Dispersion of multi-walled carbon nanotubes (MWCNTs) by ionic liquid-based phosphonium surfactants in aqueous solution

Multi-walled carbon nanotubes (MWCNTs) can be dispersed effectively in aqueous solution by ionic liquid-based phosphonium surfactants, alkyl-triphenyl phosphonium bromide (CnTPB, n = 12 and 14). The morphology of the dispersed MWCNTs was observed by TEM experiments. UV-vis-NIR spectra were performed to determine the dispersion of MWCNTs and the optimum concentration of CnTPB. Compared with tetradecyl trimethyl ammonium bromide, C14TPB is more effective to disperse MWCNTs, reflecting that the multi-benzene moieties on the head group of C14TPB is propitious to disperse MWCNTs. It is also found that CnTPB with a longer hydrocarbon chain exhibits the stronger dispersion ability. The adsorption of CnTPB molecules on MWCNTs was investigated by zeta-potential, surface tension measurements and fluorescence spectroscopy. The zeta-potential results show that the MWCNTs dispersed in CnTPB aqueous solution have relatively high positive charges. It is the electrostatic repulsion force between MWCNTs that makes MWCNT dispersion stable. The significant fluorescence quenching demonstrates that there is pi-pi stacking interaction between multi-benzene head moieties and the pi-networks of MWCNTs. Furthermore, a possible mechanism was proposed. (C) 2012 Elsevier B.V. All rights reserved.