Oil or Water Droplets Containing an Adsorbed Polymeric Surfactant Steric Stabilisation

Interaction between Oil or Water Droplets Containing an Adsorbed Polymeric Surfactant Steric Stabilisation 

Consider the case of two water droplets containing an adsorbed polymeric surfactant such as PHS-PEO-PHS (the molecule used to prepare the primary W/O emulsion). 

For the W/O case, the maximum surface pressure approached 51mNm , implying an interfacial pressure approaching zero. These interfacial tension results show that the PHS-PEO-PHS block copolymer is an excellent W/O emulsifier. Indeed, W/O emulsions with a water volume fraction p of 0.9 can be prepared using this block copolymer. 

The PHS chains are highly solvated by the oil molecules and extend at the W/O interface, giving a layer thickness on the order of 7-8 nm, as confirmed using film thickness measurements [3]. A thin film consisting of oil plus surfactant was formed between two aqueous droplets when they were brought into contact, and the thickness of the film was measured using light reflectance a fihn thickness of 

2 nm indicated a layer thickness of the PHS chains on the order of 7 nm. The layer thickness could also be measured using rheological measurements. When shear stress (rj-shear rate (y) curves were obtained for W/O emulsions at various volume fractions of water, the results indicated a change from Newtonian to non-Newtonian flow behaviour as the volume fraction of the water in the emulsion was increased. The non-Newtonian flow reflects the droplet-droplet interaction as these approach to closer distances. The data were analysed using the Bingham model [4], 

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