Stokes–Einstein and desorption-mediated diffusion of protein molecules at the oil–water interface

We observe the diffusive trajectories of individual bovine serum albumin molecules at the oil–water interface over more than two orders of magnitude of oil viscosity. At low oil viscosities, the molecular mobility is dominated by the expected Stokes–Einstein mode of interfacial diffusion, where the effective diffusion coefficient is inversely proportional to both the protein hydrodynamic radius and the oil viscosity. However, for high viscosity oils, a different diffusive regime is observed, 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. We propose that in this regime of high oil viscosity, interfacial diffusion is dominated by desorption-mediated diffusion, which also dominates at many solid–liquid interfaces.