Carmelo Sbriziolo

Spatio-Temporal Perturbation of the Dynamics of the Ferroin Catalyzed Belousov−Zhabotinsky Reaction in a Batch Reactor Caused by Sodium Dodecyl Sulfate Micelles.

The effects of the anionic surfactant sodium dodecyl sulfate (SDS) on the spatio-temporal and temporal dynamics of the ferroin-catalyzed Belousov-Zhabotinsky (BZ) reaction have been studied over a wide surfactant concentration range. For the first time, investigations were performed also for unstirred systems. The presence of SDS in the reaction mixture influences the oscillatory parameters to an extent that significantly depends on the surfactant concentration. The trend of the wave speed v upon the increasing amount of SDS was found to have a maximum at [SDS] = 0.075 mol dm (-3) ( v = 0.071 mm s (-1)), after which the speed decreased to 0.043 mm s (-1) at [SDS] = 0.5 mol dm (-3), which is below the value found in the absence of the surfactant ( v = 0.055 mm s (-1)). The response of the oscillatory BZ system to the addition of SDS has been ascribed to two different causes: (a) the peculiar capability of the organized surfactant assemblies to affect the reactivity by selectively sequestering some key reacting species and (b) the modifications induced by SDS on the physical properties of the medium. These hypotheses have been corroborated by performing spectrophotometric investigations on the stirred BZ system. Complementary viscosity measurements gave useful hints for the clarification of the surfactant role.

Effects of non-ionic micelles on transient chaos in an unstirred Belousov–Zhabotinsky reaction

The behaviour of the Ce(IV)-catalyzed Belousov-Zhabotinsky (BZ) system has been monitored at 20.0 degrees C in unstirred batch conditions in the absence and presence of different amounts of the non-ionic micelle-forming surfactants hexaethylene glycol monodecyl ether (C10E6) and hexaethylene glycol monotetradecyl ether (C14E6). The influence of the non-ionic surfactants on both the kinetics of the oxidation of malonic acid (MA) by Ce(IV) species and the behaviour of the BZ reaction in stirred batch conditions has also been studied over a wide surfactant concentration range. The experimental results have shown that, in unstirred batch conditions, at surfactant concentrations below the critical micelle concentration (c.m.c.) no significant change in the dynamics of the Belousov-Zhabotinsky system occurs. Beyond this critical concentration the presence of micelles forces the BZ system to undergo a chaos-->quasi-periodicity-->period-1 transition. Thus, the surfactant concentration has been considered as a bifurcation parameter for a Ruelle-Takens-Newhouse (RTN) scenario. Addition of increasing amounts of non-ionic surfactants has no significant effect on the kinetics of the reaction between MA and Ce(IV), but it influences the oscillatory parameters of the stirred BZ system. At surfactant concentrations below the c.m.c. all the oscillatory parameters are practically unaffected by the presence of surfactant, while beyond this critical value the induction period is the same as in aqueous solution but both the oscillation period and the duration of the rising portion of the oscillatory cycle decrease. In all cases, the experimental trends have been ascribed to the enhancement in the medium viscosity due to the presence of micelles.

Kinetics of oxidation of p-methylmandelic acid by cerium(IV) in aqueous sulphate media

Abstract The kinetics of the oxidation of p -methylmandelic acid (MMA) by ceric sulphate has been studied in aqueous sulphuric acid solutions and in H 2 SO 4 HClO 4 and H 2 SO 4 LiHSO 4 mixtures. In these mixtures the observed rate law is: −d[Ce(IV)] /dt = k[MMA][Ce(VI)], where k = k 1 + k 2 /[(1 + a ) 2 [HSO 4 − ] 2 = k 1 + k 2 /[(1 + 1/ a ) 2 [SO 2− 4 ] 2 ( a is constant). The temperature dependence of the oxidation rate and the effect of ionic strength have been also investigated. It is pointed out that the kinetic data obtained for the solutions containing sulphuric acid alone are to be confidered with caution because of the linear dependence existing between the stoichiometric molar concentration of H 2 SO 4 and both the HSO 2− 4 , SO 2− 4 and H + concentrations and the ionic strength over the acid range 0.050–3.97 M .

Kinetics of the reaction between substituted thioureas and the palladium (II) aquocomplex of 1, 1, 7, 7-tetraethyldiethylenetriamine in water and in aqueous micellar solutions

The kinetics of water replacement in the complex [Pd(Et4dien)(H2O)](NO3)2, where Et4 dien = Et2N[CH2]2NH[CH2]2 NEt2, by thiourea and its methyl, ethyl, n-butyl, N, N′-dimethyl, N, N′-diethyl, N, N′-di-n-butyl and p-tolyl derivatives have been studied at 25 °C and ionic strength 0.03 mol dm–3 in water and in the presence of cationic (CTAN) and anionic (SDS) micelle-forming surfactants. All the reactions studied are first order with respect to both reactant species. The reaction rates in water depend on both the steric hindrance and the inductive effects of the alkyl groups present in the parent thiourea molecule. The substitution reactions are inhibited by the CTAN micelles and take place in the bulk water, the nucleophiles being partitioned between the aqueous and micellar pseudo-phases. Anionic SDS micelles accelerate the overall reactions, which occur in the micellar phase with the same mechanism as in water. The estimated binding constants of the nucleophiles are larger with CTAN micelles than with SDS micelles and depend on the hydrophobic nature and number of the alkyl groups present in the unsubstituted thiourea. In the case of the CTAN surfactant the standard transfer free energies of the thioureas from water in the micelle suggest that binding of the solubilizates occurs in the palizade layer of the micelle.

Kinetic evidence for the solubilization of pyridine-2-azo-p-dimethylaniline in alkanediyl-α,ω-bis(dimethylcetylammonium nitrate) surfactants. Role of the spacer chain length

The incorporation of the bidentate ligand pyridine-2-azo-p-dimethylaniline (PADA) into micellar aggregates of the dimeric cationic surfactants propanediyl-, hexanediyl- and dodecanediyl-α,ω-bis(dimethylcetylammonium nitrate) (16-3-16,2NO3−, 16-6-16,2NO3− and 16-12-16,2NO3−, respectively) has been studied at 25°C by examining the kinetics of the complexation reaction of the Ni(II) ion with this ligand. For comparison, cetyltrimethylammonium nitrate (CTAN), which can be considered as the “monomeric” surfactant of 16-3-16,2NO3−, has also been used. The kinetic data have shown that, for 16-3-16,2NO3− and CTAN, at a surfactant concentration below the critical micelle concentration (cmc) the rate of the complex formation reaction does not significantly depend on the surfactant concentration, while it slightly decreases in the presence of the other two gemini surfactants. Beyond this critical value, in all cases examined, the rate constant is conspicuously inhibited by the presence of surfactant. The results below the cmc have been explained in the light of conductometric measurements, which have evidenced that both 16-6-16,2NO3− and 16-12-16,2NO3− form premicellar aggregates while 16-3-16,2NO3− and CTAN do not. The kinetic data above the cmc conform to a reaction mechanism that implies partitioning of the ligand only between the aqueous and the micellar pseudo-phases. The quantitative analysis of the kinetic data allows us to estimate the binding of PADA to the cationic micellar aggregates used. Solubilization of PADA in the micelles markedly depends on the nature of the surfactant used and, in particular, decreases on either increasing the spacer chain length or changing the surfactant type, namely conventional or gemini. These trends have been ascribed to the change in the shape of the micellar aggregates and, consequently, the hydrophobic character of the micelles, which can be modulated either by insertion of the spacer in the micellar interior or by using a conventional surfactant. The incremental free energy of transfer of a methylene group in the spacer chain from the aqueous to the micellar pseudo-phase has also been determined. The present data evidence that binding of PADA to micellar aggregates is primarily governed by hydrophobic interactions and the solubilization capability of gemini aggregates is superior to that of conventional micelles.

Kinetic evidence for the effects of urea on the properties of aqueous micellar solutions of sodium dodecyl sulfate

The kinetics of the alkaline hydrolysis of the chloropentaamminecobalt(III) cation have been studied at 25.0 °C in water and in aqueous micellar solutions of sodium dodecyl sulfate (SDS) in the presence of varying amounts of urea. The rate of the alkaline hydrolysis with and without added urea is strongly retarded by the SDS surfactant above the critical micelle concentration (c.m.c.) as a result of binding of the cobalt complex to the anionic micellar surface. The effects of added urea on the c.m.c. of the surfactant have been demonstrated kinetically. Moreover, urea addition causes an enhancement of the overall hydrolysis rate both in water and, to a larger extent, in the aqueous micellar solutions, and a decrease in the estimated binding constant of the cobalt complex. The observed effects of urea on the hydrolysis rate and on the other parameters derived kinetically have been discussed and related to changes in the properties of the SDS micellar aggregates, such as reduction of the micelle surface charge density on urea addition.

Interaction of the chloropentaamminecobalt(III) cation with mixed micelles of anionic and non-ionic surfactants: a kinetic study

Alkaline hydrolysis rates of the cholropentaamminecobalt(III) cation have been studied at 25.0 °C and an ionic strength of 0.05 mol dm–3 in aqueous mixed micelles composed of anionic sodium dodecyl sulfate (SDS) and non-ionic n-dodecylpenta(oxyethylene glycol) monoether (C12E5) surfactants over a wide range of total surfactant concentration (Ct) and SDS mole fraction (α). The critical micelle concentrations (c.m.c.s) of mixtures of the two surfactants over the entire α range have also have been determined and found to follow the regular solution model for non-ideal mixing. The kinetic results have shown that interactions of the cationic cobalt complex with the mixed micelles cause a decrease of the reaction rate with increasing Ct(at constant α) and α(at constant Ct). The estimated binding constant of the complex decreases markedly as α decreases as a result of the reduction of the charge density at the mixed micellar surface. The present findings are compared with those obtained previously concerning the effect of urea and its alkyl derivatives upon the rate of the same hydrolysis reaction in SDS micellar solutions.

Interactions of tetradecyldimethylaminoxide with polyacrylic and polymethacrylic acids in aqueous solution

Abstract The interactions between the zwitterionic surfactant tetradecyldimethylaminoxide (C14DMAO) and the polyelectrolytes polyacrylic (PAA-205 and PAA-250) or polymethacrylic (PMA-100) acids have been investigated in aqueous solution. The physico-chemical properties of these systems have been monitored by measuring the solution pH, electric birefringence, zero-shear viscosity and density for increasing surfactant concentrations. The experimental results have shown that significant change in the physico-chemical properties of the polyacid/surfactant system occurs only in a narrow pH range and when the amphiphile forms rod-like micelles. Moreover, they clearly reveal the important role played by both hydrogen bonding and hydrophobic forces in governing polyacid–surfactant interactions. The change in the physico-chemical properties of the polyacid solutions upon addition of C14DMAO has been attributed to both the conformational transition of the polyelectrolyte chains and the polyacid/rod-like micelles complex formation. The suggestion is made that the C14DMAO induces a progressive conformational transition in the polyelectrolyte chains much akin to that produced by NaOH.

Photocatalytic degradation of toluene in aqueous suspensions of polycrystalline TiO2 in the presence of the surfactant tetradecyldimethylamino-oxide

The heterogeneous photocatalytic method has been used for carrying out the toluene oxidation in synthetic aqueous solutions containing also a zwitterionic surfactant, i.e. tetradecyldimethylamino-oxide (C 14 DMAO). A batch photoreactor with immersed lamp and two kinds of polycrystalline TiO 2 catalysts were used. The results indicate a substantial increase of the toluene degradation rate in the presence of the surfactant; the complete photo- oxidation of toluene was achieved in a few hours of irradiation at the used experimental conditions; at higher irradiation times also C 14 DMAO was completely degraded. The main intermediates of toluene degradation were p-cresol (4-methyiphenol) and benzaldehyde. Depending on the used catalyst, benzyl alcohol, benzoic acid, hydroquinone, trans, trans muconic acid and pyrogallol (1,2,3 trihydroxybenzene) were also detected.

Effects of non-ionic and anionic micellar aggregates on the kinetics of the iridium(IV) oxidation of monosubstituted malonic acids

The kinetics of the electron-transfer reaction between the hexachloroiridate(IV) ion and phenylmalonic acid have been studied in the absence and in the presence of non-ionic (Triton X-100, Triton X-102, Triton X-165, C10E6, C12E6, C14E6, C12E10, C12E23) and anionic (sodium dodecyl sulfate, sodium tetradecyl sulfate) micelle-forming surfactants over a wide acidity and surfactant concentration range at 25.0 °C and ionic strength 0.05 mol dm–3. A number of kinetic measurements using benzylmalonic acid as reducing agent have also been performed. The redox reactions exhibit a first-order dependence on both the reductant and oxidant concentrations. The reaction rate in the absence of surfactants decreases as the hydrogen-ion concentration increases. In the presence of non-ionic micelles the oxidation process is retarded by an increase in either the acidity or the surfactant concentration. The inhibitory effects of these micellar aggregates upon the reaction rate depend on the hydrophobic micellar environment and the number of the ethoxy groups of the non-ionic surfactants. In the presence of anionic micelles a retarding effect has been observed only with sodium tetradecyl sulfate for the benzyl derivative. Binding constants of the neutral reductant species have been estimated by kinetic measurements. A mechanism for the electron-transfer reactions examined is suggested.