(400h) Structures of Surfactant Systems at the Oil/Water Interface

Industrial formulations commonly use more than one stabiliser, but different combinations of polymeric, anionic, cationic and non-ionic surfactants may act antagonistically to each other and not stabilise the interface as well as intended. At point of use, a formulation may be subjected to significant dilution, mixed with salts or be brought into contact with further surfactants. The complex interactions between the polymers, surfactants and the electrolytes at the interface affect the performance of the final product. The performance of the colloidal product is governed by the structures of the surfactants at the interface and the repulsive potential they can generate.

We have developed a covalently anchored PDMS film as an immobilised oil layer to study the structures of surfactants adsorbed at the oil/water interface. Results from neutron reflection and QCM show that Pluronic F127 forms a loosely packed thin layer at the PDMS/water interface, while addition of SDS and CTAB causes desorption of F127 from the interface. Low concentrations of SDS are seen to partially clean the surface of F127 while high concentrations of surfactants form a mixed layer at the interface. The effect of salt on the adsorption behaviour was also investigated; both UV-Vis absorbance and QCM studies showed addition of different sodium halide salts leads to dehydration and collapse of the adsorbed F127 layer and that the effect is larger for more kosmotropic anions. The resistance of the adsorbed surfactant layer to geometrical confinement is also seen to be influenced by the mixture of surfactants present, also showing specific ion effects to halide ions. These results offer formulation rules that link self-assembly behaviour and structural measurements through to the performance of the industrial formulation.