Tuning the Surface Ordering of Self-Assembled Ionic Surfactants on Semiconducting Single-Walled Carbon Nanotubes: Concentration, Tube Diameter, and Counterions

Abstract

We report direct spectroscopic measurements of the macromolecular organization of ionic surfactants on the surface of semiconducting single-walled carbon nanotubes (SWCNTs) within solution-processed thin films. By using vibrational sum frequency generation (VSFG) spectroscopy, sensitive measurements of interfacial surfactant ordering were obtained as a function of surfactant concentration for sodium dodecyl sulfate (SDS)-encapsulated (6,5) and (7,6) SWCNTs with and without excess electrolytes. Anionic surfactants are known to effectively stabilize SWCNTs. The current models suggest a strong influence of the dispersion conditions on the surfactant interfacial macromolecular organization and self-assembly. Direct experimental probes of such an organization using nanotubes of specific chirality are needed to validate the existing models. We found that as the bulk SDS concentration increases near the surfactant critical micelle concentration, the interfacial ordering increased, approaching the formation of cylindrical-like micelles with the nanotube at the core. At the higher surfactant concentrations measured here, the (6,5) SWCNTs produced more ordered structures relative to those with the (7,6) SWCNTs. The relatively larger-diameter (7,6) chiral tubes support enhanced van der Waals (vdW) interactions between the tube carbon surface and the surfactant methylene chain groups that likely increase the density of gauche defects. A new effect arises when the precursor solution is exposed to a small concentration of divalent Ca counterions. We postulate that a salt-bridging configuration on such highly curved surfaces decreases the ordering of interfacial surfactant molecules, resulting in compact, disordered structures. However, this phenomenon was not observed with excess Na ions at the same ionic strength. Instead, a modest increase in surfactant ordering was observed with the excess monovalent electrolyte. These results provide new insights for thin film solution processing of vdW nanomaterials and demonstrate that VSFG is a sensitive probe of surfactant organization on nanostructures.

Reclined trend of alkyl chain of sodium dodecylbenzenesulfonate molecules induced by friction

“…All spectra were collected under the ). These assignments were consistent with those of previous SFG-VS studies [9,10,[41][42][43][44]. As discussed in the supporting information, in order to revise the IR pulse shape and normalization, we first measured the SFG signal of the sample, and then measured the signal of gallium arsenide (GaAs) in the same state.…”

“…In order to reveal the mechanisms of surfactant molecules adsorbed on solid surfaces, numerous studies have been taken on various surfaces at ultra-smooth surfaces such as: gold [2,3], mica [4,5], graphite [6,7], C18 [8], single-walled carbon nanotubes (SWCNTs) [9,10], and multi-walled carbon nanotubes (MWCNTs) [11], or macro scale surface such as filter paper [12,13], silica [14,15], silica gel [16], CaF 2 [17], alumina [18,19], alumina particle, carbonate materials [20,21], and stainless steel [22]. In these studies, various detection methods such as atomic force microscopy (AFM), scanning probe microscopy (SPM), dual polarization interferometry (DPI), quartz crystal microbalance † Xianzhang WANG and Yuan LIU contributed equally to this work.…”

“…Das et al [9] investigated the degree of interfacial ordering for SDS surfactants adsorbed on the surface of SWCNT and the effect of bulk concentration of surfactant solution on alkyl chain order. Algoul et al [10] investigated the interfacial ordering of SDS within solution-processed thin films under various environmental conditions. Both of found that the interfacial conformational order of SDS decreases with decreasing concentration, which results in the adsorption of surfactants on the nanotube surfaces more disordered [9,10].…”

“…Algoul et al [10] investigated the interfacial ordering of SDS within solution-processed thin films under various environmental conditions. Both of found that the interfacial conformational order of SDS decreases with decreasing concentration, which results in the adsorption of surfactants on the nanotube surfaces more disordered [9,10].…”

Imaging the reactivity and width of graphene's boundary region

“…30 and the ESI. † Details of the custom-built infrared (IR)-visible (Vis) SFG spectrometer 33,34 are also provided in the ESI, † but importantly, the IR beam was mildly focused to a spot size of B200 mm, thus determining our spatial resolution. The spatial resolution of the Raman maps is B0.5 mm.…”