Insoluble lipids and colloidal particles at the oil-water interface
View/ Open
Date
29/06/2016Author
Forth, Joseph William
Metadata
Abstract
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.
Collections
The following license files are associated with this item: