Enhanced solubilization of carbon nanotubes in aqueous suspensions of anionic-nonionic surfactant mixtures

Single-walled carbon nanotubes (SWCNTs) suspensions in aqueous solutions of sodium dodecyl sulfate (SDS) and saturated fatty acids (Cn) are studied. The quality of the dispersions is analyzed by photoluminescence spectroscopy (PL) as a function of the Cn chain length. Resonant Raman scattering (RRS) measurements and molecular dynamics (MD) simulations were also carried out in order to study the effect of the surrounding medium on SWCNTs properties in suspensions. Both PL and RRS data indicate an increased individualization of SWCNTs in the dispersions for Cn's having an alkyl chain longer than SDS. MD simulations showed the formation of mixed Cn-SDS aggregates around a nanotube in water and a Cn binding energy to the nanotube wall that increases linearly with chain length. The enhanced solubilization of SWCNTs is thus interpreted in terms of the reduced electrostatic repulsion within the surfactant aggregates and the increased binding energy to the nanotube wall. Powders prepared by the evaporation of dispersions of Cn's and SWCNT bundles in ethanol were also studied by RRS in the radial breathing mode (RBM) frequency range. All the measured RBM frequencies exhibit a blue-shift (Δω) with respect to the values obtained for pristine SWCNT powders. Remarkably, nanotubes with diameters smaller than 1.0 nm show Δω in the range 2.5-4.5 cm ^-1 while those having diameters larger than 1.0 nm exhibit Δω in the range 6.5-8.0 cm^-1. MD simulations showed that large diameter nanotubes encapsulate Cn's into their cores, thus justifying the increased hardening of the RBM mode.