Aggregation kinetics of SDBS-dispersed carbon nanotubes in different aqueous suspensions

The aggregation kinetics of single-walled (SWNTs) and multi-walled (MWNTs) carbon nanotubes dispersed using sodium dodecylbenzene sulfonate (SDBS) were investigated using time-resolved dynamic light scattering (DLS), in the presence of several electrolytes, and humic acid (HA). The effects of changes in the concentration of monovalent (NaCl, 5–200 mmol/L) and divalent (CaCl2, 0.2–5 mmol/L) electrolytes and the solution pH (3–10) on the aggregation behavior were evaluated. The results showed that the carbon nanotubes (CNTs) could be effectively suspended in aqueous solution using the surfactant (SDBS), and that increased electrolyte concentrations induced aggregation. At a solution pH of 6.3 ± 0.2 (without HA), the critical coagulation concentration (CCC) values of NaCl were calculated as 90 and 120 mmol/L for SWNTs and MWNTs, respectively; the values for CaCl2 were calculated as 0.82 and 2.6 mmol/L, respectively. Increases in the solution pH from 3 to 10 led to a significant decrease in CNT aggregation, indicating the presence of functional groups on the CNTs’ surface. However, the impact of HA on the aggregation of CNTs in suspension depended on the type of the electrolyte that it was exposed to. In a NaCl solution, the stability of the CNTs was increased. For HA concentrations of 1, 5, and 20 mg/L, the CCC values of NaCl for the MWNTs increased to 130, 160, and 240 mmol/L, respectively. The effect was attributed to the steric repulsion induced by the adsorbed HA molecules. However, this effect was negligible in CaCl2 solutions. The results of this study elucidated the effects of surfactants, solution chemistry, and HA on the aggregation kinetics of CNTs, all of which may have significant influences on the behavior of CNTs in the environment.

Graphical abstractThe aggregation kinetics of CNTs, explained using DLVO theory, show that increasing electrolyte concentrations can induce the aggregation of CNTs. Moreover, the presence of HA can enhance the stability of CNTs markedly in monovalent electrolytes.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► SDBS used as a surfactant can effectively suspend CNTs in aqueous solution. ► Increasing electrolyte concentrations can induce the aggregation of CNTs. ► Increase in pH resulted in decrease in the aggregation due to the functional groups. ► The presence of HA can enhance the stability of CNTs in monovalent solution.