Carbon nanotubes and graphene in aqueous surfactant solutions : molecular simulations and theoretical modeling

摘要

This thesis describes combined molecular simulations and theoretical modeling studies, supported by experimental observations, on properties and applications of carbon nanotubes (CNTs) and graphene sheets dispersed in aqueous surfactant solutions. In particular, the role of the bile salt anionic surfactant, sodium cholate (SC), in dispersing single-walled carbon nanotubes (SWCNTs) and graphene sheets in aqueous solutions was investigated. In addition, the roles of various surfactants (SC, sodium dodecyl sulfate (SDS, anionic), and cetyl trimethylammonium bromide (CTAB, cationic)) in controlling the extent of functionalization of SWCNTs were investigated. First, the surface structure of adsorbed surfactant (SC) molecules on the SWCNT surface was studied using molecular dynamics (MD) simulations, and the interactions between two SWCNT-SC complexes were determined using potential of mean force (PMF) calculations. I found that the cholate ions wrap around the SWCNT like a ring, and exhibit a small tendency to orient perpendicular to the cylindrical axis of the SWCNT, a unique feature that has not been observed for conventional linear surfactants such as SDS. By comparing my simulated PMF profile of SC with the PMF profile of SDS reported in the literature, I found that, at the saturated surface coverages, SC is a better stabilizer than SDS, a finding that is consistent with the widespread use of SC to disperse SWNTs in aqueous media. Second, I probed the surface structure and electrostatic potential of monolayer graphene dispersed in a SC aqueous solution. Subsequently, I quantified the interactions between two graphene-SC complexes using PMF calculations, which confirmed the existence of a metastable bilayer graphene structure due to the steric hindrance of the confined SC molecules. Interestingly, one faces a dilemma when using surfactants to disperse and stabilize graphene in aqueous solution: on the one hand, surfactants can stabilize graphene aqueous dispersions, but on the other hand, they prevent the formation of new AB-stacked bilayer and trilayer graphene resulting from the reaggregation process. Finally, the lifetime and time-dependent distribution of various graphene layer types were predicted using a kinetic model of colloid aggregation, and each graphene layer type was further decomposed into subtypes, including the AB-stacked species and various turbostratic species. Third, I showed that the free energy of diazonium adsorption onto the SWCNT-surfactant complex, determined using PMF calculations, can be used to rank surfactants (SC, SDS, and CTAB) in terms of the extent of functionalization attained following their adsorption on the nanotube surface. The difference in binding affinities between linear and rigid surfactants was attributed to the synergistic binding of the diazonium ion to the local "hot/cold spots" formed by the charged surfactant heads. A combined simulation-modeling framework was developed to provide guidance for controlling the various sensitive experimental conditions needed to achieve the desired extent of SWCNT functionalization. In conclusion, molecular simulations of the type discussed in this thesis, which can be used to complement traditional continuum-based theories, provide a powerful tool to investigate nano-structured aqueous dispersions. The combined simulation-modeling methodology presented in this thesis can be extremely useful in predicting material properties and optimizing experimental procedures in order to minimize tedious and time-consuming trial-and-error experimentation when studying other nanoscale systems of interest.