Phase Behavior, Rheology, and Characterization of Lysozyme - Single-Walled Carbon Nanotube Dispersions

Abstract

Single-walled carbon nanotubes (SWNT) are one of the most promising materials in nanoscale science and technology with potential applications ranging from polymer nanocomposites to biocompatible smart materials. However, translating the extraordinary mechanical, thermal, optical and electrical properties of individual SWNT into macroscopic assemblies such as films and fibers is challenged by the aggregation of pristine SWNT. In order to achieve individual SWNT, their mutual attractive forces must be overcome. At that point, there must be a force of repulsion to stabilize the SWNT, thereby not allowing them to reaggregate. The research presented here concentrated on the application of lysozyme (LSZ) as a dispersing agent for SWNT. The structural nature of LSZ make it a well-suited dispersant for SWNT. Not only does LSZ enable the mechanical, electrical, thermal, and optical properties of SWNT to be exploited, but it also enhances the system by making it antibacterial. During this research, it was found that LSZ interacts with SWNT through π-bonding of the aromatic SWNT sidewall with the tryptophan residue within the LSZ hydrophobic core allowing for SWNT to be soluble in aqueous solutions. Furthermore, this research showed that dispersions of LSZ-SWNT exhibit phase transitions from isotropic liquid to gel as the concentration of both LSZ and SWNT are increased via evaporation. It was then determined that the phase transitions were caused by depletion interactions between LSZ and SWNT due to the loss in free energy of mixing of the system at increased LSZ and SWNT concentrations. In order to increase the concentration of SWNT at which depletion aggregation occurred, the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) was added to stabilize the system. Through stabilization of the LSZ-SWNT dispersions and the addition of synthetic or natural polymers, it was possible to assemble and process of concentrated dispersions into films and fibers which showed unique tailorability for use in novel applications.