(549c) Experimental Investigations of Transport Processes and Interfacial Phenomena in Micellar Liquid/Liquid Systems

One of the main goals of “green chemistry” is: using water as a solvent. Many substances are hardly soluble in water; hence multiphase reactions do not proceed. By using surfactants the water solubility of such substances is increased which results in an increase of the reaction rate. Due to their amphiphilic structure surfactants adsorb at interfaces. This is the place where the important transport processes occur. Therefore, an interference of the transport processes by surfactants is obvious. In previous works a reduction of the mass transfer in the presence of surfactants was attributed to two mechanisms: the physicochemical effect and the change in the fluid dynamics. Due to the adsorption at the interface an additional mass transfer resistance is created. Furthermore, the fluid dynamics will change. Quantifying the influences of surfactants on the mass transfer is the goal of this work. Therefore, the mass transfer is observed at the smallest transfer unit: at single droplets.

For the experimental investigations the system water (continuous phase), octan-1-ol (dispersed phase), Triton X-100 (surfactant) and the azo dye Pyridine-2-methylaniline (transferred component) were observed. The liquid/liquid mass transfer and the fluid dynamics were measured during a single drop-rise in special test cells. Furthermore, experiments to quantify the occurring interfacial phenomena were carried out. The determination of the interfacial tension was carried out by two different methods (pendant drop, spinning drop). A special method to determine the rigidity of liquid/liquid interfaces was developed by applying colloidal probe atomic force microscope measurements. Furthermore, oscillating drop measurements were carried out to determine the interfacial rheology.

            The results show: An increase of surfactant concentration causes a reduction of liquid/liquid mass transfer. The mechanisms mentioned above are responsible for this reduction, but the results of this work show that only regarding these mechanisms is not enough. For high surfactant concentrations the liquid/liquid mass transfer is reduced dramatically — even more than predicted by the known mechanisms in the literature. Therefore, other interfacial phenomena must be taken into account.

Besides the experimental investigations of mass transfer processes the focus of this works lies on the determination of interfacial phenomena in micellar liquid/liquid systems. The characteristics of the liquid/liquid interface were determined by applying the oscillating drop method as well as colloidal probe atomic force microscope measurements at the liquid/liquid interface. Both measurement techniques show that with an increase of surfactant concentration the rigidity of the liquid/liquid interface increases; although the interfacial tension decreases. This is caused by a change in the phase behaviour at the liquid/liquid interface, due to the high local surfactant concentration. The appearance of high viscous microemulsion layers, or even liquid crystalline layers, ends up in an additional mass transfer resistance. For an integrated description of liquid/liquid mass transfer in micellar systems this interfacial phenomenon of the change in phase behaviour must be regarded, otherwise the liquid/liquid mass transfer is overestimated.