Systematic Study of the Dispersion of SWNTs in Organic Solvents

Dispersions of as-produced HiPco single-walled carbon nanotubes (SWNTs) in a series of organic solvents were prepared by dilution with the aid of tip sonication. Mild centrifugation (similar to 945 g) was carried out to remove large bundles. Atomic force microscopy (AFM) studies revealed that the bundle size decreased as the dispersion was diluted. By measuring the UV-vis-NIR absorption before and after centrifugation as a function of the concentration, the dispersion limit of SWNTs in each solvent can be determined. Correlations between the dispersion limit and solvent solubility parameters, including the Hildebrand solubility parameter and three-dimensional Hansen solubility parameters, are explored, demonstrating that SWNTs are easily dispersed in solvents with Hildebrand solubility parameter range from similar to 22-24 MPa(1/2) and Hansen polarity component (delta(P)) similar to 12-14 MPa(1/2). No clear correlation between dispersion limits and the dispersion force (delta(D)) or hydrogen bonding force (delta(H)) are evident. It is found, however, that the degree of dispersion depends critically on sample preparation conditions and in particular sonication time. Increased sonication times increase the amount of SWNTs debundled and solubilized but do not appear to affect the dispersion limit. However, increased sonication also induces discernible changes to the SWNTs themselves and in itself influences their solubility, under which conditions no clear solubility parameters can be determined.