Interfacial structures of polymeric surfactants in an emulsion formulation

Abstract

As emulsions are thermodynamically unstable, stabilizers are added to prevent creaming and coalescence, and can greatly improve the customer appeal and shelf life of formulated products. Investigations of the structures of surfactants and polymers are key to understanding how they meditate the properties of the oil/water interface. Studying molecules at the oil/water interface is challenging, and only a handful of techniques are suitable for detailed structural studies of molecules at the oil/water interface. In this thesis, an easy-to-use sample environment suitable for oil/water interfacial characterization is described. End attached polydimethylsiloxane (PDMS) films mimic silicone layers in a way that permits the amounts and molecular structures of materials at the interface to be measured. Molecular conformations at the oil/water interface are seen to differ from those at the air/water and solid/liquid interfaces. Penetration of surfactant tails into the oil phase was observed. Addition of small hydrocarbon surfactants and salts was seen to affect polymeric surfactants. Displacement of the polymeric surfactants from the PDMS surface, producing poorly stabilized interfaces, was observed; co-adsorption behavior was also measured. The efficiency of steric stabilization could also be tuned by the addition of salts. A reduction in polymer adsorption and a thinner steric barrier were seen when salts were added as co-solutes. The effect was salt-identity specific. Adsorbed polymeric structures were also probed using neutron reflection in a sample environment that creates a confined geometry at the oil/water interface. Multilayer structures were formed under confinement and the polymeric surfactants were seen to be repelled from the hydrophobic surface. These are the first experiments on confinement at liquid/liquid interfaces ever attempted. Interfacial structures were seen to be highly dependent on the phases that were used, with these data from the PDMS/water interface indicating that it is difficult to draw direct conclusions from measurements at the air/water or solid/liquid interface. The PDMS brush layer is seen to be an excellent model oil surface and enables detailed structural studies of polymers and surfactants at the PDMS/water interface.