Adsorption of hydrophobic silica nanoparticles at the PDMS droplet-water interface

The adsorption of hydrophobic silica nanoparticles at the poly(dimethylsiloxane) (PDMS) droplet-water interface has been investigated through particle adsorption isotherms, with complementary studies of the adsorbed layer structure by freeze-fracture SEM. The influences of pH and electrolyte concentration as well as droplet cross-linking (deformability) are reported. Adsorption isotherms for liquid droplets are sigmoidal, whereas those for cross-linked droplets are of low affinity or affinity increase type. The surface coverage reaches values that correspond to multiple close-packed layers and are significantly influenced by droplet cross-linking, conferring extensive interfacial penetration, as confirmed by SEM. Densely packed adsorbed particle layers with interfacial aggregation are observed over a wide range of solution conditions. Both liquid and cross-linked PDMS droplets show pH-dependent adsorption, in agreement with DLVO theory; this is in contrast to hydrophilic silica adsorption [Langmuir 2003, 19, 3785]. Interfacial particle saturation occurred at a salt concentration 2 orders of magnitude less than the critical coagulation concentration (ccc) for hydrophobic silica in water. This phenomenon was independent of droplet cross-linking and indicates that particle interaction through the water phase plays a decisive role in particle packing at the interface. SEM indicated the presence of a rigid interfacial crust layer at salt concentrations corresponding to interfacial saturation and a multilayered interfacial particle wall at salt concentrations greater than or equal to ccc.