| Summary: | The current study examines the interfacial behaviour and foaming potential of poly(vinylpyrrolidone) (PVP, 40 kDa)-silica composite nanoparticles. Individually, the two components, PVP and hydrophilic silica nanoparticles, exhibit very little foaming potential. In contrast, combining the two components to form silica-PVP core-shell nanocomposites via polymer physisorption leads to good ‘foamability’ and long-term foam stability. Optical reflectivity (OR) measurements showed that the adsorption of PVP on silica can be regarded as irreversible with the saturated polymer surface excess around 1 mg/m2, in good agreement with the thermogravimetric analysis (TGA) data. The adsorbed 40 kDa PVP film is highly hydrated (contained water 40-55 wt%), behaving as a steric barrier, which helps to keep the particles apart. Addition of an electrolyte (Na2SO4) was shown to have a marked effect on the foam stability. By varying Na2SO4 concentration between 0 and 0.55 M, three regions of foam stability were observed: rapid foam collapse (≤ 0.01 M), delayed foam collapse at 0.1 M, and long-term stability (~ 10 days) at 0.55 M. The observed transitions in foam stability were better understood by studying the microstructure and rheological properties of the particle-laden interfaces using different techniques such as Langmuir trough, Interfacial Shear Rheometer (ISR400), Brewster angle microscopy (BAM) and cryo scanning electron microscope (cryo-SEM). Meanwhile, the 2D structure of particle-laden interfaces surrounding an air bubble and deposited at a planar interface were correlated using cryo-SEM and BAM images to elucidate the interfacial shear rheology of particle-stabilized bubbles and its relation to foam stability. For rapidly collapsing foams, the interfaces were characterized as being “liquid-like”. By contrast, the enhanced foam stability at 0.1 M and 0.55 M Na2SO4 was attributed to the formation of solid-like (pseudo solid-like for the 0.1 M particle layer) interfacial particle layers surrounding bubbles, at a compression state in the region of the liquid to solid (L-S) phase transition. The increased interfacial rigidity was attributed to adhesion between interpenetrating polymer layers. For the most stable foam (prepared in 0.55 M Na2SO4), particles strongly aggregated at the interface into a connected particle network, forming a strongly elastic interfacial layer. Hence, bubble-bubble coalescence was found to be significantly retarded by the aggregation of nanocomposite particles at 0.55 M, with the long term destabilization resulting from bubble coarsening. For rapidly destabilizing foams, however, the contribution from bubble-bubble coalescence was shown to be more significant. Further investigations need to be carried out to prevent bubble coarsening.
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