The effects of phenolic hydrogens and methyl substitute groups in organic dyes on their dispersion of multiple-walled carbon nanotubes

Bromocresol green (BCG) and bromophenol blue (BPB) in acidic and basic forms are used as dispersing agents to suspend multiple-walled carbon nanotubes (MWCNTs) into deionized water. The chemical structures of dye molecules are characterised by ultraviolet-visible-near infrared and nuclear magnetic resonance spectroscopies. The presence of MWCNTs in water is evidenced by transmission electron microscopy and Raman spectroscopy. All dyes exhibit critical concentrations in dispersing MWCNTs. This may result from the formation of dye micelles, which induces osmotic pressure on MWCNTs and aids nanotube re-aggregation. The critical values of the basic dyes are lower than their acidic counterparts, which can be ascribed to the stronger hydrophilicity of basic dyes. The critical value of basic BCG is lower than basic BPB and this can be attributed to the effect of methyl substitutes in the former. Basic BCG is superior to basic BPB in stabilizing MWCNTs, which may result from the extra steric hindrance provided by the methyl substitutes. * Corresponding author. Fax: +44 1483 689404. E-mail address: [email protected] (W. Zhang)