Tannic acid adsorption and its role for stabilizing carbon nanotube suspensions

Dissolved organic matter (DOM) has been reported to stabilize carbon nanotube (CNT) suspensions, which increases concern over the subsequent transport and behavior of CNTs. However, it is unknown exactly which compounds or functional groups cause the stabilization of CNTs in natural environments. Naturally occurring tannic acid (TA), which has a large number of aromatic functional groups, was used as a surrogate of DOM to investigate its interaction with CNTs. CNT suspendability in TA solution increased with increasing CNT diameter without the aid of sonication. Sorption affinity of CNTs for TA increased with decreasing CNT diameter, positively related to their surface area. A two-stage sorption model was proposed to illustrate the interaction between CNTs and TA. TA molecules may be adsorbed first onto CNTs with aromatic rings binding to the surface carbon rings via pi-pi interactions, until forming a monolayer; the TA monolayer then further sorbed the dissolved TA by hydrogen bonds and other polar interactions. The sorbed TA increased the steric repulsion between individual CNTs, which might disperse the relatively loose CNT aggregates and result in the stabilization of large-diameter CNTs in TA solution. The sorption and suspending process were also examined by transmission electron microscopy, providing further evidence for the above proposed CNT-TA interactions. This study implies that widely distributed TA may promote the mobility and transport of CNTs in natural aqueous environments.