Order–disorder transition induced by surfactant micelles in single-walled carbon nanotubes dispersions

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Review by Bryan Hassell: AP 255 Fall 11

From: Order–disorder transition induced by surfactant micelles in single-walled carbon nanotubes dispersions, E. Nativ-Roth et. al, Soft Matter, 2010, 6, 5289–5292

Keywords: surfactant, micelle, dispersion

Single walled carbon nanotubes (SWNT) with their all-carbon composition and interesting geometries are characterized by a unique combination of electrical and mechanical properties. To utilize their properties, you have to disperse them in aqueous media for various applications. It's becoming evident that the coupling between molecular and colloidal interactions leads to non-trivial structures when the dimensions and geometry of the nanostructures are commensurate. The results that this group showed used transmission electron microscopy at cryogenic temperatures (Cryo-TEM) and was of spontaneously formed microns-long islands of orientationally ordered SWNT in macroscopically homogeneous dispersions of ionic surfactant micelles.

Fig. 1 Aligned SWNT in aqueous solutions of CTAB are observed in Cryo-TEM images of individual SWNT (1 wt%, dark lines in the image) dispersed in aqueous solutions of CTAB (the granular background) at CTAB concentrations of (a) 2 wt% and (b) 7 wt%. (c) Mean values of the inter-tube spacing as a function of CTAB concentration, for SWNT (diamonds) and DWNT (squares). (d) A higher magnification presenting aligned SWNT in a dispersion of 5 wt% CTAB. The typical distance between the surface structures seen on the SWNT surface is 5 nm.
Fig. 2 Cryo-TEM images of SWNT (1 wt%) dispersed in aqueous solutions of (a) 5 wt% SDS, showing a mean inter-tube spacing of 19 nm and (b) 2 wt% CTAC, showing a mean inter-tube spacing of 16 nm.
Fig. 3 Cryo-TEM images of SWNT (1 wt%) dispersed in 2 wt% (55 mM) CTAB and (a and b) 3.5 mM KBr (b presents a higher magnification of (a)), (c) 7 mM KBr and (d) 20 mM KBr. A transition from aligned to randomly oriented SWNT phase was measured at KBr concentrations of 70 mM in CTAB dispersions of 5 wt% (137.5 mM).
Fig. 4 Cryo-TEM images of dispersed CNT. (a) DWNT (1 wt%) dispersed in 2 wt% CTAB, (b)MWNT(1 wt%) dispersed in 2 wt% CTAB, (c) SWNT (1 wt%) in a mixed dispersion of 2 wt% CTAB and 2 wt% Brij-78. (d) Inter-tube spacing as a function of the overall surfactant concentration in CTAB–Brij-78 mixtures: (squares ) 0.3% CTAB + 3.3 wt% Brij-78, (triangles) 2% CTAB + 2 wt% Brij-78.

In Fig. 1a, b and d they present images taken from dispersions of SWNT(0.5–1 wt%) in solutions of spherical micelles of an ionic surfactant, cetyl trimethyl ammonium bromide (CTAB). The solution concentrations are above the critical micelle concentration (cmc) of the native surfactant but within the isotropic phase of the spherical micelles and far from their liquid crystalline (LC) transition. The SWNT dispersions are optically isotropic, and the viscosity of the dispersion is similar to that of the native surfactant solution.