Direct observation of dynamic molecular behavior at a water/nitrobenzene interface in a chemical oscillation system

Nonlinear behavior of ionic surfactant (SDS) molecules at a water/nitrobenzene (W/NB) interface in a chemical oscillation system was investigated by monitoring interfacial tension (Deltay) using time-resolved quasi-elastic laser scattering (QELS). Simultaneous measurement of electrical potential (DeltaV) allowed extraction and discussion of the time course of Deltay. which represents the contribution of the interfacial molecular behavior to Deltay. No change of Deltay(a) was observed in the induction period which lasted about 10-10(3) s. This indicated that no efficient adsorption of surfactants onto the W/NB interface occurred although the surfactants diffused toward the interface. After this induction period, Deltay(a) showed a sudden decrease with electrical potential oscillation; i.e., the molecular adsorption occurred rapidly and collectively with changes of interfacial tension and electrical potential. Chemical oscillation was brought about by this unusual adsorption behavior. It appeared that some hindrance effect toward adsorption was induced at the liquid/liquid interface, which resulted in the formation of a large concentration gradient at the sub-interfacial region and led to the collapse and sudden adsorption of SDS. After the chemical oscillation, which was a relaxation process, Ay, increased gradually. Ay. was fitted by combining Langmuir adsorption-desorption and Frumkin adsorption isotherms. It was concluded that the relaxation process was governed by molecular desorption and had an extraordinarily long time scale (similar to10(3) s). The mechanism of these features was considered in terms of deceleration of counterion-binding because SDS molecules could not be desorped to the NB phase without the formation of ion pairs. These unusual molecular behaviors indicated that some barrier-like effect which hindered adsorption and desorption played a key role in the nonlinear oscillation behavior induced at the liquid/liquid interface.