Study of the stabilization of carbon nanotubes by ionic liquids acting as surfactants in aqueous media and as pure solvents

• Grant number: 24/17078-7
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Start date until: November 01, 2024
• End date until: October 31, 2025
• Field of knowledge: Physical Sciences and Mathematics - Chemistry - Physical-Chemistry
• Principal Investigator: Kalil Bernardino
• Grantee: Ana Yumi Oliozi Kanegawa
• Host Institution: Centro de Ciências Exatas e de Tecnologia (CCET). Universidade Federal de São Carlos (UFSCAR). São Carlos , SP, Brazil

Abstract

Despite their incredible physical properties, the difficulty in generating stable dispersions of carbon nanotubes is often a limiting factor for their applications. Recently, ionic liquids with a pronounced amphiphilic character have attracted attention due to their ability to stabilize dispersions of various nanomaterials both as pure solvents and as additives in aqueous media. The stabilization mechanism, however, is completely different in these two situations. In aqueous media, they act similarly to usual ionic surfactants, presenting preferential adsorption of one of the ions on the nanomaterial and establishing an electrical double-layer profile in solution that causes long-range repulsion between dispersed nanostructures. The properties of surfactants render these ionic liquids the denomination SAILs (Surface Active Ionic Liquids). SAILs can also stabilize dispersions by acting as pure solvents. Still, in this case, given the high concentration of ions, the formation of an electrical double layer is not expected to be the dominant effect, but rather the formation of alternating polar and nonpolar domains around the nanostructure, which results in complex long-range interactions, with multiple barriers to aggregation resulting from the unfavorable mixing between solvent layers of opposite natures formed around each nanostructure as they approach each other. The present project aims to compare the two stabilization mechanisms by studying, through molecular dynamics simulations, the organization of the same set of SAILs around a single-walled carbon nanotube when acting both as a pure solvent and as a surfactant in aqueous media. Three SAILs based on the 1-octyl-3-methylimidazolium cation will be studied, varying the anion between tetrafluoroborate, dicyanamide and bistriflimide. The cation is expected to be preferentially adsorbed on the nanotube surface by the hydrophobic alkyl chain. However, the nature of the cation-anion interaction should regulate the degree of association between the two, which will define the effective charge acquired by the nanotube with the layers of ions adsorbed in aqueous medium and also the intensity of domain segregation when acting as a pure solvent, factors that, in turn, will determine the stability of the generated nanotube dispersions. This study should contribute to a greater understanding of the mechanisms of stabilization of nanomaterial dispersions by ionic liquids and to the choice or even the planning of new ionic liquids for this purpose. The proposed simulations are feasible to be executed within a 12-month schedule for a scientific initiation project and will contribute to the student's training both by acquiring knowledge of the proposed simulation techniques, experience in working with supercomputers and clusters, deepening their knowledge of liquid structure and aggregation thermodynamics, and understanding the steps for the preparation and execution of a research project.