Dispersion and stability of multi-walled carbon nanotubes in water as affected by humic acids

Abstract The effects of two different humic acids (HAs), commercial Aldrich HA (AHA) and HA extracted from forest soils (FHA), on the dispersion and stability of three multi-walled carbon nanotubes (MWCNTs) in water were investigated. The amount of MWCNTs suspended in pure water increased with increases in the MWCNT surface oxygen content and solution pH, and the amount decreased as the ionic strength increased. These results indicated that electrostatic repulsion drove the dispersion of MWCNTs. Adsorbed HA greatly enhanced the dispersion and stability of MWCNTs due to the increased steric hindrance and electrostatic repulsion between individual MWCNTs. Suspended MWCNT concentrations and HA adsorption to MWCNTs were positively correlated with low ionic strengths (0–40 mM) and/or pH values between 3 and 7, which were independent of the change in electrostatic repulsion, suggesting that the dispersion of MWCNTs was primarily controlled by steric hindrance. At higher ionic strengths, MWCNTs rapidly precipitated due to the reduction of the electrostatic repulsion between individual MWCNTs. AHA could suspend more MWCNTs than FHA, which was ascribed to the higher molecular weight and aromatic carbon content of AHA that led to greater steric hindrance between individual MWCNTs. This result verified that steric hindrance was the dominant mechanism by which HAs disperse MWCNTs.