Hydrophilic Silica Nanoparticles at the PDMS Droplet−Water Interface

Abstract

The adsorption behavior of hydrophilic silica nanoparticles (fumed Aerosil 380) at the polydimethylsiloxane (PDMS) droplet−water interface has been investigated through particle adsorption isotherms, with complementary studies of the adsorbed layer structure by freeze−fracture SEM. The influence of solution conditions (pH and electrolyte concentration) has indicated the magnitude of particle−droplet and particle−particle interactions, and the influence of droplet cross-linking (deformability) indicated the role of particle penetration at the droplet−water interface. The silica particles adsorb onto PDMS droplets with plateau surface coverages that correspond to their effective particle size (hard sphere radius + double-layer thickness), i.e., lateral silica−silica interaction controls particle packing. Free energies of adsorption (ΔGads) are in the range −15 to −23 kJ mol-1 and concordant with a physical adsorption mechanism. The plateau surface coverages ΔGads and particle packing at the interface are only weakly influenced by pH, but significantly influenced by salt addition. Droplet cross-linking results in a reduction in particle adsorption, but only at higher salt concentrations: this was attributed to the increased likelihood of silica particles wetting PDMS and interfacial penetration. Freeze−fracture SEM revealed that in the low-salt regime individual silica particles are adsorbed at the droplet interface with negligible interfacial aggregation. Densely packed adsorbed particle layers are only observed when the double-layer thickness is reduced to a few nanometers, and even in the presence of a closely packed particle layer, droplets are not resistant to coalescence by excess salt. These findings lead to an improved understanding of particle adsorption to stabilize emulsion droplets.