Rate Constant Keff Research Articles

Effect of Chitosan and Amphiphilic Polymers on the Photosensitizing and Spectral Properties of Rose Bengal.

The influence of chitosan (CS) and amphiphilic polymers (AP: pluronic F108 and polyvinylpyrrolidone (PVP)) on the photocatalytic activity of rose bengal (RB) in a model reaction of tryptophan photo-oxidation in phosphate-buffered saline (PBS) was studied. It was shown that in the presence of CS, the effective rate constant keff of tryptophan photo-oxidation catalyzed by RB in PBS solution decreases by a factor of two. This is due to the ionic interaction of the RB with the chitosan. Rose bengal in a slightly acidic environment (pH 4.5) passes into a neutral lactone form, which sharply reduces the photosensitizing properties of the dye. It was demonstrated that the introduction of AP into a solution containing RB and CS prevents direct interaction between RB and CS. This is evidenced by the presence of photocatalytic activity of the dye in the RB-AP-CS systems, as well as bathochromic shifts of the main absorption bands of the dye, and an increase in the optical density and luminescence intensity of the RB when AP is introduced into a buffer solution containing RB and chitosan. The presence of RB-CS and RB-AP interaction in aqueous and PBS media is confirmed by the increase in the degree of fluorescence anisotropy (r) of these binary systems. In an aqueous solution, the value of r for the RB-F108-CS system decreases by a factor of 3.5 (compared to the value of r for the RB-CS system), which is associated with the localization of the dye in pluronic micelles. In PBS, the fluorescence anisotropy is practically the same for all systems, which is related to the stability of the dye structure in this medium. The presence of interaction between RB and AP in aqueous solutions was confirmed by the proton NMR method. In addition, the formation of RB-F108 macromolecular complexes, which form associates during solution concentration (in particular, during evaporation), was shown by AFM. Such RB-AP-CS systems may be promising for practical application in the treatment of local foci of infections by aPDT.

The Influence of Effect of Polysaccharides and Polyvinylpyrrolidone on the Photocatalytic Activity of Chlorin e6 in Tryptophan Oxidation

The effect amphiphic polymer poly-N-vinylpyrrolidone (PVP) and polysaccharides (PSs), sodium alginate (SA) and hyaluronic acid (HA), have on the photosensitizing activity of chlorin e6 (Ce6) in the reaction of tryptophan oxidation is established. This reaction is considered a model in searching for the most efficient pharmaceutical formulations of photosensitizers for antimicrobial photodynamic therapy. It is shown that the initial photosensitizing activity of Ce6, characterized by effective rate constant keff of tryptophan oxidation, grows as binary (Ce6–PVP and Ce6–SA(HA)) and (to a greater extent) ternary systems (Ce6–PVP–SA and Ce6–PVP–SA(HA)) form in aqueous solutions. This rise in activity is found to be related to disaggregation of the associates of photosensitizer (PhS) molecules initially present in aqueous solutions, resulting from the formation of complex bonds between PhS molecules and fragments of polymers added to the solution. This conclusion is confirmed for the considered binary and ternary systems by the accumulated data of electronic absorption spectra and fluorescence spectra of chlorin e6 in the absence of polymers (PVP, SA, and HA) and after they are added to the reaction system, and by 1H NMR data. In light of the most recent findings (according to which each polymeric component (PVP, SA, and HA) affects the activity of PhS, while almost no direct interaction between PVP and PS is detected in the 1H NMR spectra), ternary photosensitizing systems form in aqueous media during the formation of intertwining PVP–PS polymeric chains upon exposure to local hydrodynamic flows, followed by complex bonding between PhS molecules and fragments of both polymers.

Developing superior catalysts engineered by multichannel healing strategy for advanced oxidation

Catalyst deactivation derived from slow conversion of Fe(III)/Fe(II) is the critical problem restricting catalytic efficiency of advanced oxidation. Herein, a novel multichannel healing strategy is proposed for fast healing structural damage by designing rationally smart platform (TiO2-CDs-FeOOH hollow sandwich hybrid). This ingenious system shows remarkable catalytic activity (0.77 min−1) and stability towards the removal of organic contaminants. Systematic studies confirm that the excellent catalytic activity is attributed to multiple electron transfer (MET) behavior at interface between photocatalytic substrate and FeOOH, leading to highly efficient interfacial charge separation and then fast recovery of Fe(II) on FeOOH surface during H2O2 activation process. Notably, multichannel healing procedure and MET kinetic information are unveiled by UV–vis/scanning electrochemical microscopy (SECM) through an in situ probe scanning technique. A higher ET rate constant keff (0.92 × 10-2 cm s-1) was revealed over TiO2-CDs-FeOOH, two times that of TiO2-FeOOH, further confirming that multi-channel electron transfer could achieve fast Fe(III)/Fe(II) conversion. This work offers a new insight to design highly efficient catalysts for the practical applications of sustainable environmental remediation and energy conversion.

Determining Li+-Coupled Redox Targeting Reaction Kinetics of Battery Materials with Scanning Electrochemical Microscopy.

The redox targeting reaction of Li+-storage materials with redox mediators is the key process in redox flow lithium batteries, a promising technology for next-generation large-scale energy storage. The kinetics of the Li+-coupled heterogeneous charge transfer between the energy storage material and redox mediator dictates the performance of the device, while as a new type of charge transfer process it has been rarely studied. Here, scanning electrochemical microscopy (SECM) was employed for the first time to determine the interfacial charge transfer kinetics of LiFePO4/FePO4 upon delithiation and lithiation by a pair of redox shuttle molecules FcBr2+ and Fc. The effective rate constant keff was determined to be around 3.70-6.57 × 10-3 cm/s for the two-way pseudo-first-order reactions, which feature a linear dependence on the composition of LiFePO4, validating the kinetic process of interfacial charge transfer rather than bulk solid diffusion. In addition, in conjunction with chronoamperometry measurement, the SECM study disproves the conventional "shrinking-core" model for the delithiation of LiFePO4 and presents an intriguing way of probing the phase boundary propagations induced by interfacial redox reactions. This study demonstrates a reliable method for the kinetics of redox targeting reactions, and the results provide useful guidance for the optimization of redox targeting systems for large-scale energy storage.

Radical Polymerization of Vinyl Acetate with Bis(tetramethylheptadionato)cobalt(II): Coexistence of Three Different Mechanisms

The complex [Co(II)(tmhd)(2)] (4; tmhd = 2,2,6,6-tetramethylhepta-3,5-dionato) has been investigated as a mediator for controlled radical polymerization of vinyl acetate (VAc) and compared with the analogue [Co(II)(acac)(2)] (1; acac = acetylacetonato). A relatively well controlled process occurs, after an induction time, with 2,2'-azobis(4-methoxyl-2,4-dimethylvaleronitrile) (V-70) as radical initiator at 30 degrees C. However, whereas the polymerization essentially stops after about six initiator half-lives in the presence of 1, it continues with a first-order rate law in the presence of 4. The successful simulation of the kinetic data shows that 4 operates simultaneously by associative (degenerative transfer, DT) and dissociative (organometallic radical polymerization, OMRP) mechanisms. The occurrence of OMRP was confirmed by an independent polymerization experiment starting from an isolated and purified [Co(tmhd)(2)](PVAc) macroinitiator. The polymer molecular weight evolves linearly with conversion in accordance with the expected values for one chain per Co atom when DT is the predominant mechanism and also during the pure OMRP process; however, observation of stagnating molecular weights at long reaction times with concomitant breakdown of the first-order rate law for monomer consumption indicates a competitive chain-transfer process catalyzed by an increasing amount of Co(II). In the presence of external donors L (water, pyridine, triethylamine) the DT pathway is blocked and the OMRP pathway is accelerated, and polymerization with complex 4 is then about five times slower than with complex 1. The reversal of relative effective OMRP rate constants k(eff) (4>1 in the absence of external donors, 4<1 in their presence) is rationalized through competitive steric effects on Co(III)-C and Co(II)-L bond strengths. These propositions are supported by (1)H NMR studies and by DFT calculations.

Fluoropolyethers end-capped by polar functional groups. II. Effect of catalyst and reagents concentration, solvent nature, and temperature on reaction kinetics of ?,?-bis(hydroxy)-terminated fluoropolyethers with cycloalyphatic and aromatic diisocyanates

A comparative kinetic study of the dibutyltin dilaurate (DBTDL) and 1,4-diazabicyclo[2,2,2]octane (DABCO) catalyzed reactions of α,ω-bis(hydroxy)-terminated fluoropolyethers (FPEs)—Z-DOLs and Z-DOL TXs—of various molecular weights and purity, with 4,4′-dicyclohexylmethane diisocyanate (H12MDI), isophorone diisocyanate (IPDI) and 2,4-toluene diisocyanate (TDI) was carried out in different solvents. An analytical method was used to follow the kinetics of the reactions at four different temperatures. The rate of NCO disappearance measured by two independent methods—IR spectroscopy and chemical titration were found to be very close. Straight proportionality between rate constants kcat and catalyst concentration was found. But in some cases for the DBTDL catalyzed reactions effect of catalyst saturation along with appearance of the limiting DBTDL concentration Clim below which the rate of reaction was close to zero were observed. Reactivity of Z-DOLs in the tin-catalyzed urethane reactions was found to decrease with their storage time at RT due to the slow hydrolysis of the end COOR groups impurities, which give the corresponding acids that act as a strong inhibitor of the DBTDL activity. These acid admixtures have no influence on the DABCO catalyzed reactions. For the DBTDL and DABCO catalyzed reactions of Z-DOLs with IPDI the dependence of effective rate constants keff (where keff = kcat · 0.01/[DBTDL] and catalyst concentration is taken in mol % based on IPDI) on total reagents concentration were found to be described by curves with a maximum. Critical reagents concentration, after which the relationship keff = f (C) changes from proportional to inverse proportional, seems do not substantially depend on the solvent nature. Hydrogenated analog poly(ethylene glycol) MW 400 (PEG-400) differs greatly from Z-DOLs: only steady decrease of keff was observed with increase of reagents concentration C from 5 up to 95 wt %. Activation energies for all the studied reactions are within the range of 10.8–16.7 kcal/mol. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2579–2602, 2000

Kinetics of the oxidation of metals: Parallel circuit for point defects diffusion

A model is proposed for the oxidation of metals including both bulk crystal lattice diffusion and short-circuit diffusion paths. Assuming local equilibrium between point defects in the bulk and in grain boundaries, we obtain an effective parabolic rate constant keff=kbulk [1+θ exp-(ΔH/RT],where ΔH is the enthalpy for the reaction between point defects in the bulk and in short circuits, θ is the fraction of the short-circuit area, and T is temperature. In the case of the high-temperature oxidation of nickel coated with a thin film of vanadium pentoxide, this model yields to a rate law with a critical oxygen pressure Pc. The nickel vanadium oxide located in the short circuits of the growing oxide NiO is liquid below Pc, leading to a rapid short-circuit diffusion. Above this value, a vanadate precipitates and acts as diffusion blocks for migrating Ni ions.