Single-Molecule Studies of Surfactant Dynamics at the Oil/Water Interface Daniel K. Schwartz, Department of Chemical & Biological Engineering University of Colorado, Boulder While it is widely recognized that interfacial transport is relevant to applications such as emulsion formation and stability, experimental studies of dynamics at the oil/water interface have traditionally had a limited focus on the laterally-averaged net adsorption dynamics. We are applying novel single-molecule methods to study molecules at the oil/water interface. We have used single molecule total internal reflectance fluorescence microscopy to observe the dynamic behavior of >4,000 bovine serum albumin objects at the silicone oil-water interface. The surface residence time distribution indicated the presence of three populations at the interface. Each population had a characteristic fluorescence intensity and distinctive interfacial diffusion behavior. Larger fluorescence intensity correlated with longer residence times and slower diffusion. These combined observations of fluorescence intensity, surface residence time, and interfacial diffusion suggested that the three populations represent monomers, dimers, and trimers respectively. water silicone oil monomer dimer trimer fast slow Protein monomers were distinguished from dimers and trimers by their distinctive dynamic behavior at the oil/water interface. In the case of oligomers, multiple diffusive modes were observed, suggesting characteristic conformations and / or orientations.