Colloidal properties of aqueous suspensions of acid-treated, multi-walled carbon nanotubes.

Grafting oxygen-containing functional groups onto carbon nanotubes (CNTs) by acid treatment improves their dispersion in aqueous solutions, but there is a lack of quantitative information on the colloidal properties of oxidized CNTs. We have studied the influence that pH and electrolytes have in determining the colloidal stability of oxidized multiwalled carbon nanotubes (O-MWCNTs), prepared by refluxing pristine MWCNTs in nitric acid. The acid-treated MWCNTs contained oxygen predominantly in the form of carboxyl groups. Colloidal suspensions of O-MWCNTs were prepared by low-power sonication and contained negatively charged, individual MWCNTs with an average length of approximately 650 nm. Time-resolved dynamic light scattering revealed that the aggregation rate of O-MWCNTs exhibited both reaction and mass-transport limited regimes in the presence of different electrolytes and as a function of pH. Particle stability profiles constructed from aggregation rate data allowed for the determination of critical coagulation concentrations (CCC), a metric of colloidal stability. The CCC values of O-MWCNTs varied with counterion concentration and valence in a manner consistentwith DLVO theory. Potentiometric measurements of surface charge correlated well with the observed pH-dependent variations in the O-MWCNT's colloidal stability. Electrophoretic mobility was also a diagnostic of particle stability, but only in neutral and acidic conditions.