics of cis-polybutadiene in dilute solutions have been investigated using natural abundance ¹³C NMR spectroscopy. The time integral of the C? H orientation autocorrelation function 〈σ〉 is extracted from T₁ measurements in five solvents covering a viscosity range of two decades. The hydrodynamic Kramers' equation, which is generally assumed to be appropriate for local polymer dynamics in dilute solution, cannot describe these results. 〈σ〉 is found to depend upon the solvent viscosity η raised to the 0.33 power whether η is varied by changing the temperature or the solvent. This apparent power law behavior has been observed for other polymers in dilute solution and can be rationalized if the solvent is viewed as viscoelastic. This view is supported by experimental results which show that local polymer dynamics can occur as much as an order of magnitude faster than solvent reorientation. Trends for a mixed microstructure polybutadiene are similar to those of cis-polybutadiene. © 1994 John Wiley & Sons, Inc.     

Coordination Mode and Kinetic Behavior of the Tetracarbonatozirconate Ion

The behavior of [Zr(CO₃)₄]⁴⁻ complex in solution is studied by determining the coordination mode of the carbonato ligands and the kinetic law of the equilibrium between free and complexed carbonates. Raman polarization and dynamic ¹³C-NMR experiments show that complexed carbonates are bidentate. Magnetization-transfer experiments give the values of pseudo-first-order rate constants for the equilibrium with no net chemical transformation. The treatment of these parameters suggests a second-order rate law. As no solvent effect is noticed, an associative mechanism of ligand exchange is proposed.     

Application of a bisindocarbocyanine reagent for dispersive liquid–liquid microextraction of silver with subsequent spectrophotometric determination

A novel, simple and environmentally friendly procedure for silver determination has been developed. The method is based on ion associate formation of AgI₂⁻ and bisindocarbocyanine chloride (BDIC) reagent, followed by dispersive liquid–liquid microextraction (DLLME) of the ion associate formed and subsequent UV–Vis spectrophotometric detection. The structure of BDIC and the reaction mechanism were investigated by MS and NMR measurements and quantum chemical calculations.The optimum experimental conditions were found to be: pH 6; 0.1 mol L^{− 1} KI; 5 × 10^{− 5} mol L^{− 1} BDIC. The DLLME procedure was carried out using an auxiliary solvent for adjustment of solvent density. A 0.5 mL mixture of toluene as extraction solvent, carbon tetrachloride as auxiliary solvent and ethanol as disperser solvent was used. Ultrasonication of the organic phase was applied to decrease the value of the blank test. The absorbance of the coloured extracts obeys Beer's law in the range 0.07–2.1 mg L^{− 1} of Ag at 566 nm wavelength. The limit of detection, calculated from a blank test (n = 10) based on 3 s, is 0.03 mg L^{− 1} of Ag. The developed procedure was applied to the determination of silver in real samples such as Nano Silver and semi-conductor thin films.     

Retracted: Acid‐Catalyzed Aquation of Ni(II)‐Hydrazone Complexes: Kinetics and Solvent Effect

Complexes of the type [Ni(L)(H₂O)]Cl₂·nH₂O, where L = 2‐pyridyl‐3‐isatinbishydrazone ligands, have been synthesized and characterized on the bases of elemental analysis, molar conductance, IR, electronic spectra, and thermal analysis (TGA and DTA). Acid‐catalyzed aquation of the Ni(II) isatin‐bishydrazone complexes was followed spectrophotometrically in various water–methanol and water–acetone mixtures at temperature 298 K. Kinetic behavior of the acid aquation is a linear rate law, indicating that the acid‐catalyzed aquation of these complexes in water–methanol and water–acetone mixtures follows a rate law with k_obs = k₂[H⁺]. The effect of the mole fraction of the ganic solvent, i.e., methanol and acetone, on the acid aquation has been analyzed; the decrease in the rate constant values with increasing of the methanol or acetone ratios is attributable to the effect of the co‐organic solvent on the initial states of the acid aquation by the destabilization of the H⁺ ion.     

Kinetics of oxidation of some α-amino acids L(−) arginine,L(−) histidine,L(+) ornithine,L(−) tryptophan,L(−) threonine by sodium N-chloro-4 methyl benzene sulphonamide (chloramine-T) in alkaline medium

Kinetic studies of oxidation of L(?) arginine, L(+) ornithine, L(?) histidine, L(?) tryptophan, L(?) threonine have been carried out in alkaline medium. The reaction showed an inverse fractional order in OH^? and first-order dependence on both amino acid and chloroamine-T concentration. The effect of varying ionic strength (KCl) on the rate of oxidation is negligible. A general mechanism for the oxidation has been suggested by considering interaction between anionic species of amino acid and p-toluene-sulphochloramide. The effect of solvent and temperature have been also discussed. The fractional order obtained in OH^? is due to the fact that a fraction of overall reaction proceeds via an alternative OH^? independent path. The combined rate law in accordance to observed kinetics is derived as The rate constants predicted by the derived rate law as the concentration of OH^? ions change, are in excellent agreement with the observed rate constants, thus further justifying these rate laws and hence the proposed mechanistic schemes.     

Ion-Solvation Behavior of Pyridinium Dichromate in Water–N,N-Dimethyl Formamide Solvent Mixtures

The electrical conductances of pyridinium dichromate have been measured in N,N-dimethyl formamide–water mixtures of different compositions in the temperature range 283–313 K. The limiting molar conductance, Λ₀, association constant of the ion pair, K _A, and dissociation constant K _C have been calculated using the Shedlovsky and Kraus–Bray equations. The effective ionic radii (r _i ) of C₅H₅NH⁺ and Cr₂O₇ ^-\mathrm{Cr}_{2}\mathrm{O}_{7}^{ -} have been determined from the L_i⁰\Lambda_{i}^{0} values using Gill’s modification of Stokes’ law. The influence of the mixed solvent composition on the solvation of ions is discussed with the help of the ‘R’-factor ( R = \frachL _±⁰(solvent)hL _±⁰(water)R = \frac{\eta \Lambda_{ \pm}^{0}(\mathrm{solvent})}{\eta\Lambda_{ \pm}^{0}(\mathrm{water})}). Thermodynamic parameters are evaluated and reported. The results of this study are interpreted in terms of ion–solvent interactions and solvent properties.     

Kinetics and mechanism of anation of cis -diaquo- bis -oxalatochromate(III) ion byDL -alanine in ethanol-water mixtures of varying dielectric constant

The kinetics of the anation reaction of cis-diaquo-bis-oxalatochromate(III) ion by DL-alanine has been studied spectrophotometrically in the pH range 3.8 to 7.3, where DL-alanine remains in zwitterionic form. A second-order rate law has been established. Reaction rates in three different ethanol-water mixtures were measured. In each solvent medium the anation rate is higher as compared to water exchange reaction at a particular temperature. The activation parameters (gDH^# and ΔS^#) in different ethanol-water mixtures were obtained from Eyring plots. ΔG^#(ΔH^# −TΔS ^#) values were calculated in each solvent medium and compared with that of the isotopic water exchange process. A reaction mechanism involving theS _N2 path has been suggested.     

Adducts of Niobium (V) and Tantalum (V) Halides. XV. Ligand Exchange of the Adducts with Some Phosphoryl Compounds

The ligand exchange MX₅·L + *L?MX₅·*L + L for the octahedral adducts MX₅·L, in an inert solvent (CH₂Cl₂ or CHCl₃) with neutral ligands, proceeds via a dissociative D mechanism when M = Nb, X = Cl and L = phosphoryl compound. A dissociative interchange I_d mechanism is suggested when M = Nb or Ta, and X = F. A first order rate law and positive values for ΔS* (+4 to +14 cal K^?1 mol^?1) are observed for the exchanges on the pentachloride adducts. However, a second order rate law and large negative values for ΔS* (-15 to -24 cal K^?1 mol^?1) are found for the intermolecular neutral ligand exchange (measured by ¹H-NMR.) and for the intramolecular fluorine exchange (measured by ¹⁹F-NMR.) reactions on the pentafluoride adducts. The fluorine exchange is 2 to 5 times faster than the ligand exchange. The exchanges, on the pentachloride and on the pentafluoride adducts, are slowed down with increasing donor strength of the phosphoryl compound.     

Kinetics of the solvolysis of [Co(Rpy)4Cl4]+ with R = 4-t-Bu, 3-Me or 3-Et in mixtures of urea with water

Summary The kinetics of the solvolysis of complex ions trans-[Co(Rpy)₄Cl₂]⁺, with R = 4-t-Bu, 3-Me and 3-Et, have been investigated in mixtures formed by adding urea to water, which enhances the dielectric constant and decreases solvent structure. Differential effects of the changes in solvent structure on the initial and transition states are found to be important factors controlling changes in the rate constant with solvent composition. The variation of the enthalpy and the entropy of activation with solvent composition are contrasted with their variations found for the solvolysis of [Co(Rpy)₄Cl₂]⁺ in mixtures where solvent structure is enhanced by additions of a co-solvent to water. The application of a free energy cycle to the process of the initial state going to the transition state suggests that the Co³⁺ cation in the transition state is more stable than the Co³⁺ cation in the initial state in the water + urea mixtures.     

Oxidative Degradation of l‐Isoleucine by Au3+ Complexes in Weakly Acid Medium: A Kinetic and Mechanistic Investigation

Oxidative degradation of l ‐isoleucine (Ileu) by Au³⁺ complexes has been studied spectrophotometrically in weakly acid medium (acetic acid–sodium acetate buffer, pH range 3.72–4.80) in the temperature range 288–308 K. The reaction is first order with respect to Au^III but complex order (<1) with respect to isoleucine. Ionic strength has no significant effect on the reaction kinetics. Both H⁺ and Cl⁻ ions have been found to show inhibiting effect on the reaction rate. Decreasing solvent polarity exerts an adverse effect on the reaction. Au³⁺ complexes react with the zwitterion form of isoleucine in a one‐step two‐electron transfer redox process. The reaction passes through intermediate formation of iminic cation, which hydrolyzes to produce 2‐methyl butanal, identified by ¹H NMR. The activation parameters ΔH ^≠ and ΔS ^≠ related to the rate‐limiting step of the reaction are evaluated. The derived rate law is in excellent agreement with the experimental results. The kinetic and activation parameters of this investigation have been compared and analyzed with those of the oxidation of l ‐leucine by gold(III).     

Grafted polymers under strong shear: Scaling and non-equilibrium Monte Carlo studies

End-grafted polymer chains exposed to strong shear solvent flow in the x-direction are investigated by a non-equilibrium Monte Carlo method using the bond-fluctuating model. The solvent flow is modelled by an enhanced jump rate of monomers in the flow direction. Under strong enough shear flow, the shear force is non-linear with the blob size ζ depending on the shear force and not on the monomer volume fraction ϕ. For the case of a grafted single chain, our data on the end-to-end distance of the polymer for a wide range of shear rate γ agree well with the proposed scaling form and compare reasonably well with the Langevin force law in the extreme strong shear case. Also the scaling for the blob size is derived with ζ = aN^vK(γN^2+v) for some scaling function K, and verified by our simulation data. For the case of a polymer brush, we derive the crossover force scale F^* separating the weak and strong shear regime with F^*a/(kT) = ϕ^v/(3v−1) where v is the usual self-avoiding walk exponent. For the case of a polymer brush under strong shear, the velocity profile, v(z), inside and above the brush is calculated analytically by treating the flow as in a porous medium and solving the Brinkman equation. The solution of the velocity profile is then combined with non-equilibrium Monte Carlo simulation data which allow a self-consistent determination of the chain end-to-end length and the incline angle of the chain. Also we derive the scaling form for the positions x(n) of the nth monomer and verify it by our simulation data. We further obtain an analytical expression for x(n) in terms of the effective viscosity η of the model, given by F = ηav(z). The effective viscosity is also found from our simulation to be ηa²/(kT) ≃ 0.02 ± 0.005.     

Kinetics and mechanism of oxidation of dimethyl sulfoxide by chloramine-T in aqueous solution

The kinetics of oxidation of dimethyl sulfoxide (DMSO) by chloramine-T (CAT) is studied in HClO₄ and NaOH media with OsO₄ as a catalyst in the latter medium. In acid medium, the rate law is -d [CAT]/dt = k [CAT][DMSO][H⁺]. Alkali retards the reaction and the rate law takes the form -d [CAT]/dt = k [CAT][DMSO][OsO₄]/[NaOH], but is reduced to -d [CAT]/dt = k [CAT][DMSO] at higher alkali concentrations. The reaction is subjected to changes in (a) ionic strength, (b) concentrations of added neutral salts, (c) concentrations of added reaction product, (d) dielectric constant, and (e) solvent isotope effect, and the subsequent effects on the reaction rate are studied. The reaction mechanism in acid medium assumes an electrophilic attack by the free acid RNHCl (CAT′) at the sulfur site in DMSO, forming a reaction intermediate which subsequently decomposes to dimethyl sulfone on hydrolysis. Formation of a cyclic complex between RNHCl and OsO₄ which interacts with the substrate in a slow step explains the observed results in alkaline medium. The simplification of the rate equation at higher alkali concentrations is attributed to a direct reaction between chloramine-T and the substrate.     

A spectrophotometric method for manganese determination in water samples based on ion pair formation and dispersive liquid–liquid microextraction

In this work, a simple, inexpensive, and environmentally friendly extractive spectrophotometric method for the determination of manganese is suggested. The method is based on the formation and dispersive liquid–liquid microextraction (DLLME) of a violet-coloured ion pair of Mn(II) with 1,3,3-trimethyl-2-[3-(3-methyl-3H-benzothiazol-2-ylidene)-propenyl]-3H-indolium (BTIC) in the presence of 1-nitroso-2-naphthol (HL) as ligand, and subsequent UV-VIS spectrophotometric detection at 560?nm of the ion pair formed. The appropriate experimental conditions for the DLLME procedure were found to be: a pH of 9.5; 0.12?mmol?L^?1 of BTIC; extraction solvent – toluene containing 1.75?mmol?L^?1 of HL; disperser solvent – methanol; auxiliary solvent – tetrachloromethane. Beer's law is obeyed in the range 0.055–0.88 µg?mL^?1 of Mn(II). The limit of detection (LOD), calculated based on three times of the standard deviation of the blank test (n?=?10), was found to be 0.004?µg?mL^?1 of Mn(II). The precision (as relative standard deviation, RSD%) and accuracy (as recovery percentage, R%) of the method were examined by performing five replicate determinations at four concentration levels over two days and varied between 1.2 and 3.8, and 97.7 and 104.5, respectively. The suggested method was successfully applied to the analysis of various water samples (mineral water, spring water and drinking water).     

Multivariate statistical treatment of solvent effects in the photoreduction of CoIII(En)2(Br)(RC6H4NH2)2+ complexes in aqua-organic solvent media

Ultraviolet photolysis (λ = 254 nm) studies were carried out for a series of cobalt(III) complexes, Co^III(En)₂(Br)(RC₆H₄NH₂)²⁺, where R = m-OMe, p-F, H, m-Me, p-Me, p-OEt, and p-OMe, in various compositions of water-methanol/1,4-dioxane mixtures (0, 5, 10, 15, 20, 25, and 30 vol % organic cosolvent) at two different temperatures (287 and 300 K). The ligand-to-metal charge-transfer excited state produced in the excitation of the complex initially generated a solvent-caged {Co^II; ligand radical} pair, which eventually undergoes recombination/separation into products. The quantum yield sharply increased from a mixture containing a lower mole fraction of organic cosolvent (x _org) to higher one. In other words, when x _org in the mixture increases, a steady increase in the quantum yield is observed. The quantum yield of Co^III(En)₂(Br)(RC₆H₄NH₂)²⁺ in various solvent mixtures is found to exhibit a linear (logΦ_Co(II) − 1/ε_r) dependence. This is consistent with solvation or solvent cage effect, which may be nonspecific, specific, or both. In order to throw light on these effects, a phenomenological model of solvent effects was applied. Therefore, the quantum yield values have been correlated statistically with some success containing different solvent parameters. The solvent parameters considered in this work are Grunwald-Winstein’s Y, Krygowski-Fawcett’s E _T ^N and DN ^N along with Kamlet-Taft’s α, β, π* parameters. The regression model proposed is Y _S = Y ₀ + Σ _{i = 1} ⁿ a _i X _i , where Y_S is the solvent-dependent property (here logΦ_Co(II)) in a given solvent; Y ₀ is the statistical quantity corresponding to the value of property in the reference solvent; X ₁, X ₂, X ₃ … are explanatory variables, the solvent parameters, which can explain the various solvation effects on reactants/{Co^II; ligand radical} pair, and a ₁, a ₂, a ₃… etc., are the regression coefficients. The coefficient values can be quantified to measure the relative importance of solvent effects on the physicochemical quantity, that is, the photoreduction yields in the present investigation. The text was submitted by the author in English.     

Cobalt(II) chloro complexation in N-methylformamide–N, N-dimethylformamide mixtures

Cobalt(II) chloro complexation has been studied by titration calorimetry and spectrophotometry in solvent mixtures of N-methylformamide (NMF) and N,N-dimethylformamide (DMF). It revealed that a series of mononuclear CoCln_n ^(2–n)+ (n=1–4) complexes are formed in the mixtures of NMF mole fraction x _NMF=0.05 and 0.25, and the CoCl⁺, CoCl₃ ^– and CoCl₄ ^2– complexes in the mixture of x _NMF=0.5, and their formation constants, enthalpies and entropies were obtained. As compared with DMF, the complexation is markedly suppressed in the mixtures, as well as in NMF. The decreasing formation constant of CoCl⁺ with the NMF content is mainly ascribed to the decreasing formation entropy. DMF is aprotic and thus less-structured, whereas NMF is protic to form hydrogen- bonded clusters. In DMF-NMF mixtures, solvent clusters in neat NMF are ruptured to yield new clusters involving DMF, the structure of which depends on the solvent composition. The entropy of formation of CoCl⁺ will be discussed in relation to the liquid structure of DMF, NMF and their mixtures.     

The triplet state of 4-nitro-N,N-dimethynaphthylamine

Nanosecond laser photolytic studies of 4-nitro-N,N-dimethylnaphthylamine (4-NDMNA) in nonpolar and polar solvents at room temperature show a transient species with an absorption maximum in the 500-510-nm range. This species is assigned to the lowest triplet excited state of 4-NDMNA. The absorption maximum of this state is independent of solvent polarity, and its lifetime is a function of the hydrogen donor efficiency of the solvent. In n-hexane the lifetime 1/k of the triplet state is 9.1 × 10^?6 sec, while in acetonitrile 1/k is 2.0 × 10^?7 sec. The hydrogen abstraction rate constant k_H of the triplet state with tributyl tin hydride (Bu₃SnH) in n-hexane is 1.7 × 10⁷M^?1·sec^?1, while in the case of isopropyl alcohol as hydrogen donor, k_H is 4.0 × 10⁷M^?1·sec^?1. The activation energy for the hydrogen abstraction by the triplet state from Bu₃SnH in deaerated n-hexane is 0.6 kcal/mol. The lack of spectral shift with increasing solvent polarity, and the appreciable hydrogen abstraction reactivity of the triplet state, also independent of solvent polarity, seem to indicate that this excited state is an n-π* state which retains its n-π* character even in polar media.     

Salt Effects in the Cononsolvency of Poly(N‐isopropylacrylamide) Microgels

Poly(N‐isopropylacrylamide) (PNIPAM) is well known to exhibit reentrant behavior or cononsolvency in response to the composition of a mixed solvent consisting of water and a low‐chain alcohol. Since the solvent structure plays an important role in this phenomenon, the presence of structure‐breaking/structure‐making ions in solution is expected to have a dramatic effect on the cononsolvency of PNIPAM. The present work examines the way that the presence of different salts can modify the reentrant‐phase diagram displayed by a cationic PNIPAM microgel in the mixed ethanol/water solvent. The effects of four Hofmeister anions—SO₄^2?_, Cl^?, NO₃^? and SCN^?—with different abilities to modify the solvent structure are analyzed. The species with kosmotropic or structure‐making character show a clear competition with ethanol for the water molecules, intensifying the nonsolvency of the PNIPAM with the EtOH volume fraction (?_e). However, striking results are found with the most chaotropic or structure‐breaking anion, SCN^?. In contrast to what happens in water‐rich solutions, the presence of SCN^? in alcohol‐rich solvents enhances the solubility of the polymer, which macroscopically results in the microgel swelling. Moreover, this ion displays great stabilizing properties when ?_e> is 0.2. These results have been explained by considering how chaotropic or structure‐breaking ions interact with water and ethanol molecules.     

Colorimetric Solvent Indicators Based on Nafion Membranes Incorporating Nickel(II)‐Chelate Complexes

To develop solvent‐recognition films, Nafion membranes incorporating cationic nickel‐chelate complexes, that is, [Ni(L¹)(L²)]⁺ (HL¹=acetylacetone, 2,2,6,6‐tetramethyl‐3,5‐heptanedione; L²=N,N‐diethylethylenediamine, N‐butyl‐N,N′,N′‐trimethylethylenediamine), were prepared. Immersion of the films in various solvents effected the color changes varying from red to pale blue green depending on the donor number of the solvents. The color change is based on an equilibrium shift between square‐planar and solvent‐coordinated octahedral geometries of the cations. The degree of the color change depended on the affinity of the incorporated complex to the solvent molecules. The films were robust and exhibited a reversible solvent response. The films exhibited thermochromism when a small amount of appropriate solvents were incorporated and changed from pale blue green at low temperatures to red at high temperatures.