Effects of Temperature and Shear on the Adsorption of Surfactants on Carbon Nanotubes
MD Vo and DV Papavassiliou, JOURNAL OF PHYSICAL CHEMISTRY C, 121, 14339-14348 (2017).
Carbon nanotube suspensions in water can be stabilized with the use of surfactants. However, when a suspension is exposed to harsh conditions, the surfactants might detach from the carbon nanotube (CNT) surface leading to CNT agglomeration and precipitation. In order to explore temperature effects on the suspension, the surfactant adsorption isotherms of an ionic surfactant (alkylpropoxy sulfate, alfoterra 123-8s) and a nonionic surfactant (secondary alcohol ethoxylate, tergitol 15s-40) on a CNT are obtained at various temperatures using dissipative particle dynamics (DPD) simulations. The effects of shear stresses, such as those experienced by the CNT-surfactant suspension when it passes through pumps, are also explored by applying different levels of constant shear on the suspension. It is seen that there is good agreement between the DPD results and the Langmuir model for the adsorption of both alfoterra and tergitol on CNTs. The detachment of the surfactant from the nanotubes occurs with the detachment of the hemimiCelles formed by the surfactant for both alfoterra and tergitol. However, the desorption of tergitol occurs at lower shear rates than the desorption of alfoterra, mostly due to the size of the surfactant molecules. There are therefore two states of the suspension: one where the surfactants are fully adsorbed and one where they are fully detached from the CNTs.
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