Reduction of soluble colloidal MnO<Subscript>2</Subscript> by <Emphasis Type="SmallCaps">DL</Emphasis>-malic acid in the absence and presence of nonionic TritonX-100

Kinetics of oxidation of DL-malic acid by water soluble colloidal MnO₂ (prepared from potassium permanganate and sodium thiosulfate solutions) have been studied spectrophotometrically in the absence and presence of nonionic Triton X-100 surfactant. The reaction is autocatalytic and manganese(II) (reduction product of the colloidal MnO₂) may be the autocatalyst. The order of the reaction is first in colloidal [MnO₂] as well as in [malic acid] both in the absence and presence of the surfactant. The reaction has acid-dependent and acid-independent paths and, in the former case, the order is fractional in [H⁺]. The effect of externally added manganese(II) is complex. The results show that the rate constant increases as the manganese(II) concentration is increased. It is not possible to predict the exact dependence of the rate constants on manganese(II) concentration, which has a series of reactions with other reactants. In the presence of TX-100, the observed effect on k is catalytic up to a certain [TX-100]; thereafter, an inhibitory effect follows. The catalytic effect is explained in terms of the mathematical model proposed by Tuncay et al. (in Colloids Surf A Physicochem Eng Aspects 149:279 3). Activation parameters associated with the observed rate constants (k_obs/k) have also been evaluated and discussed.     

Effect of ionic and non-ionic surfactants on the reduction of water soluble colloidal MnO<Subscript>2</Subscript> by glycolic acid

Kinetics of the redox reaction between colloidal MnO₂ and glycolic acid have been studied spectrophotometrically by monitoring the decay in the absorbance of colloidal MnO₂ in absence and presence of surfactants. Anionic sodium dodecyl sulfate has no effect, non-ionic Triton X-100 catalyzed the reaction and experiments were not possible in presence of cationic cetyltrimethylammonium bromide due to the precipitation of MnO₂.The reaction followed the same type of kinetic behavior, i.e., fractional-, first- and fractional-order dependencies, respectively, in [glycolic acid], [MnO₂] and [H⁺ ] in both the media. Effects of gum arabic and manganese(II) have also been studied and discussed. Mechanisms in accordance with the experimental data are proposed for the reaction.     

Kinetics of colloidal MnO<Subscript>2</Subscript> reduction by L-arginine in absence and presence of surfactants

The kinetics of the oxidation of L-arginine by water-soluble form of colloidal manganese dioxide has been studied using visible spectrophotometry in aqueous as well as micellar media. To obtain the rate constants as functions of [L-arginine], [MnO₂] and [HClO₄], pseudo-first-order conditions are maintained in each kinetic run. The first-order-rate is observed with respect to [MnO₂], whereas fractional-order-rates are determined in both [L-arginine] and [HClO₄]. Addition of sodium pyrophosphate and sodium fluoride enhanced the rate of the reaction. The effect of externally added manganese(II) sulphate is complex. It is not possible to predict the exact dependence of the rate constant on manganese(II) concentration, which has a series of reactions with other reactants. The anionic surfactant SDS neither catalyzed nor inhibited the oxidation reaction, while in presence of cationic surfactant CTAB the reaction is not possible due to flocculation of reaction mixture. The reaction is catalyzed by the nonionic surfactant TX-100 which is explained in terms of the mathematical model proposed by Tuncay et al. Activation parameters have been evaluated using Arrhenius and Eyring equations. On the basis of observed kinetic results, a probable mechanism for the reaction has been proposed which corresponds to fast adsorption of the reductant and hydrogen ion on the surface of colloidal MnO₂.     

A kinetic study of the oxidation of <Emphasis Type="SmallCaps">l</Emphasis>-methionine by water soluble colloidal MnO<Subscript>2</Subscript>

Aqueous solution of water soluble colloidal MnO₂ was prepared by Perez-Benito method. Kinetics of l-methionine oxidation by colloidal MnO₂ in perchloric acid (0.93 × 10⁻⁴ to 3.72 × 10⁻⁴ mol dm⁻³) has been studied spectrophotometrically. The reaction follows first-order kinetics with respect to [H⁺]. The first-order kinetics with respect to l-methionine at low concentration shifts to zero order at higher concentration. The effects of [Mn(II)] and [F⁻] on the reaction rate were also determined. Manganese (II) has sigmoidal effect on the rate reaction and act as auto catalyst. The exact dependence on [Mn(II)] cannot be explained due to its oxidation by colloidal MnO₂. Methionine sulfoxide was formed as the oxidation product of l-methionine. Ammonia and carbon dioxide have not been identified as the reaction products. The mechanism with the observed kinetics has been proposed and discussed.     

Kinetics of metribuzin degradation by colloidal manganese dioxide in absence and presence of surfactants

The kinetics of the degradation of metribuzin by water-soluble colloidal MnO₂ in acidic medium (HClO₄) were studied spectrophotometrically in the absence and presence of surfactants. The experiments were performed under pseudo-first-order reaction conditions in respect of MnO₂. The degradation was observed to be of the first order in respect of MnO₂ while of fractional order for both metribuzin and HClO₄. The rate constant for the degradation of metribuzin was observed to decrease as the concentration of MnO₂ increased. The anionic surfactant, sodium dodecyl sulphate (SDS), was observed to be ineffective whereas the non-ionic surfactant, Triton X-100 (TX-100), accelerated the reaction rate. However, the cationic surfactant, cetyltrimethyl ammonium bromide (CTAB), caused flocculation with oppositely-charged colloidal MnO₂; hence further study was not possible. The catalytic effect of TX-100 was discussed in the light of the available mathematical model. The kinetic data were exploited to generate the various activation parameters for the oxidative degradation of metribuzin by colloidal MnO₂ in the absence as well as the presence of the non-ionic surfactant, TX-100.     

Mechanism of the oxidation of d-glucose onto colloidal MnO2 surface in the absence and presence of TX-100 micelles

Aqueous colloidal manganese dioxide (MnO₂) was prepared via titration by using potassium permanganate and sodium thiosulphate in aqueous neutral medium. The kinetics of oxidation of d-glucose onto the surface of colloidal MnO₂ have been studied spectrophotometrically. The results show that the rate of initial stage (nonautocatalytic path) increases with increasing the [d-glucose], [H⁺], and temperature and also upon addition of nonionic surfactant Triton X-100 (TX-100), which indicates that the surfactant enhances the concentration of d-glucose at the surface of the colloidal MnO₂. Hydrogen bonding interaction seemingly arises between –OH groups of d-glucose and oxygen of the ether linkages of polyoxyethylene chain of TX-100. A possible mechanism of the oxidative degradation of d-glucose is discussed in terms of d-glucose/TX-100 and colloidal MnO₂ interaction.     

Water-soluble colloidal manganese dioxide as an oxidant for l-tyrosine in the absence and presence of non-ionic surfactant TX-100

Water-soluble colloidal manganese dioxide has been used to oxidize l-tyrosine in aqueous-acidic medium. The kinetics of the reaction was studied in the absence and presence of non-ionic surfactant (TX-100) using a spectrophotometric technique. As the reaction was fast under pseudo-first-order conditions ([l-tyrosine]  [MnO₂]), the rate constants as a function of [l-tyrosine], [MnO₂], [HClO₄] and temperature were obtained under second-order conditions. The rate of the reaction increased and decreased with the increase in [l-tyrosine] and [MnO₂], respectively. Perchloric acid, sodium pyrophosphate and sodium fluoride showed catalytic effect. The effect of externally added manganese(II) sulphate is complex. It is not possible to predict the exact dependence of the rate constants on manganese(II) concentration, which has a series of reactions with other reactants. The reaction is inhibited by the non-ionic surfactant TX-100. Activation parameters have been evaluated using Arrhenius and Eyring equations. Based on observed kinetic results, a probable mechanism for the reaction has been proposed which corresponds to fast adsorption of the reductant and hydrogen ion on the surface of colloidal MnO₂ followed by one-step two-electron transfer rate limiting process.     

Kinetics of the Degradation of l-Cysteine at Freshly Prepared Nano-sized MnO2 Surfaces in the Absence and Presence of TX-100

Nano-sized colloidal manganese dioxide was synthesized at room temperature by a chemical method in neutral medium without a stabilizing agent. The obtained MnO₂ nano-sized colloid was found to be stable for several months and was characterized by means of UV–Vis spectroscopy, energy-dispersive X-ray spectrometer (EDX) and transmission electron microscopy. The EDX analysis confirmed the presence of Mn and O in the sample. The paper reports on the use of nano-sized colloidal manganese dioxide as an oxidant in the oxidation of cysteine (Cyst) in the absence and presence of surfactant (TX-100) at 35 °C. The study was carried out as functions of [MnO₂], [Cyst], [HClO₄] and temperature. The results show that the reaction proceeds through fast adsorption of Cyst onto the surface of the colloidal MnO₂. Pseudo-first-order rate constants were found to increase with the increase in [TX-100]. This paper reports values of the reaction rates and activation parameters in the absence and presence of surfactant and proposes a plausible mechanism.     

Oxidative degradation of dipeptide (glycyl–glycine) by water-soluble colloidal manganese dioxide in the aqueous and micellar media

The kinetics of the oxidative degradation of dipeptide glycyl–glycine (Gly-Gly) by water-soluble colloidal MnO₂ in acidic medium has been studied by employing visible spectrophotometer in the aqueous and micellar media at 35 °C. To obtain the rate constants as functions of [Gly-Gly], [MnO₂] and [HClO₄], pseudo-first-order conditions were maintained in each kinetic run. The first-order-rate is observed with respect to [MnO₂], whereas fractional-order-rates are determined in both [Gly-Gly] and [HClO₄]. The addition of sodium pyrophosphate and sodium fluoride has composite effects (catalytic and inhibition). The reaction proceeds through the fast adsorption of Gly-Gly on the surface of the colloidal MnO₂. The observed results are discussed in terms of Michaelis–Menten/Langmuir–Hinshelwood model. The Arrhenius and Eyring equations are found valid for the reaction over a range of temperatures and different activation parameters have been evaluated. A probable reaction mechanism, in agreement with the observed kinetic results, has been proposed and discussed. The influence of changes in the surfactant concentrations on the observed rate constant is also investigated and the reaction followed the same type of kinetic behavior in micellar media. The pseudo-first-order rate constant (k_ψ) is found to increase about two-fold with increase in [TX-100]. The catalytic effect of nonionic surfactant TX-100 is explained in terms of the mathematical model proposed by Tuncay et al.     

Oxidation of Methionine by Colloidal MnO2 in Aqueous and Micellar Media: A Kinetic Study

The oxidation of methionine by freshly prepared colloidal manganese dioxide in aqueous as well as micellar media was studied spectrophotometrically at 35°C. The reaction between methionine and MnO₂ in both media exhibits 1:1 stoichiometry (methionine:MnO₂). The oxidation reaction is first order with regard to the MnO₂ concentration, but is fractional-order in the methionine concentration and HClO₄ concentrations. A catalytic effect of nonionic surfactant TX-100 on the rate of oxidation was observed and reaction rate was found to be proportional to {k′ + k″ [TX-100]}, where k′ and k″ are the rate constants in absence and presence of surfactant, respectively. The use of surfactant micelles is highlighted as, in favorable cases; the micelles help the redox reactions by bringing the reactants in a close proximity through hydrogen bonding. The oxidation reaction in aqueous and micellar media is shown to proceed via methionine–MnO₂ and methionine–MnO₂–TX-100 complexes, respectively, which decomposes slowly in a rate determining step to give methionine sulfoxide as the product. A suitable mechanism is proposed for these observations.     

Kinetics and mechanism of the reduction of colloidal MnO2 by glycyl-leucine in the absence and presence of surfactants

The kinetics of the reduction of water-soluble colloidal manganese dioxide by glycyl-leucine (Gly-Leu) has been investigated in the presence of perchloric acid both in aqueous as well as micellar media at 35 °C. The study was carried out as functions of [MnO₂], [Gly-Leu] and [HClO₄]. The first-order-rate is observed with respect to [MnO₂], whereas fractional-order-rates are determined in both [Gly-Leu] and [HClO₄]. Addition of sodium pyrophosphate and sodium fluoride enhanced the rate of the reaction. Further, the use of surfactant micelles is highlighted as, in favourable cases, the micelles help the redox reactions by bringing the reactants into a close proximity due to hydrogen bonding. While the ionic surfactants SDS and CTAB have not shown any effect on the reaction rate, the nonionic surfactant TX-100 has catalytic effect which is explained in terms of the mathematical model proposed by Tuncay et al. (1999). The Arrhenius and Eyring equations are valid for the reaction over the range of temperatures used and different activation parameters (E_a, ΔH^#, ΔS^# and ΔG^#) have been evaluated. Kinetic studies show that the redox reaction between MnO₂ and Gly-Leu proceeds through a mechanism combining one- and two-electron pathways: Mn(IV) → Mn(III) → Mn(II) and Mn(IV) → Mn(II). On the basis of the observed results, a possible mechanism has been proposed and discussed.     

Effect of microheterogeneous environments of CTAB,Triton X-100, and Tween 20 on the oxidative degradation of d-fructose by nanoparticles of MnO2

The kinetics of the oxidative degradation of d -fructose by nanoparticles of MnO₂ has been studied in dilute sulfuric acid medium and also in the presence of surfactants of cetyl trimethyl ammonium bromide (CTAB), Triton X-100 (TX-100), and Tween 20. Amorphous nanoparticles of MnO₂ in the form of spherical particulates of size 50–200 nm, as detected by a transmission electron microscope, have been found to exist, supported on two-dimensional gum acacia sheets. The reaction is first order in MnO₂ but complex order with respect to fructose and H⁺. The reaction is inhibited due to adsorption of reaction products on the surface of MnO₂ nanoparticles. The reaction takes place through an intermediate complex formation between β-d -fructopyranose and protonated MnO₂. A one-step two-electron transfer reaction ultimately leads to the formation of an aldonic acid and formic acid. The entropy of activation plays the key role for the reaction in the absence of surfactants. In the surfactant-mediated reaction, partitioning of both the reactants takes place between the aqueous and micellar pseudophases and reaction occurs following Berezin's model. Binding of fructose with the surfactants in the Stern/palisade layer takes place through the ion–dipole interaction and H-bonding while protonated MnO₂ remains at the outer side of the Stern/palisade layer within the micelle. Both the enthalpy and entropy changes associated with the fructose–water interaction, fructose–micelle interaction, and micelle–water interaction finally control the fructose–micelle binding.     

Reactivity of some sulphur- and non-sulphur-containing amino acids towards water soluble colloidal MnO<Subscript>2</Subscript>. A kinetic study

Kinetic data for the oxidation of glutathione (reduced, GSH), cysteine, glycine and glutamic acid by colloidal manganese dioxide, (MnO₂) _n are reported. Colloidal MnO₂, oxidized glutathione to disulphide (glutathione, oxidized), was reduced to manganese (II). Glycine and glutamic acid (structural units of glutathione) are not oxidized by colloidal MnO₂, but the other structural unit, cysteine, is also oxidized by the same oxidant under similar experimental conditions. This is interpreted in terms of the rate-determining colloidal MnO₂-S bonded intermediate. The reactivity of GSH towards colloidal MnO₂ is very much higher than cysteine. Kinetics of oxidation of GSH and cysteine by colloidal MnO₂ were performed spectrophotometrically as a function of [GSH], [cysteine], colloidal [(MnO₂) _n ], [HClO₄], temperature and trapping agents sodium fluoride and manganese (II) (reduction product of colloidal MnO₂). The purpose of this work was to study the role of –NH₂, –COOH, –SH groups present in the carbon chain of the above amino acids. It was found that the reactivity of –SH group is higher than –NH₂ and –COOH groups. The mechanisms, involving a colloidal MnO₂ complex with GSH and cysteine, are proposed. The complexes decompose in a rate-determining step, leading to the formation of free radical and manganese (III), which is also an intermediate. The dimerization of radicals takes place in a subsequent fast step to yield the products.     

Influence of surfactants on the adsorption and elektrokinetic properties of the system: guar gum/manganese dioxide

The adsorption of non-ionic polysaccharide—guar gum (GG) in the presence or absence of the surfactants: anionic SDS, cationic CTAB, nonionic TX-100 and their equimolar mixtures SDS/TX-100, CTAB/TX-100 from the electrolyte solutions (NaCl, CaCl₂) on the manganese dioxide surface (MnO₂) was studied. The increase of GG adsorption amount in the presence of surfactants was observed in every measured system. This increase results from formation of complexes between the GG and the surfactant molecules. This observation was confirmed by the determination of the influence of GG on surfactants adsorption on the MnO₂ surface. The increase of GG adsorption on MnO₂ was the largest in the presence of the surfactant mixtures (CTAB/TX-100; SDS/TX-100) which is the evidence of the synergetic effect. The smallest amounts of adsorption were obtained in the presence of TX-100, which results from non-ionic character of this surface active agent. In the case of single surfactant solution CTAB has the best efficiency in increasing the amount of GG adsorption on MnO₂ which results from strong interactions with GG and also with the negatively charged surface of the adsorbent. In order to determine the electrokinetic properties of the system, the surface charge density of MnO₂ and the zeta potential measurements were performed in the presence of the GG macromolecules and the above mentioned surfactants and their mixtures. The obtained data showed that the adsorption of GG or GG/surfactants complexes on the manganese dioxide surface strongly influences the diffused part of the electrical double layer (EDL)—MnO₂/electrolyte solution, but has no influence on the compact part of the electric double layer. This is the evidence that the polymers chains are directly bonded with the surface of the solid and the surfactants molecules are present in the upper part of the EDL.     

The Kinetics of Oxidation of L-Tryptophan by Water-Soluble Colloidal Manganese Dioxide

The kinetics of the oxidation of L-tryptophan by water-soluble colloidal MnO₂ (prepared from potassium permanganate and sodium thiosulfate solutions) has been carried out in aqueous perchloric acid medium at different temperatures. Monitoring the disappearance of the MnO₂ spectrophotometrically at 390 nm was used to follow the kinetics. The first-order kinetics with respect to [L-tryptophan] at low concentrations shifted to zero-order at higher concentrations. The reaction followed first-order with respect to [MnO₂] but fractional-order with respect to [HClO₄]. Adding trapping agents enhanced the rate of the reaction. The Arrhenius and Eyring equations were found valid for the reaction between 35°C and 55°C and different activation parameters (E_a, ΔH^#, ΔS^#) have been evaluated. On the basis of various observations and product characterization a plausible mechanism has been envisaged for the reaction taking place at the colloid surface. The results suggest formation of an adsorption complex between L-tryptophan and MnO₂. The complex decomposes in a rate-determining step, leading to the formation of free radical, which again reacts with the colloidal MnO₂ in a subsequent fast step to yield products. Freundlich isotherm is used to explain the adsorption of L-tryptophan on the colloidal MnO₂.     

Kinetics,mechanism and cloud point measurements in the oxidative degradation of non-ionic Triton X-100 surfactant in acidic permanganate solutions

The polyoxyethylene chain of non-ionic surfactant Triton X-100 [4-(1,1,3,3-tetramethylbutyl) phenyl polyethylene glycol,TX-100] was degraded by permanganate in the presence of HClO₄. The oxidative degradation rate and cloud point have been obtained as a function of [surfactant], [permanganate], [HClO₄], and temperature. Dependence of the reaction rate on adding inorganic salts (Na₄P₂O₇, NaF and MnCl₂) was also examined. The oxidation rate increased with increase in [TX-100] and [H⁺]. The higher order kinetics with respect to [TX-100] at lower [H⁺] shifted to lower order at higher [H⁺]. The cloud point of TX-100 (67°C) shifted to lower temperature (23±0.5°C) after oxidative degradation of the polyoxyethylene chain. Evidence of complex formation between TX-100 and MnO ₄ ⁻ was obtained spectrophotometrically. Presence of the primary alcoholic (–OH) group in the TX-100 skeleton is responsible for the degradation of oxyethylene chain. Both monomeric and aggregated TX-100 molecules are oxidized by permanganate. A catalytic oxidation mechanism is proposed on the basis of the experimental findings.     

Synthesis of hydroxy derivatives from adamantanecarboxylic acids in the system MnO<Subscript>2</Subscript>–H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

A convenient procedure has been developed for the synthesis of mono- and dihydric cage alcohols from adamantanecarboxylic acids and their esters using the MnO₂–H₂SO₄ system. The reaction at elevated temperature involved both decarboxylation and decarbonylation of the initial acid or ester.     

Electrochemical behavior of anthraquinone in aqueous solution in presence of a non-ionic surfactant

Cyclic voltammetric behavior of anthraquinone in aqueous medium has been studied in presence of a non-ionic surfactant, Triton X-100 (TX-100) using sodium salt of anthraquinone-2-sulphonic acid (AQS) as the electro-active species. When cathodic potential is applied, the anthraquinone (AQ) group of AQS is reduced to its dianion. In the reverse scan, the oxidation of AQ²⁻ gives AQ. The electrochemical behavior shows a profound influence from the dissolved state of TX-100 in aqueous media. Spectrophotometric results indicate interaction between AQ and TX-100. A CEC (chemical–electrochemical–chemical) mechanism with the electrochemical reaction coupled with preceding interaction of AQS with TX-100 and following protonation reaction of reduced AQ has been proposed.     

Decay of H<Subscript>2</Subscript>O<Subscript>2</Subscript> under the action of the colloidal catalyst based on iron(III) oxides in a neutral medium

The catalytic activity of the colloidal catalyst based on iron(III) hydroxide was studied in the decomposition of H₂O₂ in a neutral medium (pH 6.7). A colloidal micellar solution of iron(III) hydroxide after preparation was kept at 19–20 °С for 2 or 20 h without additives or with C₂H₅OH additives. The decomposition of H₂O₂ under the action of the colloidal catalyst (20 h) proceeds via the first-order reaction with the decay rate constant k_d = 1.26?10^–4 s^–1, whereas the decay rate of the first-order reaction is k_d = 0.77?10^–4 s^–1 for the colloidal catalyst (2 h) prepared in the presence of C₂H₅OH.     

Liquid-phase MnO<Subscript>2</Subscript>-modification of clinoptilolite

Formation mechanism of the MnO₂ phase in the reaction of heterogeneous synthesis between Mn²⁺ and MnO ₄ ^- ions on a solid aluminosilicate surface in aqueous solutions was studied. It was shown that, for lowsilica forms, the Mn²⁺ ion is oxidized by the MnO ₄ ^- ion uniformly across the grain depth to give the MnO₂ phase and manganese manganites. For high-silica materials, the MnO₂ phase is formed on the outer surface of grains, with the decomposition of the MnO ₄ ^- ion and formation of the MnO₂ phase and molecular oxygen. It was found that, for the clinoptilolite rock used as a solid support, the yield of the MnO₂ phase and its distribution over the particle volume depend on the penetration capacity of the MnO ₄ ^- ion into the porous structure of this rock, determined by its composition. It is shown that the amount of the MnO₂ phase grows with increasing concentration of the MnO ₄ ^- ion and treatment duration, with the phase thickness being 15–20 and 350–1050 μm for, respectively, high- and low-silica samples.