The role of the bile salt surfactant sodium deoxycholate in aqueous two-phase separation of single-wall carbon nanotubes revealed by systematic parameter variations

Aqueous two-phase (ATP) extraction has been demonstrated as a fast, scalable, and effective separationtechnique to sort single-wall carbon nanotubes (SWCNTs) according to their diameter and chiral structure.The exact mechanism behind the chirality-dependent migration of SWCNTs between the twophases is however not completely understood, and depends on many parameters (e.g., choice of surfactantsand their concentration, pH, temperature, …), making it difficult to optimize the multivariableparameter space. In this work, we present a systematic study of the choice and concentration of specificsurfactants on the ATP sorting, by performing a series of single-step ATP separations in which each timeonly one parameter is systematically varied, while monitoring the structure-specific migration of everySWCNT chirality between both phases with detailed wavelength-dependent spectroscopy. These systematicstudies reveal that the diameter-dependent stacking of a discrete number of sodium deoxycholatemolecules fitting around the SWCNT circumference determines the separation order in the formof a periodically modulated pattern as a function of SWCNT diameter. Addition of cosurfactants can beused to compete with the bile salt surfactant to enhance the separation yields, but does not affect thesorting order. The results are afterwards directly applied to predict the parameters required to separatespecific chiral structures in just two ATP steps.