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. The trajectory shown here shows an individual protein molecule at the interface between aqueous solution and a viscous silicone oil. It initially executes slow diffusive motion corresponding to a 2D diffusion coefficient of 0.005 µm2/s (corresponding to the Stokes-Einstein prediction), and then abruptly shifting to faster diffusion, with a coefficient of 0.025 µm2/s. This behavior suggests the presence of multiple diffusive modes, that may be due to different molecular conformations, or simply different penetration depths into the oil phase. This illustrates the ability of the single-molecule approach to extract molecular level mechanisms. 1 µm Trajectory of a single bovine serum albumin molecule at the interface between an aqueous phase and a viscous silicone oil (~12,000 cP).