WHY ARE CARBON NANOPARTICLES NOT STICKY IN POROUS MEDIA?

Nanoparticles have a diffusion constant at least an order of magnitude greater than inert chemical tracers such as KBr, and this means that they can potentially be used to measure the degree to which subsurface flow occurs through fractures. A prerequisite for tracer applications is that the particles are not retained in the porous media as the result of aggregation or sticking to mineral surfaces regardless of water chemistry. By screening eight nanoparticles (3–100 nm in diameter) for retention when passed through calcium carbonate packed laboratory columns in artificial oil field brine solutions of variable ionic strength we show that the nanoparticles with the least retention are 3 nm in diameter, nearly uncharged, and decorated with highly hydrophilic polymeric ligands.

The details of these column experiments and the tri-modal distribution of zeta potential of the calcite sand particles in the brine used in our tests suggests that parts of the calcite surface have positive zeta potential and the retention of negatively charged nanoparticles occurs at these sites. Only neutral nanoparticles are immune to at least some retention. AFM measurements and DLVO theory shows the calcite surface charge is always negative for divalent salt (e.g. Na2SO4) solutions, but changes from negative to positive in a patchy fashion as the ionic strength of the monovalent salt (e.g. NaCl) solution increases. Since mixed-charge (patchy) surfaces may be common in the subsurface, nanoparticles with near-zero charge may make the best tracers. A retention comparison between two carbon nanoparticles made from different precursors indicates that the surface hydrophilicity of nanoparticles is critical determining particle non-stickiness with mineral surfaces.