Aqueous two-phase extraction (ATPE) is a widely used technique for separating sub-populations of specific (n,m) structures of single-wall carbon nanotubes (SWCNTs) from synthetic mixtures through scalable, liquid-phase, processing. Most commonly, the partitioning of different (n,m) SWCNT structures is controlled by the competition of two or more surfactants for the nanotube surface. However, validation of the surface competition model has not been directly demonstrated. Using a recently developed fluorescence-based method for rapidly determining the extraction conditions for specific (n,m) SWCNT species, we found that the change in the nature of the extraction is directly correlated with a dramatic change in fluorescence intensity of the particular (n,m) structure. Via analytical ultracentrifugation, we have now orthogonally determined the density of the bound surfactant on specific nanotube (n,m)s below, near, and above the surfactant concentrations yielding the extraction change, as well as for the two co-surfactants separately. These results show that the fluorescence and extraction conditions changes are the result of a change in the composition of the dominant adsorbed surfactant at a specific composition ratio, validating the surfactant surface competition model for surfactant-ATPE.

Determining surfactant coverage on single-wall carbon nanotubes during aqueous two-phase extraction using Analytical Ultracentrifugation

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