Quantitative Evaluation of Surfactant-stabilized Single-walled Carbon Nanotubes: Dispersion Quality and Its Correlation with Zeta Potential

Abstract

Stable dispersions of single-walled carbon nanotubes in deionized water were prepared using six common surfactants: sodium dodecylbenzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), lithium dodecyl sulfate (LDS), tetradecyl trimethyl ammonium bromide (TTAB), sodium cholate (SC), and Fairy liquid (FL). For all nanotube dispersions (C NT ) 1 mg/mL), the optimum concentration of surfactant was found to be close to C Surf ) 10 mg/mL by measuring the fraction of nanotubes remaining after centrifugation for a range of surfactant concentrations. The aggregation state of each nanotube-surfactant dispersion was characterized as a function of nanotube concentration by AFM analysis of large numbers of nanotubes/bundles deposited onto substrates. The dispersion quality could then be quantified by four parameters: the saturation value (at low concentration) of the root-mean-square bundle diameter, the maximum value of the total number of dispersed objects (individuals and bundles) per unit volume of dispersion, the saturation value (at low concentration) of the number fraction of individual nanotubes, and the maximum value of the number of individual nanotubes per unit volume of dispersion. According to these metrics, the dispersion quality of the six surfactant-nanotube dispersions varied as SDS > LDS > SDBS > TTAB > SC > Fairy liquid. It was found that each of these dispersion-quality metrics scaled very well with the measured ζ-potential of the surfactant-nanotube dispersion. This confirms that dispersion quality is controlled by the magnitude of electrostatic repulsive forces between coated nanotubes.

Liquid Phase Production of Graphene by Exfoliation of Graphite in Surfactant/Water Solutions

“…Hence, we apply the Smoluchowski approximation in our measurements 24 ; this is in line with previous work on carbon nanotube dispersions in SDBS. 17,22 The natural pH of our dispersions was 7.4, which matches a literature value for SDBS stabilised carbon nanotube dispersions. 22 We observed a zeta potential distribution for a fresh graphite/graphene dispersion centred at -44 mV ( Figure 5A).…”

“…Measurement of the fraction remaining showed a broad peak (lower inset, Figure 2), similar to those observed for nanotube-surfactant dispersions 17 , with reasonably high quantities of graphite remaining for C G,i between 0.5 mg/ml to 1 mg/ml. The fall-off in dispersed graphite below C SDBS~0 .5 mg/ml is reminiscent of the destabilisation of nanotube dispersions as the surfactant concentration is reduced below the critical micelle concentration 17,20 (~0.7 mg/ml for SDBS 21 ). Keeping the concentration of surfactant to a minimum is desirable for many potential applications, so, all subsequent experiments were performed on standard dispersions with C SDBS = 0.5 mg/ml and C G,i = 0.1 mg/ml.…”

“…This mechanism has allowed the successful dispersion of carbon nanotubes in a range of surfactants. 16,17,22,23 The zeta potential is the potential at the interface between the adsorbed surfactant molecular ions and the diffuse region of mobile counterions. As such it is a measure of the electrostatic repulsion between surfactant coated flakes.…”

“…We disperse graphite in surfactant-water solutions in a manner similar to surfactant aided nanotube dispersion [12][13][14][15][16][17] . By TEM analysis we demonstrate significant levels of exfoliation including the observation of a number of graphene monolayers.…”

Raman and FT-IR studies on dye-assisted dispersion and flocculation of single walled carbon nanotubes

“…Noncolvalent modification of SWCNTs with surfactants is also recognized as an efficient method to disperse SWCNTs in water and the suspended individual SWCNTs can be driven out of water by either high force centrifugation or mocrofiltration [7][8][9]. Both are costly, time consuming and not amenable to scale up.…”