Anomalous Attachment Behavior of Nanoparticles inside Narrow Channels

Understanding nanoparticle (NP) attachment inside narrow passages such as the pore throats of porous media and plant and animal tissues is critically important to assess the potential ecological and toxicological impact of NPs. This study investigated the attachment of a NP inside a cylindrical nano-sized channel with finite wall thickness at various ionic strengths (ISs) by calculating the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy using a modified surface element integration technique. Results show that there is a critical value of the ratio of the inner channel diameter to NP diameter (RCN) at which the repulsive energy barrier reaches a maximum at a given IS and NP transport is most favored. A non-monotonic variation of the energy barrier with IS was observed for RCNs smaller than the critical value. The repulsive energy barrier disappears at all ISs when the NP diameter is close to the inner channel diameter, resulting in favorable attachment at primary minima. The attached NP cannot be detached in these cases by a disturbance of system conditions because of increased primary-minimum depths and accordingly enhanced adhesive forces. For a given RCN, decreasing the channel thickness can increase and decrease the interaction energy barrier and primary-minimum depth for a NP inside the channel, respectively. Accordingly, NP attachment in primary minima is inhibited whereas transport is favored in channels with thin walls. These theoretical results provide plausible explanations of experimental observations that the retention of colloids in pore throats of porous media via straining is chemically favorable (i.e., no repulsive energy barrier exists) even at very low ISs and irreversible to reduction of the solution IS, and that NPs are favorably attached in narrow passages such as plant tissues and membrane pores.