Reciprocal effects of multi-walled carbon nanotubes and oppositely charged surfactants in bulk water and at interfaces

Abstract

In this work we made the complementary use of nuclear magnetic resonance spectroscopy, dynamic light scattering, electric conductivity and surface tension methods to study the micellization of cationic surfactants cetyl trimethylammonium bromide (CTAB) and dodecyltrimethylammonium bromide (DTAB), as well as anionic surfactant sodium dodecyl sulfate (SDS) in the presence of multi-walled carbon nanotubes (CNT). It was shown that the CNTs is effectively dispersed in water medium due to the adsorption of surfactant ions at CNT surface. The NMR spectral analysis (chemical shifts and line broadening) specified that surfactants interact with CNTs surface both by alkyl chains and by charged head groups. For cationic CTAB and DTAB this interaction occurs mainly through methylene (CH2)n groups of alkyl radicals, at the same time, the anionic SDS interacts with the CNTs surface by terminal methyl groups (CH3). Interaction specificity of cationic and anionic surfactants leads to their different structural organization to CNT surface. Experimental observations make it possible to propose the model where the attraction of cationic DTAB and CTAB positively charged head groups with negatively charged CNT surface results to the orientation of cationic surfactants along CNT surface. In contrast to positively charged DTAB and CTAB the repulsion of SDS with negatively charged head group from CNT surface and contacted SDS terminal groups results in the normal orientation of anionic surfactant to CNT surface. The consistency of proposed orientation model arrived from conductivity study of surfactant-CNT dispersions. According to conductivity data, the non-micellized cationic surfactants follow complete dissociation in the presence of CNT no matter are they free or bound to CNT. On the contrary the absence of complete dissociation of anionic surfactant in the pre-micellar systems may be the result of SDS ordering on CNT surface and forming the charged surface with double electric layer binding the portion of counter-ions.