Polymeric Surfactants and Emulsion Structure

 Philip Reynolds, Johann Zank, Krisztian Baranyai, Mark Henderson and John White

The structure and stability of emulsions in which the dispersed aqueous phase is 80-95% of the volume fraction, while the continuous oil phase is only 5-15% are poorly understood, and of industrial as well as scientific interest. For example, can the aqueous droplets remain spherical even though the dispersed volume fraction is well above the ca. 65% for close packing of spheres, or do the droplets become polygonised? We have conducted  extensive small angle neutron scattering experiments at many neutron contrasts and chemical compositions, We find [1] that in most cases there are polydisperse spherical  aqueous micron-scale droplets, locally significantly rough. It is the polydispersity that allows such high droplet volume fractions. This is illustrated in the electron micrograph of a section through a frozen emulsion shown above.  The SANS experiments on emulsions show that the surfactant exists as a monolayer at the aqueous droplet surface, and as nanometre scale reverse micelles within the oil phase.  Further SANS on oil-phase microemulsions [2]  also defines how much surfactant is fully dissolved to molecules in the oil. Ultra small angle neutron scattering  reveals large scale surfactant aggregates of micron scale in some emulsions. The balance of these four components can be altered by choice of  surfactants and compositions, with large effects on emulsion stability and rheology. When solids are introduced, the composition at the solid-emulsion interface is also locally changed, and we have studied this by SANS from slurries [4] and by neutron reflectometry for silicon-emulsion interfaces [3]. By synthesising our own surfactants, often by RAFT methods, we are beginning to be able to design and  control the emulsion properties both in the bulk and at surfaces.

These experiments have been supported by Linkage grants from the Australian Research Council including cash contributions from Orica Ltd., "in kind" contributions from the ANU and a Research Scholarship from UnIChe.

[1]       Reynolds, P.A., Gilbert, E.P.,  White,  J.W., J. Phys.Chem. B,  2000, 104, 7012-7022.

[2]       Reynolds, P.A., Gilbert, E. P.,  White, J.W., J. Phys.Chem. B, 2001, 105, 6925-6932.

[3]       Reynolds, P.A., Henderson,  M.J., Holt, S.A ., White,  J.W., Langmuir, 2002, 9153-9157.

[4]       P.A.Reynolds, P.A., M.J.Henderson, M.J. ,White, J.W. Colloids and Surfaces A, 2004, 232, 55-65.