  Aqueous two-phase (ATP) extraction has been demonstrated as a fast, scalable,
and effective separation technique 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 two phases is
however not completely understood, and depends on many parameters (e.g., choice
of surfactants and their concentration, pH, temperature, ...), making it
difficult to optimize the multivariable parameter space. In this work, we
present a systematic study of the choice and concentration of specific
surfactants on the ATP sorting, by performing a series of single-step ATP
separations in which each time only one parameter is systematically varied,
while monitoring the structure-specific migration of every SWCNT chirality
between both phases with detailed wavelength-dependent spectroscopy. These
systematic studies reveal that the diameter-dependent stacking of a discrete
number of sodium deoxycholate molecules fitting around the SWCNT circumference
determines the separation order in the form of a periodically modulated pattern
as a function of SWCNT diameter. Addition of cosurfactants can be used to
compete with the bile salt surfactant to enhance the separation yields, but
does not affect the sorting order. The results are afterwards directly applied
to predict the parameters required to separate specific chiral structures in
just two ATP steps.
