Effects of polarity and pH on the solubility of acid-treated carbon nanotubes in different media

The aggregation of acid-treated carbon nanotubes (CNTs) as a solid and their solubility in solvents of different polarity and in water of different pH were investigated as a function of acid treatment conditions. The CNTs were found to form solid hydrogen-bonded aggregates, with a higher content of COOH groups resulting in a denser aggregate. The untreated CNTs were non-polar in nature and could dissolve, or be easily dispersed, in non-polar or low polar solvents such as acetone and alcohols (methanol and ethanol), but precipitated from deionized water, a highly polar solvent. In contrast, the acid-treated CNTs dissolved or were well dispersed in the deionized water, but not in acetone or alcohols. The treated CNTs were insoluble in an aqueous solution of pH 0, but soluble in those of pH 4 and higher with their solubility monotonically increasing with pH up to pH 10. The considerable ionic bond strength between carboxylate anions and sodium cations could be a reason for a decrease of their solubility in an aqueous solution of pH 12.