New Responsive Surfactants for Aqueous Dispersion of CNTs and Graphene

Abstract

We have developed a flexible approach to the synthesis of surfactants with an ‘Anchor Linker-Head’ (ALH) architecture. These ALH surfactants are designed for the dispersion of multi-walled carbon nanotubes (MWNTs) and exfoliation of graphite in water. Four series of surfactants have been synthesised, all with a pyrene anchor group, which binds strongly to graphitic surfaces through π-π interactions, and hydrophilic head groups based on a carboxylate moiety, carboxylate dendron, crown ether or podand. These are joined by oligoethylene glycol (OEG) linker groups.

The anionic surfactants PyrB-PEGn-CH2COONa (n = 2, 4, 6, 12) PyrB-PEGn-CH2COG1(ONa)3 (n = 2, 4, 6) all disperse MWNTs at least as well as commercial surfactants in Millipore water and achieve higher dispersion levels than comparable amide linker surfactants. Non-ionic surfactants are more effective, dispersing up to 61% of the MWNT feedstock. Exfoliation of graphite has been achieved using anionic and non-ionic surfactants. We examined the effect of salts, including NaCl, KCl and CaCl2, on the ability of surfactants to disperse MWNTs and found the choice of linker and head group to be significant. MWNT dispersing ability in 0.6 M NaCl increases with OEG linker length. Structural variation gives surfactants which show improved, reduced, or comparable dispersion levels in 0.6 M NaCl vs. Millipore water, due to the effects of ionic screening and cation coordination.

MWNTs dispersed using anionic surfactants can be precipitated by addition of acid, and re-dispersed by addition of base. Eleven non-ionic surfactants have a lower critical solution temperature (LCST), which is tuned by structural changes. We demonstrate using PyrB-PEG4-CH2CO(15-c-5) that LCST surfactants with a pyrene anchor can be used to repeatedly and reversibly precipitate dispersed MWNTs without harsh re-processing. We believe this to be the first report of such behaviour using a small molecule dispersant.