(Invited) Establishing Thermodynamically-Stable Surfactant Structures Around Single Wall Carbon Nanotubes
Authors: Ziegler Kirk J 
Published: February 2020

The high surface area of single-walled carbon nanotubes (SWCNTs) dictates that the interface with their surroundings is important in determining their properties or functionality. For example, the interface of SWCNTs with their surroundings is important to their application in polymer composites, devices, drug delivery, bioimaging and biosensing. Understanding and ultimately controlling these surface layers is important because of its influence on reactivity, adsorption of pollutants, and interaction with biological materials. SWCNT interfaces are often altered with surfactants to improve their dispersion in aqueous suspensions. While the surfactant surrounding the nanotube provides many benefits, the inability to alter or control this interface often limits the performance or functionality of the nanotube. A lack of information on the effect of the surrounding environment on SWCNT properties further complicates the development of processes to control these interfaces.
Our group has focused on characterizing and controlling SWCNT interfaces. The ability to reorganize the surfactant structure surrounding SWCNTs is achieved through both chemical and mechanical manipulations. The goal is to learn how to control these structures around SWCNTs to influence their interaction with biological materials and other surfaces. For example, the surfactant orientation on the SWCNT surface plays an important role in their separation by nearly any method. Using our understanding of the surfactant structure surrounding SWCNTs, we have achieved high-fidelity separations based on selective desorption with a single elution profile in a single hydrogel-packed column. The selective desorption of a wide range of single-chirality (n,m) fractions only occurs once a specific ratio of sodium dodecyl sulfate (SDS)/sodium deoxycholate (DOC) co-surfactant solution is used. High-purity fractions of small diameter nanotubes are obtained at this ratio even with long elution times, different total co-surfactant concentrations, and moderate temperature changes. The elution of only one (n,m) type at a specific co-surfactant ratio while other types are exposed to more surfactant suggests that each (n,m) type forms a thermodynamically-stable surfactant structure in the co-surfactant solution. These thermodynamic equilibrium states result in entropy-driven desorption at specific co-surfactant ratios, enabling high-fidelity separations of a single (n,m) type in a single column. Although the differences between the co-surfactant ratios needed to separate two (n,m) species can sometimes be very small, the separation can be improved at higher co-surfactant concentrations at the same ratio. The elution order is nearly identical to the order of separation in the aqueous two-phase extraction, suggesting that the same processes are occurring regardless of the method used to separate the SWCNTs. The structure of the surfactant or other molecules around the SWCNTs is also shown to have important implications in toxicology and drug delivery.