Basma, Nadir S.; (2019) Structure and Ordering in Solvents and Solutions of Carbon Nanotubes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Our lack of understanding of interactions between nanoparticles in liquids is impeding our ability to controllably produce and manipulate nanomaterials. Traditionally, nanoparticle dispersions are treated using classical colloidal theories originally developed for micron-scale particles. These theories have recently been called into question as significant deviations from micro-scale models are routinely seen as dimensions of the dispersed particle are reduced. The deviations are prominent in the liquid phase where low-dimensional nanomaterials are often processed to produce individualised species with which desirable properties are associated. In this context, a few solvents have proven to more effective than others. Yet, their liquid structures, which ultimately underpins their solvation properties, has not been established. In the first part of this work, advanced neutron scattering methods was used to probe the structure of three solvents: N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF) and dimethylacetamide (DMA). Monte Carlo molecular modelling was used to analyse the neutron data. In NMP, an unusually well-developed mix of interactions is uncovered and its dipole moment was found to have a profound effect on its structure, inducing molecular organisation extending into the nanometer range. In the case of DMF and DMA, structural information pertaining to spatial and orientational correlations in each of the solvents was revealed. The results demonstrate a variety of differences between these two structurally-analogous solvents. Most notably, a higher degree of ordering was found in DMF, dictated by its polar moment, though stronger hydrogen bonding interactions were found in DMA. The intrinsic order in all of these solvents enables a range of local solvent-solute interactions, rationalising their ability to coordinate to and solvate a variety of species. The influence of a nanoparticle on both local and system solvent structures is then investigated in the second part of this work. Solutions of nanomaterials provide a unique system to explore local solvent ordering effects at smaller particle dimensions, and examine the crossover from colloidal to solution behaviour, allowing comparison to classical models. The investigation represents the first atomistically-resolved neutron scattering measurement of a nanomaterial solution. Single-Walled Carbon Nanotubes (SWCNTs) have been shown to thermodynicamlly dissolve to high concentrations in aprotic solvents, though the underpinning mechanisms to their dissolution are unknown. Thus, the system studied was of a concentrated solution of SWCNTs in DMF. The findings provide experimental evidence of enhanced solvent ordering near the nanoparticle’s surface, and contributes crucial insights that differ from conventional standpoints in understanding nanoparticle dissolution.

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