Properties and Structure of Palmitic Acid - Silica Nanoparticle Interfacial Layers at Liquid Interface

The association of nanoparticles and surfactants at the liquid interfaces of bubbles and drops is of great importance in several natural phenomena and in technologies related to the production and stabilization of emulsions and foams. In this study, the interfacial properties and the structure of mixed layers composed by hydrophilic silica nanoparticles and a lipophilic surfactant, Palmitic Acid (PA), have been investigated both at water/air and water/oil interfaces, using complementary measurement methods and diagnostics. PA layers spread on silica dispersions were investigated in the Langmuir trough where the area - surface pressure isotherms were obtained as well as the dilational viscoelasticity versus frequency according to the Oscillating Barrier method. Brewster Angle Microscopy and Ellipsometry were used to investigate the lateral and vertical structure of the interfacial layers. Drop Shape tensiometry, used for interfacial tension measurements, allowed a better understanding of the transport processes occurring in the water-oil system, driven by the interaction of PA with the nanoparticles, occurring at the interface. These combined experimental study evidenced the formation of silica nanoparticle-PA complexes which, depending on the PA concentration, may be water dispersible or increase their hydrophobicity and re-adsorb onto the interface, forming a stable mixed condensed layer. The complex behaviour of these layers, can be rationalized accounting for the adsorption of PA on the particle surface. The results evidence a threshold in PA adsorption above which particles change from hydrophilic to partially hydrophobic, promoting their incorporation into the interfacial layer