Molecular Diffusion of Ions in Nanoscale Confinement.

We measured the diffusion of an anion, fluorescein, confined to a nanoscale (10-100 nm) aqueous film between two glass walls. The two glass walls were very slightly angled to form a crack. The diffusion of fluorescein was strongly influenced by the presence of an inert electrolyte, NaCl, in the film prior to the diffusion of charged fluorescein into the crack. The time to reach an equilibrium distribution of fluorescein was 10 times longer without the inert electrolyte than when the electrolyte was present. In applications where rapid diffusion of ions is important, it would therefore be advisable to not prewet a confined space with pure water. We attribute this phenomenon to the effect of the electrical potential of the confining walls. Unscreened surface potential in a thin film severely hinders the diffusion of the fluorescein ion. As salt diffuses into the thin film, the electrostatic double layer shrinks in thickness and further diffusion of ions is less hindered. On the other hand, diffusion of ions into the film is only weakly affected by the Debye length of the solution, provided that the surface potential inside a thin film is initially screened by even a low concentration of electrolyte inside the film. The time evolution of the concentration profile for different Debye lengths matches a diffusion model developed with the finite difference method (FDM).