TOTAL INTERNAL-REFLECTION FLUORESCENCE SPECTROMETER TO STUDY DYNAMIC ADSORPTION PHENOMENA AT LIQUID LIQUID INTERFACES/

Adsorption at oil/water interfaces affects the performance of many ind ustrial systems including oil recovery, extraction processes, cosmetic products, and food technology. However, no technique currently availa ble can monitor adsorption dynamics using molecularly sensitive method s. We have constructed a novel total internal reflection fluorescence spectrometer (TIRFS) to follow dynamic adsorption events at the oil/wa ter interface. The TIRFS monitors changes in fluorescence intensity an d fluorescence spectra over time by maintaining an optical focus on th e fluid interface during adsorption and desorption processes. Kinetic adsorption phenomena are examined by altering the composition of the a queous phase and recording surface fluorescence response without mecha nically disturbing the fluid/fluid interface. The spectrometer capture s changes in the fluorescence intensity over tenths of seconds and mai ntains optical focus for periods of days. Mass transport of fluorescin g surface-active material to and from the oil/water interface is accur ately modeled using the simple one-dimensional diffusion equation. The geometry designed for this apparatus can be applied to other light-ba sed techniques studying adsorption at liquid/liquid interfaces. Here, we apply the TIRFS apparatus to the study of beta-casein adsorption an d desorption at an aliphatic oil/water interface. The observed increas e in interfacial fluorescence due to beta-casein adsorption is slower than the diffusive flux, and desorption is found to be very slow if no t irreversible. The TIRF spectrum indicates interaction of sorbed beta -casein with the oil phase and subsequent rearrangement of the native structure.