Role of surfactant structure in aqueous dispersions of carbon nanotubes

Abstract

Gemini surfactants typically consist of two single-chain surfactant schemically connected by a spacer chain. Recently, it has been shown experimentally that they are able to effectively disperse and stabilize carbon nanotubes (CNTs) in aqueous solutions at very low surfactant concentration. To aid elucidate the role of surfactant structure in the CNT dispersion process, we report herein the results of fully atomistic molecular dynamics (MD) simulations of the adsorption and surface self-assembly of a cationic single-chain surfactant, dodecyltrimethyl-ammonium bromide (C12TAB), and its related Gemini surfactant dimethylene-α,β-bis (dodecyldimethylammonium bromide) [12-2-12]Br2, on (5,7), and (10,14) single-walled carbon nanotubes (SWNTs) in aqueous solution at ambient conditions. We find that the morphology of surfactant aggregates on the SWNT is influenced by the surfactant structure. The same number of [12-2-12]Br2 Gemini surfactants adsorbed are able to cover a larger surface area of SWNT and their head-groups are protruded more extensively toward the aqueous phase, preventing water molecules from accessing the nanotube surface. These morphological results propose that [12-2-12]Br2 should be more effective than C12TAB at stabilizing aqueous dispersions of carbon nanotubes. Furthermore, the influence of SWNT diameter and length of the Gemini surfactant spacer on the structure of aggregates formed onto nanotubes are investigated. These simulation results advance our understanding of the mechanism of CNT solubilization via Gemini surfactants.