Adsorption of Triton X-series surfactants and its role in stabilizing multi-walled carbon nanotube suspensions

Surfactants can enhance the stabilization of carbon nanotubes (CNTs) in water through their adsorption, thus affecting the environmental behavior and application of CNTs. However, the quantitative relationship between adsorption and stabilization and the role of the surfactant structure in the surfactant–CNT interactions are largely unknown. Therefore, Triton X-series surfactants with a same hydrophobic functional group (4-(1,1,3,3-tetramethylbutyl)-phenyl) and different hydrophilic polyethoxyl chain lengths were selected to investigate their adsorption onto CNTs and their ability to stabilize CNT suspensions. Adsorption data were fitted well by Langmuir equation, indicating monolayer coverage on CNTs. Adsorption capacities of the surfactants increased with decreasing hydrophilic chain length: Triton-305 < Triton-165 < Triton-114 < Triton-100. Electrostatic interaction and hydrogen bond could be excluded as the main mechanism because adsorption was not significantly affected by pH change. Hydrophobic and π–π interactions between the surfactants and CNTs were the dominant mechanism for their adsorption. CNT suspension data were well fitted to a nonlinear equation with a similar form to the Langmuir equation. Suspended CNT amounts in water were positively related to the adsorption capacities of the surfactants, but negatively with the hydrophilic fraction ratio of the X-series surfactants.