A new simple theoretical-experimental approach to estimate surface charge of surfactant-covered CNTs

The power of binary mixtures of dodecyl trimethyl ammonium bromide and sodium dodecyl sulfate surfactants for dispersion of multi-wall carbon nanotubes (MWCNTs) in aqueous media was investigated. Stable dispersions were obtained at lower total surfactant concentration as compared to their concentration when used alone, indicative a synergistic effect in these catanionic mixtures. The dispersion of carbon nanotubes was characterized using UV-Vis spectroscopy and Zeta potential measurement. In this study, for the first time we introduce a new simple approach to estimate surface charge of surfactant-covered CNTs in a combined theoretical-experimental way. To do so, we have utilized the well-known Poisson-Boltzmann equation and thermo gravimetric analysis (TGA) results. According to the Poisson equation, calculation of electric potential needs charge density, a parameter which in this work has been estimated using thermo gravimetric analysis results. The TGA-based estimated potentials are in good agreement with experimental results. Ultimately, using the surface charge results, we have calculated interaction energy between two parallel tubes. Calculated results clearly show that the dispersions could be stable, which has been confirmed experimentally too by using zeta potential measurements.