Phase Behavior and Shear Alignment in SWNT‐Surfactant Dispersions

Abstract

The effect of single-walled carbon nanotubes (SWNT) on the phase behavior of the cationic surfactant cetyltrimethylammonium bromide (CTAB) in aqueous solutions is investigated at room temperature. Small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM) are used for characterization of bulk dispersions and nanometrically thin films. Additional carbonaceous additives (fullerenes, multi-walled carbon nanotubes, and carbon black) serve as reference systems. It is found that dispersions of carbonaceous additive (excluding fullerenes) at intermediate surfactant concentrations (below the liquid-crystalline region of the native surfactant) induce demixing and macroscopic phase separation in otherwise homogeneous solutions of CTAB. Two coexisting liquid phases of similar CTAB concentrations are observed, with the carbonaceous species residing within the lower phase. At high CTAB concentrations (liquid-crystal region) the SWNTs are found to incorporate into the ordered lyotropic liquid-crystalline phase while preserving the native d-spacing. Investigation of nanometrically thin films at intermediate surfactant concentrations under external shear reveals shear-induced structure (SIS) in the presence of minute amounts of SWNTs. The effect is found to be exclusive to SWNT and does not occur in dispersions of other carbonaceous additives.

Filament formation in carbon nanotube-doped lyotropic liquid crystals

“…As suggested above we propose that this is due to the ordering effect of the CNTs on the surrounding CTAB micelles. 17,18 They are generally longer and extend linearly over greater distances than in the corresponding nanotube-free CTAB solution. With extended average micelle length the critical shear rate for macroscopic alignment and thus for cross-over to a less viscous regime is reduced, just like in pristine CTAB solutions upon increasing the surfactant concentration 14 .…”

“…The crucial role of the nanotubes is to give the monomer micelles the required exceptional length and stiffness, a situation which has been confirmed through cryo electron microscopy investigations by Regev and co-workers for CTAB micelles in the vicinity of carbon nanotubes. 17,18 . The role of the nematic host phase is to provide a quadrupolar ordering field that was recognized by van der Schoot as a key factor in the divergence of the effective chain length.…”

Liquid Crystals of Carbon Nanotubes and Carbon Nanotubes in Liquid Crystals

“…Thus, while the uniform alignment was corroborated with polarized Raman spectroscopy also in this work, the fact that the director of the liquid crystal phase could easily be uniformly aligned over square centimeter areas by shearing allowed the transfer of uniaxial orientational order to the nanotubes to be verified by simple inspection by eye through a linear polarizer. Importantly, the method combining cationic and anionic surfactants works equally well for single-wall CNTs [67] as well as for multi-wall CNTs [66], whereas other work with lyotropic liquid crystal hosts reported ordering only for single-wall but not for multi-wall nanotubes, presumably because the MWCNT diameter is much greater than that of the native lyotropic phase micelles [65,75]. In the cationic system each nanotube is most likely surrounded by a 'buffer layer' of oppositely charged native micelles.…”

“…This leads to an entropy penalty that can be relieved if the CNTs aggregate, giving rise to the depletion attraction phenomenon. It seems to be of particular importance when using the commonly employed anionic surfactant sodium dodecyl sulfate (SDS) for aiding the dispersion, whereas the cationic surfactant hexadecyl trimethylammonium bromide (CTAB) turns out to be much less susceptible to induce depletion attraction [75,76]. In fact, the beautiful cryo-TEM work of Nativ-Roth and co-workers revealed that SWCNTs dispersed in CTAB tend to align uniformly within small clusters at intermediate surfactant concentrations, well above the critical micelle concentration but below the threshold for development of a macroscopically ordered liquid crystal phase.…”

Electro-conductive porous scaffold with single-walled carbon nanotubes in wormlike micellar networks

“…The diameter of the micellar bundles in the FISP is $100 ± 15 nm with a length of $350 ± 45 nm [26], while FISPSWCNTs scaffold exhibits a wider distribution of bundle diameters, ranging from 30 nm to 2 lm. We suspect that electrostatic interactions among CTAB molecules (positively charged), SWCNTs (positively charged), and SDS molecules (negatively charged) [9,10,12,13,36] introduce spatially inhomogeneously charged elements in the scaffold, yielding wider size distribution in the bundle diameter. Since an individual CTAB/NaSal wormlike micelle has a diameter of $5 nm and each thin FISP-SWCNTs bundle has an average diameter of $58 ± 12 nm and a length $100-750 nm, each bundle possibly consists of $12 CTAB/NaSal wormlike micelles and SWCNTs.…”