Self-diffusion surface structure sensitivity

Recent single-crystal studies reveal the surface-structure sensitivity and anisotropy of self-diffusion (70, 71]. Depending on the structure of the crystal face, diffusion coefficients may vary by orders of magnitude. This is shown for rhodium adatom diffusion on various rhodium crystal faces in Figure 4.14. Diffusion rates parallel to steps are greater than diffusion rates perpendicular to them. 

The self-diffusion coefficient of a molecule in the liquid phase is dominated by the tortuosity t here x characterizes the detour that the molecule has to take due to the obstacle that the gel skeleton imposes, compared to the distance that it would cover by free Brownian motion in the liquid. In particular, it is sensitive to pore accessibility. The self-diffusion coefficient is therefore a characteristic of the pore connectivity which itself is for gels a function of porosity and backbone structure. For small mesopores, the small pore volume to surface ratio is expected to result in an additional reduction of the self-diffusion coefficient due to the interaction of the liquid molecules with the pore surface. 

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