TR-QELS

In these past 10 years, it has been demonstrated that the TR-QELS method is a versatile technique that can provide much information on interfacial molecular dynamics [1-11]. In this chapter, we intend to show interfacial behaviour of molecules elucidated by the TR-QELS method. In Section 3.2, we present the principle, the historical background and the experimental apparatus for TR-QELS. The dynamic collective behaviour of molecules at liquid/liquid interfaces was first obtained by improving the time resolution of the TR-QELS method. In Section 3.3, we present an application of the TR-QELS method to a phase transfer catalyst system and describe results on the scheme of the catalytic reactions. This is the first application of the TR-QELS method to a practical liquid/liquid interface system. In Section 3.4, we show chemical oscillations of interfacial tension and interfacial electric potential. In this way, the TR-QELS method allows us to analyze non-linear adsorption/desorption behaviour of surfactant molecules in the system. 

FIGURE 3.2. Schematic diagram of the experimental set-up. (a) An enlarged sketch of the sample space, and (b) a block diagram of the TR-QELS method for liquid/liquid interface investigation. 

TR-QELS Measurements of Phase Transfer Catalytic Reactions... 

We successfully observed dynamic molecular behaviour reflecting the interfacial specificity by focusing on the phase transfer catalyst system using the TR-QELS method. Our results provided new information on the characteristic behaviour of phase transfer catalysis which gives the high yield reaction. 

Later, Yoshikawa and Matsubara [40] further studied a non-linear system and proposed a mechanism for the periodic behaviour that involved the formation of inverted micelles that suddenly moved to the oil phase after the concentration of adsorbed surfactants reached a critical value. They extended the experiment to a water/oil/water three-phase system in a U-shaped glass tube that gave spontaneous and stable oscillatory behaviour over a long period [41]. Since then, various characteristics of non-linear behaviour have been investigated and several mechaiusms for the non-linear behaviour have been proposed by many research groups including ours[2,5,10,42-48] however, the mechanism at a molecular level has not been clarified yet and no consensus has been achieved. The difficulty in the explanation seems to come from not only the complexity and diversity of the systems, but also limitations of the observation methods that enable us to monitor dynamic molecular behaviour at liquid/liquid interfaces with sufficient interfacial selectivity and time resolution. In this section, the TR-QELS method has been applied to the investigation of W/NB—sodium dodecyl sulfate (SDS) two-phase system [10]. 

---