Suttipong, M; (2016) Molecular Self-Assembly of Surfactants on Solid Surfaces. Doctoral thesis , UCL (University College London).

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Abstract

Understanding adsorption and aggregation of surfactants on solid surfaces is of great importance to many applications. The aim of this thesis is to obtain molecular-level knowledge regarding the role of (1) surfactant-assisted aqueous dispersions of single-walled carbon nanotubes (SWNTs) and (2) surfactant adsorption on heterogeneous surfaces, using computer simulations. For the first objective, molecular dynamics simulations were employed to study the morphology of surfactants self-assembled on (6,6), (12,12), and (20,20) SWNTs. The results show that the surfactant molecular architecture significantly affects the packing of surfactants on SWNTs. The branched sodium dodecyl benzenesulfonate (SDBS) is more effective in stabilising dispersions of narrow SWNTs than its linear counterpart. There is no strong effect of the nanotube diameter seen on the morphology of mixed linear and branched SDBS. Comparing the self-assembled aggregates formed by caesium (Cs+) and sodium (Na+) dodecyl sulphate surfactants, Cs+ ions yield a more compact coverage on the (6,6) SWNT, compared to Na+. These outcomes could provide physical guidelines for designing surfactant formulations to improve the quality of the aqueous SWNT dispersions. For the second objective, dissipative particle dynamics simulations were used to investigate the adsorption of surfactants near patterned surfaces. The hydrophobic patterns on which the surfactants could adsorb are surrounded by surfaces that repel the surfactants. On the surfaces containing one hydrophobic stripe, as the stripe width decreases, monolayers become hemi-cylinders, hemi-spheres, and individual surfactants, a consequence of lateral confinement. When two hydrophobic stripes are present on the surfaces, there is evidence of cooperative effects (i.e., hemi-cylindrical shells or irregular structures formed). The morphological (width and depth) and chemical (fully and partially hydrophobic) properties of the trenches predominantly affect the self-assembled surfactant aggregates. These findings could assist in understanding of surfactant adsorption on heterogeneous surfaces and perhaps in facilitating new methods for the fabrication of nano-structured surfaces.

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