Rheological investigation of single-walled carbon nanotubes – induced structural ordering in CTAB solutions† Ofra Ben-David, a Einat Nativ-Roth, ab Rachel Yerushalmi-Rozen * ab and Moshe Gottlieb * ab Received 14th November 2008, Accepted 12th February 2009 First published as an Advance Article on the web 17th March 2009 DOI: 10.1039/b820404g The rheological behavior of aqueous dispersions of single-walled carbon nanotubes (SWNTs) in solutions of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) was investigated. The steady shear viscosity as a function of the applied shear rate was monitored in different concentrations of surfactant which correspond to different mesophases. We found that the presence of SWNTs had a dramatic effect on the behavior of the combined system not observed with other additives: a significant increase in the low shear-rate viscosity of SWNT dispersions, and shear thinning replacing Newtonian behavior were observed for CTAB concentrations below the onset of the surfactant hexagonal phase. As CTAB concentration increases the rheological behavior of the SWNT-CTAB system and the native CTAB solutions become more alike. We suggest that the origin of the observed phenomena is the good size-match between SWNTs and elongated CTAB micelles. Thus dispersed SWNTs may induce the formation of size-matched elongated CTAB micelles that further orient under the action of external shear. A similar effect was not observed in dispersions of multi-walled carbon nanotubes or carbon black particles, suggesting that the cooperative behavior is not invoked when significant size-mismatch exists between the surfactant micelles and the dispersed additives. 1. Introduction Single-walled carbon nanotubes (SWNTs) are tubular structures with a diameter of 1 nanometer and a length of few tens of microns. SWNTs may be dispersed in aqueous solutions of different ionic surfactants at low surfactant concentrations. 1–3 As was shown in a variety of cases, a stable dispersion of individual tubes readily forms following sonication of a powder of pristine SWNTs in the solution. 4 In the absence of SWNTs, at surfactant concentrations well above the critical micellar concentration (cmc), a rich phase behavior of the surfactant molecules results in self-assembly on a hierarchy of length scales: micelles merge into elongated structures known as worm-like micelles that further pack and orient into lyotropic liquid crystalline (LC) phases. This hierarchical structure buildup is observed for example, in the phase diagram of the cationic surfactant cetyl- trimethylammonium bromide (CTAB) depicted in Fig. 1. CTAB solutions at 30  C are isotropic at concentrations below 23 wt% (cf. Fig. 1), with mostly spherical micelles at low concentrations. 5 As the CTAB concentration approaches the isotropic-to-nematic (I–N) transition, elongated cylindrical micelles become domi- nant. At C CTAB > 23 wt% LC phases are formed. Recently we reported that the presence of dispersed nano- structures may modify the phase behavior of self-assembled CTAB structures. 6,7 At CTAB concentrations above 5 wt% dispersions of carbonaceous particles (carbon black, CB; carbon nanotubes, CNTs) de-mix and phase-separate into two coexist- ing phases of similar CTAB concentrations: a clear upper phase that is depleted of the carbonaceous structures and a lower phase that is enriched by the additives. At higher CTAB concentrations (above 22 wt%), under conditions leading to the formation of lyotropic liquid crystalline phase in the native CTAB solutions, a re-entrant single-phase was observed. In this phase SWNTs were incorporated into the liquid crystalline phase while preserving the d-spacing of the native system. 7 Cryo-TEM imaging of thin films prepared from the additive-rich lower phase revealed that the presence of individual SWNTs resulted in the formation of an ordered cylindrical phase, as depicted in Fig. 2, at CTAB concentrations as low as 9 wt%, well below the concen- trations leading to LC formation in the native surfactant solutions. The effect was found to be exclusive to SWNTs, and similar behavior was not detected in dispersions of MWNTs or CB. Fig. 1 Phase diagram for the water–CTAB system (from optical measurements). I – isotropic, N – nematic, Ha – hexagonal. 1 a Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva, 84105, Israel. E-mail: rachely@bgu.ac.il; mosheg@ cs.bgu.ac.il b Ilze Kats Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva, 84105, Israel † Electronic supplementary information (ESI) available: Additional experimental information. See DOI: 10.1039/b820404g This journal is ª The Royal Society of Chemistry 2009 Soft Matter , 2009, 5, 1925–1930 | 1925 PAPER www.rsc.org/softmatter | Soft Matter