ABSTRACT
Experimental probes of liquid-liquid interfaces have been done mainly by spectroscopic and electrochemical techniques, which are suited to monitoring the interface between two bulk phases because their measurements result in no mechanical perturbation [1, 2]. The quasi-elastic laser scattering (QELS) method has advantages as a tool for in situ, noncontact time-resolved measurements of dynamic behavior of molecules at liquidliquid interfaces and liquid-air interfaces [3-9]. The method monitors the frequencies of capillary waves, which are spontaneously generated by a thermal fluctuation at liquidliquid interfaces. Since the capillary wave frequency is a function of interfacial tension, and the change in the interfacial tension reflects the change in the number density of surfactant molecules at the interface, the QELS method allows observation of dynamic changes of liquid-liquid interfaces such as the change in number density of surfactant molecules and the formation of a lipid monolayer. Owing to its improved time resolution, each power spectrum can be obtained in 1 ms to 1 s, so the method can be used to monitor dynamic changes at liquid-liquid interfaces in a real environment. Furthermore, this method has good interface selectivity for overcoming the problem of interference by bulk phases, because the capillary wave is a characteristic phenomenon at the interfaces and its frequency can be detected by an optical heterodyne technique.
