Stabilization of Multi-Walled Carbon Nanotube Dispersions with Dissolved Humic Substances.

Soil humic substances (HS) have been shown to stabilize carbon nanotube (CNT) dispersions in solution yet the mechanisms by which this occurs are widely misunderstood. For this paper, we hypothesize that this behavior is a property of the surfactive nature of HS. Experiments were conducted by dispersing multi-walled CNT in solutions containing a range of dissolved HS concentrations obtained from the commercial Aldrich humic acid or water-extractable HS from Catlin silt loam soil (fine-silty, mixed, mesic, superactive Oxyaquic Argiudolls). CNT dispersions demonstrated enhanced stability at 50 and 300 mg L-1 added HS from Aldrich HA and Catlin HS, respectively. Dynamic light-scattering data showed that increasing the concentration of HS decreased CNT mean particle diameter (MPD) to approx. 250 nm for Aldrich HA and to approx. 450 nm for the Catlin HS. CNT particle size polydispersivity (PD) also reached a minimum at approx. 0.3 and 0.35 with increasing Aldrich HA and Catlin HS, respectively, indicating enhanced homogeneity of particle sizes but with significant differences between the two humic materials. HS adsorption isotherms revealed that maximum dispersion stability and minimization of MPD and PD corresponded with saturation of CNT particles with HS – a behavior indicative of surfactants. To verify this conclusion, CNT dispersion potential was studied in the presence of two nonionic (Brij 35 and Triton X) and one anionic (SDS) surfactants. Trends in CNT MPD and PD minimas, and surfactant adsorption were observed with increasing dispersion stability. Results showed that the highly surfactive nature of dissolved HS readily stabilized CNT dispersions. It is our opinion that natural levels of HS present in most waters is sufficient to readily disperse CNT in the environment.