Correlation between electrokinetic potential, dispersibility, surface chemistry and energy of carbon nanotubes

This paper proposes the correlation between the electrokinetic potential, dispersibility in solvents, surface energy and oxygen content of carbon nanotubes (CNTs) affected by functionalization. Colloidal systems consisting of CNTs with varying degrees of dispersion are prepared and characterized to evaluate CNT dispersibility and suspension stability in solvents with different polarities. The results show that an absolute value of zeta potential at about 25 mV is closely related to the micro- and macroscopic dispersion of CNTs, whereas a high absolute value of 40 mV is regarded as an indication of high quality CNT dispersion with much enhanced suspension stability in solvents. The absolute zeta potential value increases consistently with increasing degree of CNT functionality, the increase being most pronounced in a hydrophilic liquid such as water. A linear correlation is established between the surface energy of a CNT film and the oxygen to carbon ratio of CNT surface. The CNT dispersibility in a liquid is determined not only by their physical states, but also by the hydrophilicity and surface functionality of CNTs, all of which are reflected by zeta potential.