Colloidal Stability of Carbon Nanotubes - A Matter of Modification

The implications of a carbon nanotube (CNT) release into the environment are majorly influenced by the colloidal stability of the CNTs in surface or ground waters. Main factors for this stability are the degree of oxidation of the CNTs and the presence of natural organic matter (NOM) such as fulvic or humic acids. Pristine CNTs show very low dispersion stability in water and quickly aggregate and sediment.
However covalent and/or non-covalent modification of the CNTs through, for example, natural oxidation processes or adsorption of NOM can considerably alter their colloidal stability, thereby affecting the fate of the CNTs upon release. For this study samples of different types of CNTs, multi wall as well as single wall CNTs, were modified covalently by a microwave assisted oxidative treatment and non-covalently by dispersion in the presence of fulvic acid. The colloidal stability of the so modified CNTs was investigated using dynamic light scattering methods to follow the aggregation and sedimentation of the CNTs as well as establishing the ζ potential of the modified CNTs in waters of different ionic strength.
The colloidal stability of the modified CNTs in distilled and low ionic strength water or in the presence of fulvic acid can exceed months. This has implications for their transport behavior upon release into the environment. A transport of CNTs in surface and ground waters can be expected under certain conditions.