(720h) Dynamic Adsorption Processes of Surfactants at Liquid/Liquid Interfaces Determined By the Fluid Dynamics of Single Droplets

In many industrial applications interfacial active impurities such as surfactants occur. These molecules adsorb at liquid/liquid interfaces where they influence the occurring transport processes, which again affects reaction rates of multiphase reactions or contact times in extraction columns etc. To quantify the influences on the given applications the interfacial coverage of the liquid/liquid interface with surfactant molecules must be known. With an increase of surfactant concentration the interfacial coverage with surfactant molecules increases. As a result the mobility of the liquid/liquid interface decreases which ends up in a higher drag coefficient and a lower drop rise velocity. Therefore, the fluid dynamics of single droplets can be applied as a useful tool to determine the interfacial coverage. Furthermore, the dynamic progress of the drop rise velocity gives important information about the adsorption kinetics of surfactants in the regarded liquid/liquid system.

For the experimental investigations different liquid/liquid systems were observed: water was always used as the continuous phase and various long chain alcohols (homologous row) were applied as dispersed phases. SDS and Triton X-100 were used as examples of ionic and non-ionic surfactants.  The fluid dynamics were measured during a single drop rise in special test cells. Furthermore, many experiments to quantify the occurring interfacial phenomena were carried out (e.g. measurements of the dynamic interfacial tension, colloidal probe atomic force microscope measurements, oscillating drop measurements etc.).

With an increase of the surfactant concentration a reduction of the terminal drop rise velocity was observed, which is referred to the mechanisms described above. For high surfactant concentrations the drop rise velocity approaches the velocity of a particle with a rigid sphere. By comparing the results gained by the interfacial tension measurements a good agreement was observed for the ionic surfactant SDS. For Triton X-100 other effects than pure adsorption must be taken into consideration. Furthermore, different adsorption behaviour of both surfactants was observed by the determination of the transient drop rise velocity. The coverage of the liquid/liquid interface with surfactant molecules increases with time during the drop rise and the droplets were slowly decelerated. Due to the different adsorption behaviour of the surfactants in various test systems the transient drop rise velocity was affected differently. For some test systems a distinctive maximum of the transient drop rise velocity could be observed, while in other systems the coverage of the liquid/liquid interface was completed during the acceleration phase of the droplet. Again, for the ionic surfactant SDS the results showed good agreement with the results gained by the dynamic interfacial tension measurements. But for non-ionic surfactants the dynamic formation of microemulsion layers (change of the phase behaviour) at the liquid/liquid interface has to be taken into consideration. Otherwise, the drop rise velocity is over predicted and the design of e.g. extraction columns would fail due to wrong contact times and mass transfer rates.