Surfactant concentration dependent spectral effects of oxygen and depletion interactions in sodium dodecyl sulfate dispersions of carbon nanotubes.

Quenching of optical absorbance spectra for carbon nanotubes (CNTs) dispersed in sodium dodecyl sulfate (SDS) has been observed to be more pronounced at higher concentrations of the surfactant. The protonation-based quenching behavior displays wavelength dependence, affecting larger diameter nanotube species preferentially. Although absorbance may be recovered by hydroxide addition, pH measurements suggest that hydrolysis of SDS does not play a major role in the short term quenching behavior at high SDS concentrations. The degree of quenching is observed to correlate well with an increase in attractive depletion as SDS concentration is increased, while the extent of depletion is found to depend heavily on the concentration of preparation in comparison to the final SDS concentration. Attractive depletion in SDS is also found to be preferential for CNTs of larger diameter. It is proposed that depletion enhances the quenching effect due to close association of CNT-SDS complexes providing higher SDS densities on the CNT surface, leading to further oxidation. In addition, the quenching behavior in SDS is found to strongly suppress the optical and Raman signal from metallic nanotube species even at high pH. Displacement of SDS by sodium deoxycholate as a secondary surfactant is able to reverse the effects of protonation of metallic species, whereas hydroxide addition is only partially effective.