(54a) Dynamics of Single-Walled Carbon Nanotubes in Water

The behavior of SWNTs in liquids is still poorly understood. This is perhaps surprising, because SWNTs are commonly described as high aspect ratio rod-like particles, and slender rigid objects in liquids have been studied for decades (Jeffery 1922; Kirkwood 1967; Onsager 1949). Processing SWNTs in liquids is key to developing scalable techniques for directed assembly or self-assembly of SWNTs, e.g., production of SWNT fibers and films, length and type separation of SWNTs. SWNTs in liquids are important in the physical, material, and life sciences; real-time visualization of SWNTs in liquids can impact each of these areas of research. A simple and convenient SWNT fluorescent tagging procedure was developed; with this technique, we studied the dynamics of individual SWNTs in water by video-microscopy. We measured the rotational diffusion coefficient of SWNTs confined between parallel plates and find it in reasonable agreement with predictions based on confined diffusion of dilute Brownian rods. The critical concentration at which SWNTs in suspensions start interacting was also determined. The fluctuating shape of SWNTs was analyzed to determine the persistence length. We found that the measurements agree with theoretical estimates. We conclude that commonly available SWNTs in liquids can be considered as rigid Brownian rods in the absence of imposed external fields.