Surfactants' Key Role in Ionic Aqueous Two-Phase Separation of Carbon Nanotubes Towards Simple Chirality Isolation

SWCNTs are known to have wide distribution in diameter/chirality when produced; leading to variability in the starting material chirality distributions with respect to production batch and method. Therefore numerous separation methods that provide nearly single-chiralities have been reported. Most of these separation techniques involved numerous iterations to achieve high purity (gel chromatography, DGU) and long separation times, thereby preventing low-cost scalability of the method. Recently aqueous two-phase (ATP) separation has been explored as a new promising method to separate single-wall carbon nanotubes (SWCNTs) by diameter. In this work, we determined that the ATP separation mechanism is driven by the hydrophobicity of the surfactant, or combination of surfactants, covering the SWCNTs, which can be tuned by mixing different surfactants in well-defined ratios. Having a three-surfactant system, with very different diameter-dependent affinities for SWCNTs, and the combination of salt to adjust the specific surfactant structure surrounding the SWCNTs, has allowed us to develop a simple strategy for obtaining highly enriched single-chirality suspensions  of (6,5) SWCNTs in only 1 or 2 steps, thereby obtaining a high separation yield and simplifying the separation technique. Single chirality solutions can now be obtained in an extremely fast, and easy-to-scale-up, cheap way.