Abstract
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 ( B), 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.
Highlights
Polyelectrolyte self-assembly is widely applicable technology which is simple in construct, yet stands to gain from further understanding of fundamental processes occurring during the assembly approach [1,2]
The approach taken in driving the adsorption of polystyrene sulfonate (PSS) onto single-walled carbon nanotubes (SWNTs) surfaces is the electrostatic layer-by-layer (LbL) approach—in which positively charged SWNT surfaces coulombically attract the PSS polyanion
The net positive charge on the SWNT surfaces are formed by adjusting the pH of the aqueous medium to a value at which the electric double layer around the carbon nanotubes (CNTs) is terminated by positive
Summary
Polyelectrolyte self-assembly is widely applicable technology which is simple in construct, yet stands to gain from further understanding of fundamental processes occurring during the assembly approach [1,2]. I.e., those whose charge has been compensated for by the presence of counterions in solution, tend to collapse into spherical morphologies [3] and adsorb onto oppositely charges surfaces with greater film thicknesses, relative to rod-like morphologies [4]. This change in film thickness is potentially associated with the adsorption of spherically collapsed morphologies. The mathematical criterion for counterion condensation is avoidance of a diverging partition sum at monomer spacing closer than the Bjerrum length, B, times the electrostatic point charges, ZcZp. That is to say, at the condition (3). Results show that PSS adsorbs at the nanotube surface in a spherically coiled morphology and disperse CNT bundles into individual tubes which remain in suspension for days
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