Dynamic Behavior of Carbon Nanotube and Bio-/Artificial Surfactants Complexes in an Aqueous Environment

We examined the characteristics and behaviors of the carbon nanotube (CNT) and surfactants complexes in an aqueous environment using computational techniques to elucidate the effects of surfactants used to disperse the CNTs in toxicity studies of CNTs. We found that the cohesive energy per one adsorbed surfactant molecule of the CNT-surfactants complex depends on the type and the amount of the adsorbed surfactant molecules on the CNT surface. The CNT-dipalmitoyl phosphatidylcoline (DPPC; a primary component of human lung surfactants) complex was more energetically stable than the CNT-Tween 80 (an artificial surfactant often used to disperse CNTs) complex by 20-60 kJ/mol. The average cohesive energy of the complexes was 330 kJ/mol. Such high cohesive energy suggests that the CNT molecule that was once covered by surfactant molecules is unlikely to return to its original bared form. Furthermore, we found that reactions of adsorption and desorption of surfactant molecules occur on the CNT surface in a time scale of milliseconds. Hence, the CNTs are thought to be coated by the surfactant corona composed of amphiphilic molecules, which is similar to a protein corona in the biological system. Moreover, CNT-surfactants complexes are believed to convert to some other more energetically stable CNT-surfactants complexes through a surfactant exchange due to the adsorption and desorption of surfactants on the CNT surface in the biological system. The specific surface area of the CNTs, which is one of the most important parameters for assessing the toxicity of a nanomaterial, is thought to change because of the surfactant exchange.