Discovery of a universal mechanism for acid-modulated surfactant-based chiral separation of carbon nanotubes

Abstract

The acid modulation technique, which employs acids to facilitate the chiral separation of surfactant-dispersed single-wall carbon nanotubes (SWCNTs), is recognized as one of the most promising approaches due to its simplicity, high resolution, and efficiency. A comprehensive understanding of the underlying mechanisms is crucial for the rational optimization and further advancement of this technique. However, these mechanisms remain poorly understood, significantly hindering its advancement. Here, we elucidate the mechanisms by systematically investigating the interactions between acids and various surfactants including sodium dodecyl sulfate (SDS), sodium cholate (SC) and sodium deoxycholate (DOC), as well as between acids and SWCNTs, in conjunction with the selective adsorption of SWCNTs within the gel matrix induced by acid modulation. In single surfactant system of SDS, hydrogen ions from the acid selectively protonate SWCNTs, enhancing the attraction of the anionic surfactant SDS to their surfaces, thereby weakening their selective adsorption onto the gel matrix. In a binary surfactant system comprising bile salt surfactants (SC or DOC) and SDS, the added acids preferentially react with bile salt surfactants to form bile acid surfactants, which modulate the surfactant coating on SWCNT surfaces and drive the separation process by forming compound micelles with SDS. Based on this new understanding, we have advanced gel chromatography to achieve efficient enantiomer-level separation of multiple single-chirality SWCNTs by combining chirality-selective enrichment through acid modulation with subsequent selective desorption using a stepwise elution process. Our research establishes a significant foundation for the development of advanced surfactant-based separation techniques and future applications of SWCNTs.