Akihisa Shioi

Effect of solid walls on spontaneous wave formation at water/oil interfaces.

The regulation of spontaneous waves at water/oil interfaces was investigated, focusing on effects of materials and sizes of containers. Trimethylstearylammonium chloride was dissolved in an aqueous phase. Nitrobenzene with potassium iodide and iodine was used as an organic phase. Rotation of interfacial waves with almost triangular shape was observed only in containers made of glass. The nature of interfacial waves is sensitive to container size. There was no interfacial wave in PFA (Teflon) containers. However, when a glass plate was soaked vertically to the interface, oscillation of contact angles of water/oil interfaces to glass plates was observed. The oscillation generated wave propagation along the plate. Dynamic interfacial tension was measured by Wilhelmy method and the pendant drop technique. Results with the Wilhelmy method in small glass containers exhibited spontaneous oscillation. However, oscillations in dynamic interfacial tension were not observed for other cases, i.e., the Wilhelmy method for large glass containers, for PFA containers, and for the pendant drop technique. It was concluded that all nonlinear behavior such as wave generation and apparent tension oscillation could be attributed to the effect of the sidewalls of container on the adsorption/desorption kinetics of the surfactant. We propose a possible scenario which can explain all of the qualitative features of the present experimental findings.

Mechanism of protein solubilization in sodium bis(2-ethylhexyl) sulfosuccinate water-in-oil microemulsion

Abstract We selected α-chymotrypsinogen A (CTN) as a model protein, and the mechanism for protein solubilization in a sodium bis(2-ethylhexyl) sulfosuccinate (AOT) water-in-oil microemulsion was investigated using the two-phase transfer method. The extraction of CTN by the microemulsion is composed of two processes; a fast extraction and a subsequent slow back-extraction process. The adsorption of AOT onto the surface of CTN through the electrostatic interaction between CTN cations and AOT anions converts CTN from a hydrophilic to a hydrophobic state. This adsorption is responsible for the fast extraction process. The adsorption of AOT due to a hydrophobic interaction with CTN in turn makes the CTN-surface hydrophilic. The slow back-extraction process is attributed to the adsorption due to the latter interaction. The presence of two adsorption modes is ascertained by cation-exchange and hydrophobic-interaction chromatography, and spectroscopic measurements. In the fast extraction stage, CTN is extracted to the microemulsion accompanied with large amounts of AOT and water when the CTN-to-AOT mole ratio is comparatively large, suggesting the formation of large clusters composed of many AOT, water and CTN molecules. With a decrease in the ratio, the large cluster is divided into finer aggregates. The distribution of CTN between the microemulsion and the aqueous phase was examined by altering the salinity and organic solvent species of the microemulsion phase in the fast extraction stage. Both effects of the salinity and the solvent species were ascribed to the size effect of the microemulsion droplets. The weak interaction between the microemulsion droplet surface and the tails of the AOT chian adsorbed on the protein plays an essential role in the solubilization of the guest proteins. Since the energy of the droplet deformation required for uptake of the guest protein exceeds this weak interaction, the droplet has the ability to recognize the size of the guest molecules.

Catalytic micromotor generating self-propelled regular motion through random fluctuation.

Most of the current studies on nano∕microscale motors to generate regular motion have adapted the strategy to fabricate a composite with different materials. In this paper, we report that a simple object solely made of platinum generates regular motion driven by a catalytic chemical reaction with hydrogen peroxide. Depending on the morphological symmetry of the catalytic particles, a rich variety of random and regular motions are observed. The experimental trend is well reproduced by a simple theoretical model by taking into account of the anisotropic viscous effect on the self-propelled active Brownian fluctuation.

Selective separation of trypsin from pancreatin using bioaffinity in reverse micellar system composed of a nonionic surfactant.

Selective separation of trypsin from a mixture involving many kinds of contaminating proteins, i.e., pancreatin, was achieved using trypsin inhibitor immobilized in the reverse micelles, which were composed of a nonionic surfactant, tetra-oxyethylene monodecylether. To determine the efficient operations throughout the whole separation process we examined the operating conditions, which affect the immobilization efficiency of trypsin inhibitor and also the forward and backward extractions of trypsin. Fifty percent of the recovery of trypsin from pancreatin was realized with no loss of activity of the recovered trypsin.

Autonomous motion of vesicle via ion exchange.

The autonomous motion of vesicle is observed in a simple chemical system. A vesicle composed of didodecyldimethylammonium bromide DDAB breaks down by ion exchange from Br(-) to I(-). When an electrolyte is supplied to vesicles, some of them begin to move after an induction period. They continue to move, leaving behind the reaction products on the trail. The ion exchange decreases the vesicle size, and smaller vesicles remain after the motion. We examine the characteristics of this motion. The surface tension of the DDAB-containing aqueous phase depends on the KI concentration. Considering this result carefully, we conclude that vesicles can move when the ion exchange from Br(-) to I(-) proceeds irreversibly. Then, inhomogeneity in the vesicle membrane develops because of the coagulating nature of the product, didodecyldimethylammonium iodide (DDAI), which is sparingly soluble in water. Inhomogeneous properties of vesicle membranes are then generated, which induce surface transport of the reaction product and flow in the water pool. As a result, a couple of convection rolls appear in the water pool of the vesicle. The convection rolls drive vesicle motion. A simple model for the semiquantitative description is proposed.

Ion-Selective Marangoni Instability Coupled with the Nonlinear Adsorption/Desorption Rate

An oil/water interface containing bis(2-ethylhexyl)phosphate and Ca(2+) or Fe(3+) exhibits spontaneous Marangoni instability associated with the fluctuation in interfacial tension. This instability rarely appears for oil/water systems with Mg(2+), Sr(2+), Ba(2+), Cu(2+), or Co(2+). The same ion selectivity is observed for n-heptane and nitrobenzene despite their significant differences in density, viscosity, and the dielectric constant of oil. We studied this instability under acidic pH conditions to avoid the neutralization reaction effects. The result of the equilibrium interfacial tension and the extraction ratio of cations indicates that a large number of oil-soluble complexes form at the interfaces of Ca(2+)-containing systems and probably for Fe(3+)-containing systems. The results obtained by oscillating drop tensiometry and Brewster angle microscopy indicate that desorption, rather than adsorption, is more significant to the onset of instability and that the resulting complex tends to form aggregates in the interface. This aggregation gives the nonlinear desorption rate of the oil-soluble complex. Then, exfoliation of the aggregating matter occurs, which triggers the Marangoni instability. The induced convection removes the oil-soluble complex accumulated at the interface, creating a renewed interface, which is necessary for the successive occurrence of the Marangoni instability. For the other cations, the oil-soluble compounds are insignificant, and they rarely form aggregates. In such cases, adsorption/desorption proceeds without instability.

The evolution of spatial ordering of oil drops fast spreading on a water surface

The design of dynamically self-assembled systems is of high interest in science and technology. Here, we report a unique cascade in the self-ordering of droplets accompanied by a dewetting transition. The dynamic self-emergent droplets are observed when a thin liquid layer of an immiscible fluorocarbon oil (perfluorooctyl bromide, PFOB) is placed on a water surface. Due to the gradual evaporation of PFOB, a circular PFOB-free domain appears as a result of a local dewetting transition. A circular pearling structure is generated at the rim with the growth of the dewetting hole. As the next stage, linear arrays of droplets are generated in a radial manner from the centre of the hole. These one-dimensional arrangements then evolve into two-dimensional hexagonal arrays of microdroplets through collective rhythmical shrinking/expanding motions. The emergence of such dynamic patterns is discussed in terms of the nonlinear kinetics of the dewetting transition under thermodynamically dissipative conditions.

Ion-selective Marangoni instability—Chemical sensing of specific cation for macroscopic movement

Spontaneous motion and tension oscillation of an oil/water interface responding to specific cation Ca(2+) or Fe(3+) were observed when the oil phase containing the anionic surfactant bis(2-ethylhexyl) phosphate came in contact with the cation-containing water. Both the dynamics were the results of Marangoni instability. Complex formation between the anionic surfactant and cation caused the instability. The results showing the level of cation extraction and degree of interfacial tension revealed that the surfactant-cation combination forms an oil-soluble complex with reduced surface activity. Brewster angle microscopy indicated that molecules of the complex tend to aggregate at the interface. This aggregation affected the desorption rate of the complex. We were able to generate ion-selective instability by imposing mechanical and electrochemical perturbations to the interface at equilibrium. The results from these efforts suggested that the aggregation is a type of thermodynamic transition and is required for the onset of instability: Desorption probably occurs as an exfoliation of the aggregated complex, which generates the gradient of interfacial tension. For the standard experiment of biphasic contact, two neighboring interfacial flows compress the local interface between them. We considered that this compression provides mechanical work to the local interface, resulting in desorption of the aggregates and occurrence of instability. Both complex formation and aggregation are possible in the presence of the specific cation. The interface detects the cation via the chemical and thermodynamic processes in order to develop the macroscopic movement, a form of biomimetic motion of the oil/water interface.

Autonomously Moving Colloidal Objects that Resemble Living Matter

The design of autonomously moving objects that resemble living matter is an excellent research topic that may develop into various applications of functional motion. Autonomous motion can demonstrate numerous significant characteristics such as transduction of chemical potential into work without heat, chemosensitive motion, chemotactic and phototactic motions, and pulse-like motion with periodicities responding to the chemical environment. Sustainable motion can be realized with an open system that exchanges heat and matter across its interface. Hence the autonomously moving object has a colloidal scale with a large specific area. This article reviews several examples of systems with such characteristics that have been studied, focusing on chemical systems containing amphiphilic molecules.

Rhythmic shape change of a vesicle under a pH gradient

A vesicle that exhibits oscillatory structural changes under a pH gradient is reported. This vesicle is composed of oleic acid and sodium oleate and appears to exhibit a stomatocyte-type or a seemingly double spherical shape initially. The inner water is located at one side of the double spherical structure (i.e., the swollen part). The other side of the vesicle is nearly free of water. When the diffusion of hydroxide anions causes a pH gradient, the double spherical structure rotates such that its swollen part faces the higher-pH side. After this event, a small hole in the center of the swollen part begins to open. The vesicle changes to a torus-like structure: the swollen part is formed at the periphery, and a disk-like part is formed in the central part of the torus. The shape change proceeds via the torus-like shape, and the vesicle closes again to form a double spherical shape with a curvature opposite to the initial curvature. At this stage, the swollen part faces the lower-pH side. After this event, the vesicle rotates back to its initial state, where the swollen part faces the higher-pH side. This rhythmic transformation repeats numerous times. The size of the vesicle remains nearly constant during these numerous transformations. Thus, this vesicle continuously generates mechanical work from a pH gradient. In this study, experimental observations under an optical microscope and a model for the transformation are reported.