Chemical Oscillation with Periodic Adsorption and Desorption of Surfactant Ions at a Water/Nitrobenzene Interface

Abstract

An interface formed by two immiscible liquids is a specific area where chemical composition and physical properties steeply change on the nano-meter scale.1 Some characteristic phenomena are found: for example, adsorption and desorption of molecules, molecular recognition, 2,3 solvent extraction. 4 Chemical oscillations at a liquid/liquid interface have also been drawing attention from researchers in terms of not only the nonlinear chemistry but also for studies of biomimetic system. Since Dupeyrat and Nakache 5 first reported the electrochemical potential oscillation at the water/nitrobenzene interface in the 1970's, many kinds of chemical oscillation systems, not only water/oil two-phase systems 5-11 but also water/oil/water threephase systems 12-28 or gas/liquid interfaces, [29][30][31][32] have been developed and extensively investigated.Recently, we found a new-type interfacial oscillatory phenomenon with periodic adsorption and desorption of sodium alkyl sulfate at a water/nitrobenzene interface. [23][24][25][26] As previously reported in other interfacial oscillations, our system also shows an oscillation of electrical potential between the aqueous phase and the oil. The electrical potential is generated by unbalanced distribution of charged elements, which is not a simple phenomenon because such a situation is caused by adsorption of ions at the liquid/liquid interface, differences of concentration of the ions between the two liquid phases, and differences of ion mobility across the interface. As a result, understanding the chemical oscillation only from an electrical potential measurement is quite difficult. That is why, while most experiments showed a periodic oscillation pattern of electrical potential as a common property, the mechanisms proposed were not always similar. Agriculture and Technology, Japan Chemical oscillations with periodic adsorption and desorption of surfactant ions, alkyl sulfate ions, at a water/nitrobenzene interface have been investigated. The interfacial tension was measured with a quasi elastic laser scattering (QELS) method and the interfacial electrical potential was obtained. We found that this oscillation consists of a series of abrupt adsorptions of ions, followed by a gradual desorption. In addition, we observed that each abrupt adsorption was always accompanied by a small waving motion of the liquid interface. From the analysis of the video images of the liquid interface or bulk phase, we could conclude that each abrupt adsorption is caused by nonlinear amplification of mass transfer of ions from the bulk phase to the liquid interface by a Marangoni convection, which was generated due to local adsorption of the surfactant ions at the liquid interface that resulted in the heterogeneity of the interfacial tension. In the present paper, we describe the mechanism of the chemical oscillation in terms of the hydrodynamic effect on the ion adsorption processes, and we also show the interfacial chemical reaction with ion exchange during the ion desorption process.

Quasi-elastic Laser Scattering for Measuring Inhomogeneous Interfacial Tension in Non-equilibrium Phenomena with Convective Flows

“…QELS method is based on the angle-and frequency-resolved detection of quasi-elastic light scattering by capillary waves at an interface, 4,5,[29][30][31][32][33][34][35][36][37][38] which is a kind of Brillouin scattering by ripplons. The capillary waves, which can have different frequencies, are generated by thermal fluctuations at the liquid surface or interface.…”

“…In non-equilibrium open systems involving surfaces or interfaces, macroscopic molecular transport is observed even in isothermal processes with the concomitant spontaneous and rhythmical formation of regular spatio-temporal patterns and the motion of objects curried in the fluid. Such systems, called dissipative dynamic systems, occur, for example, under macroscopic agitation, during the flip-motion of a liquid/liquid interface, [3][4][5][6][7][8] or when objects display various modes of motion. [9][10][11][12][13][14][15][16][17][18] The BelousovZhabotinsky (BZ) reaction, which generates chemical oscillations in a bulk material, also produces sophisticated spatio-temporal patterns in an unstirred system.…”

“…These techniques often perturb the effects that they seek to measure. Quasi-elastic laser scattering (QELS) 4,5,[29][30][31][32][33][34][35][36][37][38] method involves light scattering by capillary waves, and is one of the most useful tools for measuring interfacial tension in such non-equilibrium systems. This article describes the principle behind the QELS method, and reviews some applications of the method.…”

Characteristics of a Liquid/Liquid Optical Waveguide Using Sheath Flow and Its Application to Detect Molecules at a Liquid/Liquid Interface

“…Figure 5 shows the fluorescent spectra of ANS with the LLW with various linear velocities of the organic phase (112 to 250 cm s -1 ) at a detection position of 1 mm from the tip of the inner capillary. ANS is well-known as a solvatochromic fluorescent reagent, and the peak wavelength of the fluorescent spectrum is 532 nm in an aqueous solution, 525 nm in 10% ethanol, 470 nm in 95% ethanol, or 460 nm in heptane; 12 i.e., the ANS spectrum shows a strong blue shift in a hydrophobic solvent. Thus, we could use this compound as a probe to examine the hydrophobicity of the medium.…”

“…9 Recently, the time-resolved quasi-elastic laser scattering (QELS) method has been applied to study the behavior of a phase-transfer catalyst at the interface. 11,12 These techniques have been applied to study the phenomena in the equilibrium state or a slow reaction process at a static liquid/liquid interface. However, it is difficult to apply these techniques to measure the fast kinetics of reactions at the interface.…”