Isotropic-nematic phase transition of single-walled carbon nanotubes in strong acids

We present the first quantitative assessment of the maximum amount of nanotubes that can exist in the isotropic phase (phi(iso)(max)) of single-walled carbon nanotubes (SWNTs) in Bronsted-Lowry acids. We employ a centrifugation technique in conjunction with UV-vis-nlR spectroscopy to quantify phi(iso)(max), which is also the critical concentration of the isotropic-nematic transition of SWNTs in strong acids. Centrifugation of biphasic dispersions of SWNTs, that is, acid dispersions consisting of an isotropic phase in equilibrium with an ordered nematic liquid crystalline phase, results in a clear phase separation, where the isotropic phase is supernatant. Dilution of the isotropic phase with a known amount of acid followed by UV-vis-nIR absorbance measurements yields phi(iso)(max), that is, the maximum concentration of SWNTs that can exist in the isotropic phase in a given acid for a given SWNTs' length distribution. At low SWNT concentration (below 200 ppm) in superacids, light absorbance in the range from 400 to 1400 nm scales linearly with concentration. This Beer's law behavior yields calibration curves for measuring SWNTs' concentration in acids. We find that the critical concentration of the isotropic-nematic transition increases with acid strength in accordance with the previously proposed sidewall protonation mechanism for dispersing SWNTs in acids.