Abstract
The interaction (adsorption process) of commercial ionic surfactants with non-functionalized and functionalized carbon nanotubes (CNTs) has been studied by potentiometric measurements based on the use of ion-selective electrodes. The goal of this work was to investigate the role of the CNTs’ charge and structure in the CNT/surfactant interactions. Non-functionalized single- (SWCNT) and multi-walled carbon nanotubes (MWCNT), and amine functionalized SWCNT were used. The influence of the surfactant architecture on the CNT/surfactant interactions was also studied. Surfactants with different charge and hydrophobic tail length (sodium dodecyl sulfate (SDS), octyltrimethyl ammonium bromide (OTAB), dodecyltrimethyl ammonium bromide (DoTAB) and hexadecyltrimethyl ammonium bromide (CTAB)) were studied. According to the results, the adsorption process shows a cooperative character, with the hydrophobic interaction contribution playing a key role. This is made evident by the correlation between the free surfactant concentration (at a fixed [CNT]) and the critical micellar concentration, cmc, found for all the CNTs and surfactants investigated. The electrostatic interactions mainly determine the CNT dispersion, although hydrophobic interactions also contribute to this process.
Highlights
Carbon nanotubes (CNTs) are a carbon allotrope synthesized for the first time by Oberlin et al by pyrolising benzene and hydrogen at 1100 ◦ C [1] and Iijima by using arcdischarge evaporation of carbon [2]
These results show that the electrostatic interactions do not play an important role in the adsorption of the ionic surfactants investigated and, as in the case of anionic single-walled carbon nanotubes [36], the mechanism of the adsorption process involves random interactions
Neither the type of the nanotubes nor their charge practically influence the formation of the CNT/surfactant complexes. These results show that the hydrophobic interactions are the driving force in the adsorption process of the surfactant molecules on the CNTs surfaces, these hydrophobic surfaces attracting the hydrophobic tails of the surfactants
Summary
Carbon nanotubes (CNTs) are a carbon allotrope synthesized for the first time by Oberlin et al by pyrolising benzene and hydrogen at 1100 ◦ C [1] and Iijima by using arcdischarge evaporation of carbon [2]. They have been used in electronics, polymer composites, energy storage materials, catalysis, gas storage materials, sensors, environment and biomedicine [4,5,6,7,8,9,10,11,12] These applications are limited due to a low and poor solubility in solution because of the existence of strong Van der Waals forces and π-π stacking interactions among the tubes, which provoke their agglomeration into ropes and/or bundles [13,14,15]. Chemical methods include covalent and non-covalent approaches [18] The former consists in the functionalization of the CNTs walls with different chemical groups, increasing their charge and, decreasing their agglomeration. This method can affect the electrical and mechanical properties of nanotubes [19,20]. The honeycomb lattice is preserved and the π bonds of the nanotubes remain undisturbed [21]
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