Wide-frequency dilational rheology investigation of mixed silica nanoparticle-CTAB interfacial layers

The interfacial properties of aqueous dispersions are strictly connected to the transfer/accumulation of particles into the surface layer, driven by the particle hydrophobicity. The addition of surfactants adsorbing on the particles and tuning their hydrophobicity represents therefore an attractive route to control the properties of these liquid interfaces. These mixed systems present however rather complex behaviours, which are still difficult to understand, deserving both theoretical developments and experimental investigations. Here the results of a wide study on the interfacial properties of a silica nanoparticle dispersion added with CTAB are presented, which allows to elucidate some basic aspects. A set of different techniques and methodologies have been utilised to measure the dilational viscoelasticity in a wide frequency range: from 10(-3) to 10(3) Hz. The study concerned both particle layers obtained after the spontaneous accumulation/segregation at the dispersion interface, and layers obtained after surface compression. The analysis of these data in the framework of available models provides qualitative and quantitative information about kinetic and structural features of these complex mixed layers. The results evidence different relaxation mechanisms effective on different timescales, which involves both the particles and the surfactant adsorbed on them. Besides the specific results concerned with the particular investigated system, the study confirms the surface dilational rheology investigation as a powerful method for surface science, in particular, as far different techniques are utilised to explore dilational properties in a broad frequency window.