Total Internal Reflection Fluorescence Spectrometer To Study Dynamic Adsorption Phenomena at Liquid/Liquid Interfaces

Adsorption at oil/water interfaces affects the performance of many industrial systems including oil recovery, extraction processes, cosmetic products, and food technology. However, no technique currently available can monitor adsorption dynamics using molecularly sensitive methods. We have constructed a novel total internal reflection fluorescence spectrometer (TIRFS) to follow dynamic adsorption events at the oil/water interface. The TIRFS monitors changes in fluorescence intensity and fluorescence spectra over time by maintaining an optical focus on the fluid interface during adsorption and desorption processes. Kinetic adsorption phenomena are examined by altering the composition of the aqueous phase and recording surface fluorescence response without mechanically disturbing the fluid/fluid interface. The spectrometer captures changes in the fluorescence intensity over tenths of seconds and maintains optical focus for periods of days. Mass transport of fluorescing surface-active material to and from the oil/water interface is accurately modeled using the simple one-dimensional diffusion equation. The geometry designed for this apparatus can be applied to other light-based techniques studying adsorption at liquid/liquid interfaces. Here, we apply the TIRFS apparatus to the study of β-casein adsorption and desorption at an aliphatic oil/water interface. The observed increase in interfacial fluorescence due to β-casein adsorption is slower than the diffusive flux, and desorption is found to be very slow if not irreversible. The TIRF spectrum indicates interaction of sorbed β-casein with the oil phase and subsequent rearrangement of the native structure.