(147b) Stokes-Einstein and Desorption-Mediated Diffusion of Protein Molecules At the Oil-Water Interface

We directly observe the diffusion of individual bovine serum albumin molecules at the oil-water interface over more than two orders of magnitude of oil viscosity using total internal reflection fluorescence microscopy. Our system consists of silicone oils, and fluorescently labeled proteins.  At low oil viscosities, on the order of 100 cP, the molecular mobility is dominated by the expected Stokes-Einstein mode of interfacial diffusion as the effective diffusion coefficient is inversely proportional to both the protein hydrodynamic radius and the oil viscosity. However, at higher oil viscosities, we observe a different diffusive regime, with anomalously large effective diffusion coefficients that are relatively insensitive to the oil viscosity and are similar to the measured diffusion coefficient of the same protein at a hydrophobic solid-liquid interface. This transition occurs at approximately 2000 cP. We propose that at high oil viscosity, interfacial diffusion is dominated by desorption-mediated diffusion. This has been previously observed at many solid-liquid interfaces.