Effect of surfactant concentration on the rheological and aggregation behavior of a carbon nanotube suspension

We investigated the effect of surfactant concentration on the rheological behavior of suspensions containing multi-walled carbon nanotubes (MWNTs) dispersed in N-methyl-2-pyrrolidone (NMP) mixed with nonionic surfactant, polyoxymethylene sorbitan monooleate (Tween 80). The surfactant concentration ranged from 0 to 10 wt.% relative to the MWNT particles. As the surfactant concentration increased, the shear viscosities initially decreased, reaching a minimum at 6 wt.%, and then increased. Similar trends were observed in the behavior of the yield stress and the power-law index. In oscillatory shear tests, storage modulus increased with the surfactant concentration up to 6 wt.% and then decreased. Aggregation structure was analyzed using a scaling theory, based on the suspension’s elasticity obtained from linear viscoelastic measurements. The dependence of elastic modulus on MWNT concentration at different surfactant concentrations provided fractal dimension of average aggregate. It was found that the addition of surfactant lowered the fractal dimension of average aggregates from 1.9 to 1.2–1.3. The fractal dimension at the surfactant concentration of 6 wt.% showed slightly lower values than those of 2 wt.% and 10 wt.%. which is the same trend as the rheological properties. These findings were further supported by a UV–visible-NIR spectroscopy method, which measured the light absorbance of individual MWNTs in diluted suspensions. The UV–visible-NIR spectroscopy analysis showed maximum absorbance at a surfactant concentration of 6 wt.%, indicating optimal dispersion of the MWNTs. This behavior aligned with the observed trends in shear viscosity and fractal dimension.

Graphical Abstract