Behavior of na⁺-polystyrene sulfonate at the interface with single-walled carbon nanotubes (SWNTs) and its implication to swnt suspension stability

The assembly of sodium polystyrene sulfonate (Na⁺-PSS) at the surface of single-walled carbon nanotubes (SWNTs) in pH 3 aqueous solution is described. Rather than forming linear or sheet-like chain morphologies over SWNT surfaces, Na⁺-PSS adopts a spherically collapsed conformation believed to be the result of cation (either Na⁺ or H⁺) condensation onto the ionized polymer chain. It is well reported that cations (and also anions) adsorb preferentially onto single-walled and multi-walled carbon nanotube surfaces leading to an increased ion concentration in the near surface regions relative to the bulk solution. This work provides experimental evidence for preferentially absorbed cation condensation onto PSS anions until those cations are spaced at distances corresponding to the Bjerrum length (ℓB), as defined by the Manning theory of ion condensation, at the SWNT surface. The resulting electrostearic repulsions allow the SWNTs to remain suspended for days. Furthermore, coulombic repulsion among SWNT bundles after cation adsorption alone is not sufficient to form stable suspensions-but rather the stearic repulsions associated with spherically collapsed PSS at the nanotube surface is responsible for suspension stability. It is believed that the ultrasonic agitation drives cations into the small spaces between SWNT bundles and coulombic potential attracts the PSS to those regions.