In addition to changes in ion self-diffusion induced by variations in the solution density, we also observe that the mechanism for ion diffusion changes with the degree of confinement. This mechanism can be quantified by the diffusion length L Ion-Water describing the characteristic distance for an ion to diffuse together with a water molecule in its first solvation shell. This diffusion length, illustrated schematically and plotted for both ions in Fig. , allows us to classify ion transport as either vehicular or structural. When L Ion-Water is large, an ion travels an appreciable distance with its first solvation shell intact, and its diffusion is classified as vehicular. In contrast, structural diffusion is characterized by lower L Ion-Water , in which water molecules in the first solvation shell are frequently exchanged.