(Invited) Chirality-Dependent Surfactant Interactions Unravelled By Systematic Ultracentrifugal and Aqueous Two-Phase Separations

Abstract

The discovery that natural bile salt surfactants are extraordinarily efficient at solubilizing individual, intact single-wall carbon nanotubes (SWCNTs) in water[1], soon enabled the separation of different SWCNT structures for the first time by density gradient ultracentrifugation (DGU),[2] and more recently also by aqueous two-phase extraction (ATP).[3,4] Despite very nice results for a limited number of small diameter SWCNT chiralities that have been isolated as individual species by DGU and ATP, the underlying mechanisms are not fully understood and the number of variables to be explored and optimized is enormous, preventing a systematic extension to other chiralities. Based on the unique capabilities of optical spectroscopy for characterizing SWCNTs, we present a systematic study of the structure sorting of DGU and ATP. We combine a direct in situ optical characterization of the centrifuge tubes after DGU[5] with systematic concentration-dependent ATP separations for different (mixed) surfactants and as such obtain a more detailed understanding of and control over both separation mechanisms. These systematic studies reveal that chirality-dependent surfactant interactions determine the separation order. [1] W. Wenseleers et al., Adv. Fucnt. Mater. 2004, 14, 1105 [2]. M.S. Arnold et al., Nature Nanotechnol. 2006, 1, 60 [3]. C.Y. Khripin et al., J. Am. Chem. Soc. 2013, 135, 6822 [4]. N.K. Subbaiyan et al., ACS Nano 2014, 8, 1619 [5]. S. Cambré et al., Nanoscale 2015 , 7, 20015