Effect of counter ion valence and pH on the aggregation and charging of oxidized carbon nanohorn (CNHox) in aqueous solution

Oxidized carbon nanohorn (CNHox) is one of carbon nanomaterials and has attracted attention due to its unique material properties. To clarify the dispersion, aggregation, and charging behaviors of CNHox in aqueous solutions with different valences of counter ions, K+, Ca2+, La3+, and pH, the stability ratio and electrophoretic mobility of CNHox were investigated by electrophoretic light scattering and time-resolved dynamic light scattering techniques. The CNHox showed pH-dependent negative zeta potential probably due to the de-protonation of carboxylic groups on CNHox. Increasing electrolyte concentration and counter ion valence decreased the magnitude of zeta potential by double layer screening and counter ion adsorption. The stability ratio of CNHox showed clear slow aggregation regime, fast aggregation regime, and critical coagulation concentration (CCC). The CCC decreased with increasing the counter ion valence. That is, the aggregation-dispersion of CNHox follows the Schulze-Hardy rule and the Derjaguin-Landau and Verwey-Overbeek (DLVO) theory. Furthermore, the relationship between critical coagulation ionic strength and charge density reasonably agrees with theoretical prediction by the DLVO theory for low surface potential. The DLVO theory is useful for the description of aggregation-dispersion of irreversible aggregates with colloidal size as found for CNHox aggregate, once their zeta potentials are evaluated. [Display omitted] •Electrophoretic mobility and stability ratio of oxidized carbon nanohorn were measured.•Higher counter-ion valence induced the reduction in the magnitude of zeta potential.•Higher electrolyte concentration induced the charge reversal.•Critical coagulation concentration decreased with increasing the counter-ion valence.•Coagulation properties follow Schulze-Hardy's rule and DLVO theory.