Effect of surfactant structure on the stability of carbon nanotubes in aqueous solution

The suspending behaviors of multiple-wall carbon nanotubes (MWNTs), including pristine MWNTs (p-MWNTs) and acid-mixture-treated MWNTs (MWNTCOOH), stabilized by cationic single-chain surfactant, dodecyltrimethylammonium bromide (DTAB), and cationic gemini surfactant hexyl-alpha,beta-bis(dodecyldimethylammonium bromide) (C 12C 6C 12Br 2) were studied systematically. The surfactant structure influences the suspendability of MWNTs dramatically as well as the surfactant adsorption behavior on the nanotubes. Although both the surfactants can disperse the MWNTs effectively, they actually show different stabilizing ability. DTAB is not capable of stabilizing these two MWNTs below critical micelle concentration (CMC). However, C 12C 6C 12Br 2 can suspend both the nanotubes effectively even well below its CMC. Moreover, the adsorption of these two surfactants reaches equilibrium at twice the CMC with the original MWNT concentration of 2 mg/mL, 2 mM for C 12C 6C 12Br 2, and 30 mM for DTAB. After the adsorption equilibrium, the maximum amounts of the two suspended MWNTs in C 12C 6C 12Br 2 solution are about twice as much as those in DTAB solution. The strong hydrophobic interaction among the C 12C 6C 12Br 2 molecules and between the C 12C 6C 12Br 2 molecules and the nanotubes as well as the high charge capacity of C 12C 6C 12Br 2 lead to its much stronger adsorption ability on the MWNTs and result in its superior stabilizing ability for the MWNTs in aqueous phase. The gemini surfactant provides a possibility to effectively stabilize the MWNTs in aqueous solutions even at very low surfactant concentration well below its CMC.