- Title
- Surfactant adsorption and structure at ionic liquid interfaces
- Creator
- Wakeham, Deborah
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2013
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The interfacial structure of a series of protic ionic liquids, including ethylammonium nitrate (EAN), propylammonium nitrate (PAN), ethanolammonium nitrate (EtAN), and ethylammonium formate (EAF) at the air – liquid surface has been investigated using x-ray reflectivity and vibrational sum frequency spectroscopy (EAN and EtAN only). The same liquids as well as propylammonium formate, (PAF), ethylmethylammonium formate (EMAF) and dimethylethylammonium formate (DMEAF) were also studied at the solid – liquid interface using atomic force microscopy (AFM). This series of protic ionic liquids allowed the effect of solvophobicity, amphiphilicty, hydrogen bonding, and electrostatic interactions on surface morphology to be elucidated. At both the air and solid interfaces, protic ionic liquids are shown to form structures closely related to, but more ordered than, their bulk liquid nanostructure. In general, the alkyl chains are segregated from the charged groups, forming alternating layers aligned by the surface, which gradually decay into the bulk liquid structure. However, the extent of ordering and layering is highly influenced by the ionic liquids cation and anion. EAN and PAN have a large interfacial region, which extends across several ion pairs. The degree of layer segregation is stronger in PAN than in EAN. The longer alkyl chain of PAN enhances the ionic liquid solvophobic interactions leading to stronger association. Conversely, in EtAN solvophobicity was disrupted by the inclusion of a hydroxyl group on the cation alkyl chain. This resulted in no surface layering at the air surface, and reduced layering at the solid interface. Reduced hydrogen bonding was achieved by replacing the nitrate anion with formate. EAF had decreased interfacial ordering at both the air and solid surfaces as compared to EAN. A similar effect was achieved with PAF at the solid surface. Electrostatic interactions were decreased by increasing the amount of cation alkyl chain substitution. Both EMAF and DMEAF showed no surface layering at the solid surface.At the air surface, EAN, PAN, EtAN, and EAF all showed evidence of possible dynamic aggregates formimg which consist of multiple cations and anions. The ions arrange so that the nonpolar hydrocarbon chains surround the hydrophilic ionic groups, protecting them from the hydrophobic gaseous phase. Nonionic surfactant layers adsorbed at the protic ionic liquid – air interface were investigated using neutron reflectivity and surface tensiometry. The adsorbed surfactant structure was similar to aqueous systems, but there were some significant differences. In EAN the ethylene oxide groups formed a thin compact layer, poorly solvated by EAN, while in EtAN they are slightly more extended, although not as elongated as aqueous systems. In ionic liquid mixtures, the adsorbed surfactant layer structure is invariant with solvent composition. This is attributed to the solvent – solvent interactions being more favourable than the solvent – surfactant interactions.
- Subject
- surfactant adsorption; protic ionic liquids; surface morphology; interfacial structure; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/938498
- Identifier
- uon:12626
- Rights
- Copyright 2013 Deborah Wakeham
- Language
- eng
- Full Text
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