Connecting the Molecular and the Continuum Scales

Key Challenges: A molecular-scale understanding of structure and surface chemistry of clay mineral surface nanoparticles.

Why it Matters: With a ubiquitous presence in natural materials and strong surface reactions, nanoparticles figure importantly in a broad range of phenomena, from climate change to contaminant remediation.

Accomplishments: Used molecular dynamics (MD) simulations to determine molecular-scale diffusion coefficients of water tracers and representative cations (Na+,Cs+,Sr2+)  in Smectite and found a simple way to accurately relate this to bulk, pore-scale diffusion.  The result is that a remarkably simple expression relates Dinterlayer to the pore-scale parameter δnanopore e 1, a constrictivity factor that accounts for the lower mobility in interlayers as compared to macropores.  Using this scaling expression, one can accurately predict the apparent diffusion coefficients of tracers H2O, Na+, Sr2+, and Cs+ in compacted Na smectite-rich materials.

Investigators: Ian Bourg and Garrison Sposito (LBNL)

More Information: See, for example, Environ. Sci. Technol., 44 (6), 2085–2091 (2010)