Enhancing the Colloidal Stability and Electrical Conductivity of Single-Walled Carbon Nanotubes Dispersed in Water

In this article, the results of a focused systematic investigation of the nature, type, and concentration of cationic and anionic surfactants and proteins required to simultaneously disperse single-walled carbon nanotubes (SWNTs) and enhance their electrical conductivities in water are presented. The dispersibility of SWNTs suspended in aqueous solutions is evaluated via light scattering to characterize the average particle size, particle size dispersion, electrophoretic mobility, and ζ-potential of the dispersions. It is found that the colloidal stability of SWNT dispersions is influenced by the surfactant charge and concentration – i.e., above or below its critical micelle concentration and for proteins its charge. The amphiphilicity, concentration, and charge of the surfactant or protein determine their surface coverage on the SWNT and simultaneously increase electrostatic and steric repulsion and decrease surface chemical heterogeneity. It is found that the electrical conductivities of SWNT films stabilized with surfactant are as high as that without surfactants, with the added advantage of being homogeneously dispersed in water with significant enhancement in the long-term stability of the nanotubes in water. These findings suggest that enhancement of the electrical properties of SWNTs requires selection of a surfactant that has strong adsorption, and thus strong interactions with the nanotube – i.e., π–π stacking – and using concentrations above the CMC. Overall, these results demonstrate the importance of understanding the structure/property relationships between SWNTs and their dispersants in order to achieve high colloidal stability and electrical conductivities.