Insoluble lipids and colloidal particles at the oil-water interface
Forth, Joseph William
The films formed by tetradecylamine (TDA) at the water-dodecane interface in the presence of hydrogen phosphate ions have been studied. Between pH 5 and 8, tetradecylammonium cations bind to hydrogen phosphate anions to form needle-shaped crystallites of tetradecylammonium hydrogen phosphate (TAHP). Fourier Transform Infrared Spectroscopy (FTIR) shows that these films consist of two tetradecylammonium cations and one hydrogen phosphate anion. Light microscopy and Small Angle Neutron Scattering (SANS) results show that these crystallites self-assemble into films with a hierarchical structure. On the molecular level, the TAHP has lamellar order. On a mesoscopic level, these lamellae assemble into needle-shaped crystallites with a size on the order 0.1-100 μm. These crystallites assemble into interfacial films with a range of morphologies; below pH 7 they form continuous sheets, at pH 8 they form lace-like networks. The films have both unusually high elastic shear moduli (G's ≈ 10 N/m) and a structure that can be directly imaged using light microscopy. Varying the structure of the films affects their rheology. The lace-like networks deform plastically when sheared, whilst the continuous sheets are highly brittle. The films are temperature-responsive: when the system is heated the film thins and its rheological moduli drop. The temperature response is caused by dissolution of the film into the bulk fluid phases. The TAHP film also stabilises water-in-oil emulsions. The rheological properties of the film are directly related to the stability of the TAHP-stabilised droplets: the droplets coalesce upon being heated, suggesting potential applications in controlled release of active molecules. TAHP and colloidal PMMA particles stabilise emulsions by compatible mechanisms, and can be used to synthesise ('patchy') droplets with an inhomogeneous particle coverage, and ('hairy') droplets, from which long, tendril-like aggregates extend. Finally, the temperature-responsive nature of TAHP is applied to the composite interfaces. As the droplets are heated the lipid film dissolves, and the PMMA particles become mobile at the interface. The electrostatic interactions between the particles lead to competition between the ordering that the electrostatics would favour, and the defect structure favoured by the topology of the spherical surface. I investigate the freezing transition on a spherical surface, and show the emergence of hexagonal order as particle density is increased.
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