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Title: Sum-frequency spectroscopy of surfactant monolayers at the air-water and oil-water interfaces
Author: Bell, Graham Ronald
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2012
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In order to improve understanding of the relationship between molecular structure and interfacial structure, surfactant monolayers at the air-water (A-W) and oil-water (O-W) interfaces, and counterions bound to surfactant monolayers, have been studied using sum-frequency spectroscopy (SFS) and complementary techniques, including neutron reflection (NR) and ellipsometry. The non-linear optical technique of SFS is applied first to the study of surfactants at the A-W interface. Various theoretical models used to explain peak intensities in the resulting vibrational spectra are critically assessed. Values of mean tilt and degree of conformational disorder of adsorbed surfactant molecules are inferred. It is observed that the molecular conformation of monolayers of different surfactants cannot be predicted solely from the area per hydrocarbon chain, depending also on the nature of the head-group. At constant area per molecule, monolayer structure is found to be unaffected by chain length. The behaviour of binary mixtures of a non-ionic and an anionic or zwitterionic surfactant is found to be indicative of ideal mixing thermodynamically in the monolayer, whereas strong synergistic interactions are observed for an anionic-zwitterionic mixture. Two counterions known to induce viscoelasticity in surfactant solutions are then investigated. The orientations of benzoate and p-tosylate ions bound to surfactant monolayers at the A-W interface are quantified. Both are found to be located deep within the hydrophobic region of the monolayer. The presence of p-tosylate causes structural changes in the monolayer, and p-tosylate is found to be acting as both ion and cosurfactant. Specifically, through the joint application of SFS and NR, the structure of a monolayer of hexadecyltrimethylammonium p-tosylate is established, revealing that the hydrocarbon chains of the surfactant rise steeply away from the water before turning over to lie flatter in the region above the p-tosylate ions. These findings are related to micelles in solution to provide an insight into the sphere-to-rod transitions thought to be responsible for dramatic changes in the physical properties of surfactant solutions in the presence of specific counterions. Finally, an entirely new experimental arrangement is developed to address the significant technical problems standing in the way of obtaining SF spectra of surfactants adsorbed at the O-W interface. The same basic design was later used to acquire the first SF spectra of surfactant monolayers at this interface.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available