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Formation, properties and functionalization of polyelectrolyte/ nucleotide coacervate droplets

The central themes of this thesis are the spontaneous formation of polymer/nucleotide complex coacervate droplets and self-assembly of a fatty acid membrane on the surface of such droplets, along with the characterization of the droplets with and without membrane. The spontaneous formation of poly-diallydimethylammonium chloride (polycationic) and adenosine 5'triphosphate (anionic) (PDDA/ATP) coacervate droplets was studied using changes in turbidity and (-potentials. The droplets' formation was determined at critical concentration for coacervation (CCC) when approximately 60 % PDDA fixed charges were neutralized by ATP binding. The CCC was dependent on PDDA concentrations, ionic strengths and PDDA molecular weight. DLS measurement showed complexation prior to droplet formation, suggesting that droplets were formed via nucleation and growth mechanism. At CCC droplets had net positive surface charges, were spherical and polydisperse under optical microscope and kinetically stable to coalescence and thermally stable up to 85°C. Conversely, at complete charge neutralization droplets were non-charged and unstable to coalescence and sedimentation. The dynamic properties and structure of coacervate phase were measured with rheological experiments. The transparent, condensed coacervate phase possessed liquid-like viscoelastic properties which was highly dependent on PDDA-ATP interactions: the stronger the interactions, the more viscous the phase. Viscoelastic behaviour was attributed to the present of ATP bound to PDDA chain, which formed a highly interconnected chain. The structure of coacervates was identified as heterogeneous domains, composed of a mixture of elastic-like component embedded into liquid-like domains, which displayed pronounced liquid-like behaviour. Surface tension measurements exposed that the coacervate phase was less polarised than water. Contact angle measurements presented the hydrophilicity of coacervate phase with the structure of coacervate was composed of ATP-rich at the surface and PDDA-rich at the interior and for charged droplets. Given these properties, coacervate demonstrated compartmentalization within the droplets' interior. The self-assembly of a fatty acid membrane was performed on the surface of coacervate droplets by a simple addition of negatively-charged fatty acid molecules into positively-charged droplets. The membrane assembly occurred at the surface of droplets at a specific molar ratio of oleate:PDDA:ATP as monitored via(-potentials and fluorescence microscopy using BODIPY FL C16- tagged fatty acid fluorescence. Multichannel confocal imaging then showed oleate membrane was at the surface while kiton red pre-stained droplets remained stable inside the membrane, thus confirming that the membrane was encapsulated by the droplets. The mechanism of membrane assembly was driven by an increase in thermodynamic for bilayer formation at the droplets' surface. The membrane was highly net negative charges and was multi-lamellar bilayer. The viscosity (FUM) was higher at the membrane than in coacervate region. The membrane was stable at higher ionic strength while coacervate was destroyed, which induced t he oleic acid vesicle fusion and growth. The membrane encapsulation showed marked changes in the sequestration properties than uncoated droplets, indicating that the semi-permeability of the membrane-coated droplets was evident.
Date January 2013
CreatorsChe Hak, Cik Rohaida
PublisherUniversity of Bristol
Source SetsEthos UK
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation

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