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Chapter 3Interfacial effects during reactive transport ∗The mass transfer coupled with hydrolysis and gelation in the aqueous phase givesrise to an intriguing motion of the interface between both phases. This phenomenonwas analyzed in detail both theoretically and experimentally. The theory wasdeveloped assuming quasi-static conditions and provides a non-linear second orderboundary value problem, which was treated numerically. Magnetic Resonance Imaging(MRI) experiments are used to quantify the fluids in the oil and water phasesand capture the interface profiles between the oil and the water phases at differenttime intervals. An excellent fit of the theoretical interface profiles to the experimentalones at various times reveals that the interfacial tension increases gradually duringthe extent of the mass transfer.3.1 IntroductionThe coupled mass transfer and gel reaction of TMOS in two-phase bulk systems werestudied using Magnetic Resonance Imaging (MRI). Nuclear magnetic relaxation times(T 1 and T 2 ) were used to determine the concentration of TMOS in oil and to monitorthe progress of gelation (see Chapter 2). The images revealed that the reactive masstransfer in these systems gives rise to an intriguing motion of the interface between theoleic and the aqueous phases. Figure 2.9 shows two MRI images of a two-phase bulksystem within a small Teflon vial. Initially, the oleic phase, consisting of n-hexadecane,is mixed with TMOS at a concentration of 40 vol%. During the experiment the TMOSgradually transfers to the aqueous phase, while the volumes of the phases change and,as a result, the interface moves upwards. In addition, the shape of the curved interfaceand the contact angles at the vial material show subtle changes during the reactive masstransfer, suggesting gradual changes in interfacial tension (IFT). The main question iswhether the IFT increases or decreases during the process. This is also of interest in lightof the application of the chemical. For example, when a mixture of TMOS and oil isinjected in a water-saturated porous material the IFT determines the capillary forces andhence the displacement and distribution of the fluids.Interfacial tension between organic liquids and water has been extensively studied inthe past, both experimentally and theoretically [73–77]. Also, the IFT between mixturesof organic compounds and water has been investigated [78]. Common methods to measuresurface or interfacial tension are either based on a force balance, using a Wilhelmy plate[79] or a du Noüy ring [80], or on visualization of droplet surfaces, such as the pendant* Adapted from Castelijns et al. , Colloid Surf. A, in press (2007)31