Renato Lombardo

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.

Adsorption of triblock copolymers and their homopolymers at laponite clay/solution interface. Role played by the copolymer nature.

The adsorption thermodynamics of copolymers, based on ethylene oxide (EO) and propylene oxide (PO) units, at the laponite (RD) clay/liquid interface was determined at 298 K. The copolymer nature was tuned at molecular level by changing the hydrophilicity, the architecture and the molecular weight (Mw) keeping constant the EO/PO ratio. Polyethylene (PEGs) and polypropylene (PPGs) glycols with varying Mw and their mixture were also investigated to discriminate the role of the EO and the PO segments in the adsorption process. Enthalpies of transfer of RD, at fixed concentration, from water to the aqueous macromolecule solutions as functions of the macromolecule molality were determined. They were treated quantitatively by means of a model based on two equilibria: (1) one-to-one binding between the macromolecule and the site on the solid and (2) two-to-one binding following which one macromolecule interacts with another one adsorbed onto the solid. The good agreement between the equilibrium constants obtained from calorimetry and those determined from kinetic experiments confirmed the reliability of the experimental and theoretical approaches. Almost all of the systems investigated are highlighted by the one-to-one binding; the L35 and 10R5 systems present both equilibria. The insights provided by the thermodynamics of adsorption of their homopolymers onto RD were fruitful in obtaining detailed information on the nature of the forces involved between RD and the copolymers. The data obtained in the present work clearly evidenced that for comparable polymer Mw, PPG is more suitable in building up a steric barrier around the RD particles and, indeed, exhibits several advantages and no drawbacks. Moreover, the parent copolymers may properly functionalize the RD surface by exploiting both their high affinity to the solid surface and the ability to self-assemble onto it as L35 and 10R5 clearly showed.

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.

Effect of pseudo-gravitational acceleration on the dissolution rate of miscible drops

The effect of pseudo-gravitational acceleration on the dissolution process of two phase miscible systems has been investigated at high acceleration values using a spinning drop tensiometer with three systems: 1-butanol/water, isobutyric acid/water, and triethylamine/water. We concluded that the dissolution process involves at least three different transport phenomena: diffusion, barodiffusion, and gravitational (buoyancy-driven) convection. The last two phenomena are significantly affected by the centrifugal acceleration acting at the interface between the two fluids, and the coupling with the geometry of the dissolving drop leads to a change of the mass flux during the course of the dissolution process.

The apparently anomalous effects of surfactants on interfacial tension in the IBA/water system near its upper critical solution temperature

We studied the effect of anionic, cationic, and nonionic surfactants on the interfacial tension between the two phases of the isobutyric acid/water system using spinning drop tensiometry. It has been found that interfacial tension decreases with increasing concentration of the surfactant in the case of sodium dodecyl sulfate (SDS) and of dodecyltrimethylammonium chloride (DTAC). However, in the case of Triton X-100 an increase of surfactant concentration leads to an increase of the interfacial tension. Such results are consistent with the upper critical solution temperature (UCST) in the presence of these surfactants: the UCST decreases with increasing SDS and DTAC concentrations while it increases in the case of Triton X-100. This apparently anomalous effect has been explained by considering the molecular structure of Triton X-100, which possesses a short polymeric chain that promotes its partitioning toward one of the two phases making them less compatible and thus raises the UCST and the interfacial tension.