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In our previous simulation study [J. Phys. Chem. C, 2011, 115, 17286], branched sodium dodecyl benzene-sulfonate (SDBS) surfactants showed self-assembled structures on single-walled carbon nanotubes (SWNTs) that were strongly dependent on tube diameter. Those results suggested that branched SDBS, as opposed to their linear counterparts, could specifically stabilize SWNTs of narrow diameter. Experimental data, however, show that SDBS stabilizes aqueous SWNTs of many diameters. This discrepancy between simulated and experimental results could be explained by the fact that experimental SDBS samples are isomeric mixtures. To test this possibility we report here molecular dynamics (MD) simulation results for equimolar mixtures of aqueous linear and branched SDBS on (6,6) and (20,20) SWNTs at ambient conditions. Our results suggest that there is no strong effect due to nanotube diameter on the morphology of mixed SDBS surfactant aggregates, although the adsorbed aggregate structure strongly depends on surfactant coverage. In-plane radial distribution functions suggest that linear and branched molecules distribute evenly onto the surfaces of (6,6) SWNTs, while some evidence of segregation, in which branched SDBS predominantly pack near other branched molecules, was obtained on (20,20) SWNTs at high surface coverage. These results suggest that the lack of specificity in stabilizing aqueous dispersions of carbon nanotubes using SDBS surfactants is probably due to the presence of multiple isomeric molecules in commercial surfactant samples. Perhaps more importantly, these simulations suggest that using mixtures of surfactants could affect the structure of the adsorbed aggregates, and the stability of aqueous dispersion of carbon nanotubes.

Equimolar mixtures of aqueous linear and branched SDBS surfactant simulated on single walled carbon nanotubes