Electrochemistry at micro- and nanoscopic liquid/liquid interfaces

In this tutorial review, we will briefly introduce the history and basic concepts of micro- and nanoscopic liquid/liquid interfaces (size from nm to μm) in electrochemical studies of charge (electron and ion) transfer reactions at soft molecular interfaces. Their advantages and problems are usually compared with those of conventional liquid/liquid interfaces (size from mm to cm); and with solid/electrolyte interfaces. Three methods of fabrication of micro-liquid/liquid interfaces and one approach to support a nano-liquid/liquid interface are surveyed. The experimental and theoretical aspects are discussed along with possible approaches to characterize these micro- and nanoscopic liquid/liquid interfaces, and the methods to modify them with new functionality. Unique examples of applications of electrochemistry at micro- and nanoscopic liquid/liquid interfaces are provided. Some novel and potential research interests in the future in this field are discussed.     

Pulsed laser ablation in liquid for micro-/nanostructure generation

Pulsed laser ablation in liquid is an approach for micro-/nanostructure generation directly from bulk materials. It has grown rapidly as a research field of photochemistry and physical chemistry in the last decade, and represents a combinatorial library of constituents and interactions, but the understanding of this library is still insufficient. This review attempts to build up a comprehensive mechanistic scenario of pulsed laser ablation in liquid and illustrate the underlying principles to micro-/nanostructure generation. Various structures produced by this method have been summarized that provide prototypes for potential applications in sensing, optoelectronics, and biomedicine, etc.     

Controlled, rapid deposition of structured coatings from micro- and nanoparticle suspensions

The objective of the study was to develop the operational basis for rapid and controlled deposition of crystal coatings from particles of a wide size range. We deposited such structured coatings by dragging with constant velocity a small volume of liquid confined in a meniscus between two plates. Two types of structured coatings were characterized: latex colloidal crystals and thin layers from metallic nanoparticles. The crystal deposition was sped up by use of preconcentrated suspensions. Crystal coatings larger than a few square centimeters were deposited in minutes from aqueous suspension volumes of approximately 10 microL. The governing mechanism of crystal deposition is convective assembly at high volume fractions. The two major process parameters that allow control over the coating thickness and structure were the deposition speed and particle volume fraction. The evaporation rate was not found to affect the process to a large extent. A volumetric flux balance was used to relate the deposition parameters to coating structure and properties. Operational "phase" diagrams were constructed, relating the crystal layer thickness and packing symmetry to the process parameters. These diagrams could be instrumental in transforming the convective colloidal deposition into a robust scaleable technology.     

Tensions at liquid interfaces: a general filter for the separation of micro-/nanoparticles

In this letter, we put forward a new strategy for the separation of micro-/nanoparticles with different sizes and densities by using the tensions at liquid interfaces. The interactions between particles and a liquid-liquid interface have been analyzed. Furthermore, we applied our strategy to the separation of two size-distributed Cu2O particles by using the water and n-pentanol interface, which demonstrates the feasibility of the proposed separation method.     

Formation of mesoscopic silver particles at micro- and nano-liquid/liquid interfaces

Silver particles have been deposited at externally polarised 1,2-dichloroethane (DCE)/water interfaces supported at the tip of micro- and nanopipettes. The electrochemical process involved the reduction of silver ion in the aqueous phase by an organic-phase electron donor (butylferrocene). The silver nucleation and growth process was investigated using potential step chronoamperometry, and the resulting current–time transients were analysed to extract nucleus numbers. At larger pipettes, with diameters of several micrometers, multi-particle nucleation was observed and optical microscopy provided direct evidence for metal electrodeposition at the liquid/liquid interface. For pipettes with radii of 0.5 μm or smaller, the current–time behaviour was consistent with single particle generation. Under some conditions, detachment of the particle from the liquid/liquid interface was inferred from the current–time characteristics, and it is suggested that controlled-detachment from micropipettes could provide a method for the deposition of small metal structures on surfaces.     

Modeling of diffusive transport of benzoic acid through a liquid membrane

A model of diffusive transport of benzoic acid through a liquid membrane (LM) separating two aqueous solutions, based on diffusion layers and the assumption of a steady state, has been developed and tested using experimental results. It has been found that a model with the apparent partition coefficient dependent on the concentration is able to describe the time dependence of acid concentration in LM with and without a maximum on that dependence. The quality of the model fit with the single apparent diffusion coefficient of benzoic acid is the same as the one which takes into account the diffusion of benzoic acid in different forms (undissociated and dissociated form in aqueous phase, monomer and dimer in organic phase); however, in the second case, the model becomes overparameterized. Assuming that the partition and diffusion coefficients are constant, the diffusion layer model corresponds to the model of reversible consecutive reactions. Analytical solution for such case is given. Apart from the partition equilibrium, also kinetics of partitioning was considered. It was shown that in some basic situations both cases yield identical results.     

Critical behaviour of nematic liquid crystals in oblique magnetic fields

The static critical behaviour of a bulk nematic liquid crystal sample in an oblique magnetic field is analysed. When a magnetic field is applied at a suitable angle alpha with respect to the initially homogeneous nematic director, a spatially inhomogeneous director pattern can be formed. The transition to the deformed state and the formation of walls between the domains resulting from the two equally stable configurations above the transition are studied. The width of the walls is found to diverge at the transition. The critical exponents corresponding to the transition and wall formation are shown to be characteristic of a mean field second order phase transition.     

Electrodes in stripping voltammetry: from a macro- to a micro- and nano-structured surface

A correlation between the morphology of the solid surface and electrochemical response was found in microscopic and electrochemical investigations. A shift of the oxidation potentials of metals to more negative values was observed on electrodes with microstructured surface with respect to similar processes on macrostructured electrodes. The formation of passivating films, causing reverse current and deteriorating the analytical signal, was not observed, and the performance characteristics of voltammetric procedures were improved. The experimental data indicated the increased electrochemical activity of modifying metal particles with a decrease in the particle size. As a result of the deliberate change of the surface composition and the formation of a micro- and nano-structured surface, a new generation of electrodes was developed with excellent electroanalytical characteristics.     

Confinement effect on the adsorption from a binary liquid system near liquid/liquid phase separation

The preferential adsorption of one component of a binary system at the inner surfaces of mesoporous silica glasses was studied in a wide composition range at temperatures close to liquid/liquid phase separation. Confinement effects on the adsorption were investigated by using three controlled-pore glass (CPG-10) materials of different mean pore size (10 to 50 nm). For the experimental system (2-butoxyethanol+water), which exhibits an upper miscibility gap, strong preferential adsorption of water occurs, as the coexistence curve is approached at bulk compositions, at which water is the minority component. In this strong adsorption regime the area-related surface excess amount of adsorbed water decreases with decreasing pore width, while the shift in the volume-related mean composition of the pore liquid shows an opposite trend, i.e., greatest deviation from bulk composition occurring in the most narrow pores. A simple mean-field lattice model of a liquid mixture confined by parallel walls is adopted to rationalize these experimental findings. This model reproduces the main findings of the confinement effect on the adsorption near liquid/liquid phase separation.     

Optical second harmonic generation in ferroelectric liquid crystals under oblique incidence

The second harmonic generation (SHG) in ferroelectric liquid crystals under oblique incidence was studied by using a numerical band matrix method that uses boundary conditions directly. No approximations were used except for the nondepleted fundamental wave. By matching the edge of the full-pitch band with that of the half-pitch band, the SH intensity increased significantly. The thickness (L) dependence of the SH intensity showed L ^6.93–L ^7.09 dependence without the limitation of the thickness.     

Adhesives and coatings from phenol-formaldehyde/resorcinol-formaldehyde resins

Resole phenol-formaldehyde resins were prepared and modified with the prepared resorcinol-formaldehyde resins. The optimum conditions of formulation and curing processes were studied to obtain modified wood adhesives characterized by high-tensile shear strength values. This study indicated that the more suitable conditions are equal weight ratios of phenol-formaldehyde to resorcinol-formaldehyde resins in the presence of paraformaldehyde (10 or 15 wt%) of the resin content as a curing agent at 80 or 25°C for 40 min or 100–110 days, respectively. The activation energies of the curing reactions for phenol-formaldehyde/resorcinol-formaldehyde resin samples were determined. Metallic and glass coatings from the previous pure resins and their formulated cured mixtures were prepared and evaluated as varnishes or paints. Copyright © 1998 John Wiley & Sons, Ltd.     

Spatiotemporal complexity in a diffusive Brusselator model

Journal of Mathematical Chemistry - Turing–Hopf bifurcation of the diffusive Brusselator model with homogeneous Neumann boundary conditions is considered in this paper. By stability analysis,...     

Compressed exponential relaxation in liquid silicon: universal feature of the crossover from ballistic to diffusive behavior in single-particle dynamics

We report a first-principles molecular-dynamics study of the relaxation dynamics in liquid silicon (l-Si) over a wide temperature range (1000-2200 K). We find that the intermediate scattering function for l-Si exhibits a compressed exponential decay above 1200 K including the supercooled regime, which is in stark contrast to that for normal "dense" liquids which typically show stretched exponential decay in the supercooled regime. The coexistence of particles having ballistic-like motion and those having diffusive-like motion is demonstrated, which accounts for the compressed exponential decay in l-Si. An attempt to elucidate the crossover from the ballistic to the diffusive regime in the "time-dependent" diffusion coefficient is made and the temperature-independent universal feature of the crossover is disclosed.     

Surface-enhanced Raman scattering from oleate-stabilized silver colloids at a liquid/liquid interface.

Oleate-stabilized silver colloids of 5-nm-diameter were adsorbed to a toluene/water interface, and surface enhanced Raman scattering (SERS) spectra from these colloids were measured under the total internal reflection (TIR) condition. From the observed spectra, we examined the states of oleate ions and toluene molecules on silver colloids at the liquid/liquid interfacial region. The TIR-SERS spectra of oleate ions showed stronger peaks of the carboxylate group and the ethylene group than those of alkyl chains. From these results, it was found that the oleate ions were adsorbed on the silver surface in two different ways at the liquid/liquid interface; the carboxylate group adsorbed in the organic phase side, while the ethylene group adsorbed in the aqueous phase side. The shifts of the toluene in the interfacial SERS spectra were identical to those of bulk toluene, though the relative intensities among the peaks were not same. This result suggested that the toluene was adsorbed with a weak interaction, but was significantly enhanced by the local electromagnetic field at the colloid surface.     

Polymer coatings as stationary phases in high-performance liquid chromatography

Modern biochromatography demands highly sophisticated packing materials in terms of biocompatibility, (substrate-) selectivity and recovery. Polymers can be designed in a wide variety and therefore deliver solutions to specific chromatographic problems. Thus, tailor-made polymer coatings are an alternative to the classical chemically bonded stationary phases.     

Rayleigh scattering of a new lyotropic nematic liquid crystal system: crossover of propagative and diffusive behaviour

In this paper we present to our knowledge the first light-scattering measurements on a rather new lyotropic system potassium laurate, decylammonium chloride, H₂O. Relative concentrations were chosen in order to obtain the discotic N_D phase over a large temperature range. Measurements involve digital recording of the autocorrelation function resulting from selfbeating of the depolarized scattered light. They were carried out at a fixed temperature in the N_D phase range, by varying the scattering angle. A strong tendency to oscillations, superimposed on the usual relaxation signal, was found. This behaviour is quite surprising, as special precautions had been taken to minimize any eventual propagative signal. Such precautions have been tested previously, while investigating the older system potassium laurate, decanol, D₂O. Nevertheless, we have found that in the new lyotropic system, a propagative component may occur in a significant way, or even become dominant for some particular situations. Actually, the characterization of such situations is the main purpose of the present paper. Based upon a detailed analysis of photocorrelation data, it will be shown that the observed behaviour is consistent with the expected wave vector dependences of propagative and diffusive modes. Finally, a discussion is given on possible causes for the tendency to instability of lyotropic nematic liquid crystals.     

Rayleigh scattering of a new lyotropic nematic liquid crystal system: crossover of propagative and diffusive behaviour

In this paper we present to our knowledge the first light-scattering measurements on a rather new lyotropic system potassium laurate, decylammonium chloride, H₂O. Relative concentrations were chosen in order to obtain the discotic N_D phase over a large temperature range. Measurements involve digital recording of the autocorrelation function resulting from selfbeating of the depolarized scattered light. They were carried out at a fixed temperature in the N_D phase range, by varying the scattering angle. A strong tendency to oscillations, superimposed on the usual relaxation signal, was found. This behaviour is quite surprising, as special precautions had been taken to minimize any eventual propagative signal. Such precautions have been tested previously, while investigating the older system potassium laurate, decanol, D₂O. Nevertheless, we have found that in the new lyotropic system, a propagative component may occur in a significant way, or even become dominant for some particular situations. Actually, the characterization of such situations is the main purpose of the present paper. Based upon a detailed analysis of photocorrelation data, it will be shown that the observed behaviour is consistent with the expected wave vector dependences of propagative and diffusive modes. Finally, a discussion is given on possible causes for the tendency to instability of lyotropic nematic liquid crystals.     

Controllable gas/liquid/liquid double emulsions in a dual-coaxial microfluidic device

This article presents a simple and novel approach to prepare monodispersed gas-in-oil-in-water (G/O/W) and gas-in-water-in-oil (G/W/O) double-emulsions in the same dual-coaxial microfluidic device. The effects of three phase flow rates on the sizes of microbubbles and droplets and the number of the encapsulated microbubbles were systematically studied. We successfully synthesized two different types of gas/liquid/liquid (G/L/L) double emulsions with different inner structures in the same geometry by adjusting the flow rates sequentially. Mathematical models were developed to predict the size and structures of the double emulsions. This simple approach gives a new idea for preparing hollow and porous microspheres with microbubbles as the direct core/pores templates.     

Poly(ionic liquid) complex with spontaneous micro-/mesoporosity: template-free synthesis and application as catalyst support

A facile, template-free synthetic route is reported toward poly(ionic liquid) complexes (PILCs) which for the first time exhibit stable micro-/mesoporous structure. This is accomplished via in situ ionic complexation between imidazolium-based PILs and poly(acrylic acid) in various alkaline organic solvents. The PILC can be highly loaded with copper salts and can be used as a catalytic support for effective aerobic oxidation of activated hydrocarbons under mild conditions.