Aqueous two-phase (ATP) separation has been demonstrated as a fast, highly-scalable separation technique to sort single-wall carbon nanotubes (SWCNTs) by diameter, chirality, metallicity, etc.[1-2] In ATP, two water-soluble, yet immiscible polymers are mixed together at sufficiently high concentration after which they spontaneously form two phases with different hydrophilicity. In a typical separation protocol, SWCNTs are added to this two-polymer mixture with a combination of different surfactants, resulting in separation of different chiral species among both phases. While very promising separations have been achieved, the separation mechanism is not well understood

In this paper, we present a series of ATP separations, in which surfactant combinations and concentrations are systematically varied. Absorption spectroscopy, multi-wavelength resonant Raman scattering and wavelength-dependent photoluminescence-excitation spectroscopy, combined with detailed two-dimensional spectral analysis, is applied to characterize the full composition of the resulting separated fractions and as such study the underlying separation mechanism. These systematic studies reveal that chirality-dependent surfactant interactions determine the separation order.

[1]. C.Y. Khripin et al., J. Am. Chem. Soc. 2013, 135, 6822

[2]. N.K. Subbaiyan et al., ACS Nano 2014, 8, 1619