Effects of Surfactants and Electrolytes on Chemical Oscillation at a Water/Nitrobenzene Interface Investigated by Quasi-elastic Laser Scattering Method

Abstract

We used a time-resolved interfacial tension measurement method with quasi-elastic laser scattering to investigate the effects of electrolytes and various surfactants on the nonlinear dynamics of the chemical oscillation that occurred at a water/nitrobenzene interface when a surfactant was added to the interface through a capillary. For both cationic and anionic surfactants, an electrolyte in the water phase was required for slow desorption of the surfactant from the interface. In the absence of an electrolyte, repulsion between the polar head groups of the ionic surfactants hindered adsorption of the surfactant molecules at the interface, resulting in their rapid desorption. In contrast, the presence of an electrolyte induced adsorption of the surfactant ions by screening their charged polar heads. Zwitterionic and nonionic surfactants were also examined and we deduced that the salting out effect of the surfactant produced by the presence of an electrolyte results in strongly attractive interactions between the surfactant molecules and the water/nitrobenzene interface.

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.…”

“…A system displaying regular and repeated flip motion at an interface during the continuous addition of a surfactant solution by a capillary [4][5][6]35,37 is one of the simplest reproducible interfacial oscillatory systems. Ikezoe et al [4][5][6] reported that when an aqueous solution of 10 mmol L -1 SLS surfactant was introduced at a rate of 3.0 μL min -1 through a capillary at a water/ nitrobenzene interface, the electric potential oscillated in phase with the interfacial tension.…”

“…We focused our study on the first stage of the above mechanism, i.e., direct observations by the QELS method of heterogeneity in the interfacial tension of this system. 35,37 The role of electrolyte species in water was also investigated. We then compared the time course of the interfacial tension at two locations: one just below the capillary and the other 15 mm away from it, as shown in Fig.…”

“…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.…”

“…In the donor/membrane/ acceptor oscillator system, oscillations in the electric potential were in phase with those in tension at the membrane/acceptor interface, indicating that local adsorption of cetyltrimethylammonium bromide (CTAB) at membrane/ acceptor interface induces Marangoni convection, followed by their rapid adsorption. 35,37,38 The propagating chemical wave in an unstirred BZ reaction system showed an evolution in the interfacial tension, which was compared with the evolution of the ratio of concentrations of Fe(III) to Fe(II) in the spatiotemporal pattern of the bulk phase. 36 Finally, we investigated the propulsion mechanism of two different self-propelled objects: a camphor boat and an alcohol droplet floating on an aqueous phase.…”