Observations of the Effect of Anionic, Cationic, Neutral, and Zwitterionic Surfactants on the Belousov−Zhabotinsky Reaction

Measurement of a Bubble-Free Chemical Oscillator Using QCMs Treated with Self-Assembled Monolayers

The oscillation of a 1,4-cyclohexanedione-bromate (CHD-bromate) system was investigated by using a quartz crystal microbalance (QCM) without or with a self-assembled monolayer (SAM), where gold and platinum were used for QCM electrodes and SAMs were composed of HS(CH2)11CH3, HS(CH2CH2O)5H, and HS(CH2CH2O)5CH3. The CHD-bromate system is well-known as a bubble-free oscillator and oscillates without or with a catalyst. In the CHD-bromate system without a catalyst, the oscillation of a resonant frequency shift (ΔF) of the QCM was observed in the Au-electrode QCMs without a SAM or with SAMs formed from HS(CH2)11CH3 and HS(CH2CH2O)5H. On the other hand, the HS(CH2CH2O)5CH3 SAM suppressed the ΔF oscillation. The results revealed that in the CHD-bromate system without a catalyst, hydrophobic CH3 terminal and helical conformation were important to prevent nonspecific adsorption of substances on a gold surface and its dissolution. In the CHD-bromate system with a catalyst (ferroin), the ΔF oscillation was observed in the Au-electrode QCM with the HS(CH2CH2O)5CH3 SAM. The results suggest evidence that the change of the solution viscosity and density led to the ΔF oscillation. These results for Au-electrode QCMs were also corroborated by those for Pt-electrode QCMs.

Halloysite nanotubes as nanoreactors for heterogeneous micellar catalysis

HYPOTHESIS

Electrostatic attractions between the anionic head group of sodium alkylsulphates and the positively charged inner surface of halloysite nanotubes (HNTs) drive to the formation of tubular inorganic micelles, which might be employed as nanoreactors for the confinement of non polar compounds in aqueous media. On this basis, sodium alkylsulphates/halloysite hybrids could be efficient nanocatalysts for organic reactions occurring in water.

EXPERIMENTS

Sodium decylsulphate (NaDeS) and sodium dodecylsulphate (NaDS) were selected for the functionalization of the halloysite cavity. The composition, the structure and the surface charge properties of the hybrid nanotubes were determined. The actual formation of inorganic micelles was explored by studying the microviscosity and polarity characteristics of the surfactant modified nanotubes through fluorescence spectroscopy experiments using DiPyme as probe. The performances of the sodium alkylsulphates/halloysite composites as micellar catalysts for the Belousov-Zhabotinsky (BZ) reaction were investigated.

FINDINGS

The halloysite functionalization with sodium alkylsulphates generated the formation of hydrophobic microdomains with an enhanced microviscosity. Compared to the surfactant conventional micelles, the functionalized nanotubes induced larger enhancements on the rate constant of the BZ reaction. This is the first report on the surfactant/halloysite hybrids showing their efficiencies as reusable nanocatalysts, which are dependent on their peculiar microviscosity and polarity properties.

Modulation of the Belousov-Zhabotinsky Reaction with Lipid Bilayers: Effects of Lipid Head Groups and Membrane Properties

The Belousov-Zhabotinsky (BZ) reaction is an oscillating reaction due to periodic oscillations that happen in the concentration of some intermediates. Such systems can be applied together with hydrophobic membranes to create an autonomous behavior in artificial systems. However, because of a complex set of reactions happening in such systems, the interferences caused by hydrophobic membranes are not easily understood. In this study, we tested lipid membranes composed of trimethylammonium-propane (TAP) and phosphate (PA) lipids in an attempt to break down how the polar region of phosphatidylcholine (PC) lipid membranes affect the BZ reaction. According to our findings, the trimethylammonium group and membrane fluidity are crucial to change the frequency of oscillations in the reaction. In addition, the results also indicate a possible complexation of cerium ions with membranes with a phosphate head group.

Changes Caused by Liposomes to the Belousov-Zhabotinsky Reaction

The Belousov-Zhabotinsky (BZ) reaction has been applied to give autonomous dynamic behaviors to artificial systems. This reaction is conducted in an aqueous system, but it produces some hydrophobic intermediates, such as bromine. On the basis of previous works about reactions in the lipid bilayer, we investigated how liposome membranes (water-oil interface) affect the BZ reaction. Herein diacylglycerophosphocholine (PC) molecules with a variety of hydrocarbon tails were selected as components of liposomes, and the BZ reaction in the presence of the liposomes was characterized. As a result, membrane fluidity was the main characteristic leading to changes in the reaction behavior. The decrease of the frequency of oscillations was relevant to membrane fluidity, suggesting the interaction of bromine species in the hydrophobic site of the liposomes. In addition, the heterogeneous membrane (s_o+l_d) of DMPC showed a fast decrease in the amplitude of oscillations. Conclusively, characteristics of the hydrophobic environment play a role in the reaction.

Effect of Surfactants on the Belousov-Zhabotinsky Reaction with Ninhydrin as Organic Substrate

The effects of sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB) and Brij 35 on the oscillations of the cerium-catalyzed Belousov-Zhabotinsky (B-Z) reaction with ninhydrin as the organic substrate at 30 °C were described by following the change in absorbance of the reaction mixtures at 357 nm. The behavior of the oscillatory parameters was determined: (i) the induction period (IP) increases first and then decreases with increasing the concentration of all used surfactants (ii) the number of oscillations decreases with the SDS concentration, while for Brij 35 and CTAB it remains nearly constant, and (iii) the mean amplitude of the oscillations decreases with increasing concentration of CTAB and SDS, while it varies irregularly with that of Brij 35. The ability of micelles to selectively shield ions and molecules may explain their effect on the oscillation parameters of the studied B-Z system.

Interfacial hydrodynamic instabilities driven by cross-diffusion in reverse microemulsions

When two microemulsions are put in contact in the gravity field along a horizontal contact line, cross-diffusion can trigger the transport of one species in the presence of a gradient in concentration of another species. We show here theoretically that such cross-diffusion effects can induce buoyancy-driven convective instabilities at the interface between two solutions of different compositions even when initially the less dense solution lies on top of the denser one. Two different sources of convective modes are identified depending whether positive or negative cross-diffusion is involved. We evidence the two predicted cross-diffusion driven instabilities experimentally using a two-layer stratification of Aerosol-OT (AOT) water-in-oil microemulsions solutions with different water or AOT composition.

Recent aspects of self-oscillating polymeric materials: designing self-oscillating polymers coupled with supramolecular chemistry and ionic liquid science

Herein, we summarise the recent developments in self-oscillating polymeric materials based on the concepts of supramolecular chemistry, where aggregates of molecular building blocks with non-covalent bonds evolve the temporal or spatiotemporal structure.

Surfactant-induced gradients in the three-dimensional Belousov-Zhabotinsky reaction

Scroll waves are prominent patterns formed in three-dimensional excitable media, and they are frequently considered highly relevant for some types of cardiac arrhythmias. Experimentally, scroll wave dynamics is often studied by optical tomography in the Belousov-Zhabotinsky reaction, which produces CO(2) as an undesired product. Addition of small concentrations of a surfactant to the reaction medium is a popular method to suppress or retard CO(2) bubble formation. We show that in closed reactors even these low concentrations of surfactants are sufficient to generate vertical gradients of excitability which are due to gradients in CO(2) concentration. In reactors open to the atmosphere such gradients can be avoided. The gradients induce a twist on vertically oriented scroll waves, while a twist is absent in scroll waves in a gradient-free medium. The effects of the CO(2) gradients are reproduced by a numerical study, where we extend the Oregonator model to account for the production of CO(2) and for its advection against the direction of gravity. The numerical simulations confirm the role of solubilized CO(2) as the source of the vertical gradient of excitability in reactors closed to the atmosphere.

The Effect of Perturbants on the Uncatalyzed Bromate Oscillator

The effect of cationic (CTAN), anionic (SDS) and nonionic surfactants (Brij-30 and Triton X-100) as well as of tert-butanol (t-BuOH) on the uncatalyzed bromate oscillator with phenol as substrate was investigated at 25 ± 0.1oC in non-stirred batch conditions. Addition of the surfactants influenced the oscillatory parameters: a slight increase in induction period of the first series of oscillations, a significant increase in induction period of the second series of oscillations as well as a gradual decrease in oscillation numbers of both series until complete disappearance at a certain surfactant concentration. The changes in the oscillatory parameters have been ascribed to the solubilization of phenol and of bromination products in the micelles, to the inhibition of bromination of aromatic substrate due to the bromine solubilization and to the catalytic effect of charged micelle surface. The changes in the oscillatory behaviour with increasing alcohol concentration have been discussed in terms of its ability to act as a radical scavenger and to affect the 3D-structure of water.