Orthogonal Determination of Competing Surfactant Adsorption onto Single-Wall Carbon Nanotubes During Aqueous Two-Polymer Phase Extraction Fluorescence Spectroscopy and Analytical Ultracentrifugation.

A combination of analytical ultracentrifugation (AUC) and fluorescence spectroscopy are utilized to orthogonally probe compositions of adsorbed surfactant layers on the surface of (7,5) species single-wall carbon nanotubes (SWCNTs) under conditions known to achieve differential partitioning in aqueous two-phase extraction (ATPE) separations. Fluorescence emission intensity and AUC anhydrous particle density measurements independently probe and can discriminate between adsorbed surfactant layers on a (7,5) nanotube comprised of either of two common nanotube dispersants, the anionic surfactants sodium deoxycholate and sodium dodecyl sulfate. Measurements on dispersions containing mixtures of both surfactants indicate near total direct exchange of the dominant surfactant species adsorbed to the carbon nanotube at a critical concentration ratio consistent with the ratio leading to partitioning change in the ATPE separation. By conducting these orthogonal measurements in a complex environment reflective of an ATPE separation, including multiple surfactant and polymer solution components, the results provide direct evidence for the hypothesis that it is the nature of the adsorbed surfactant layer that primarily controls partitioning behavior in selective ATPE separations of SWCNTs.