Liquid-Liquid and Liquid-Air Interfacial Tension and Interfacial Rheology of Nanoparticle-Surfactant Dispersions

Solid particles have been known for long time as stabilizers of natural emulsions and foams. Based on these observations, synthetic micro/nanometric particles, sometime in association with surfactants, are being tailored as new additives in emulsion and foam technology. To this aim, understanding how the properties of the water-oil and the water-air interfaces and liquid films are modified by these surfactant/particle structures is an important and still open scientific and technological target. Here an extensive investigation on the interfacial properties of nanometric colloidal silica dispersions in the presence of a cationic surfactant is reported. These properties are the result of different phenomena such as the particle attachment/segregation at the interface, the surfactant adsorption at the liquid interface and the surfactant adsorption at the particle interface. Since the latter strongly influences the hydophobicity/lipophilicity of the particle - that is, the particle affinity for the interfacial biphasic environment - all those phenomena are closely linked in a complex interplay. The equilibrium and dynamic interfacial tensions of the liquid-air and liquid-oil interfaces have been measured as a function of the surfactant and particle concentration. For these systems the interfacial rheology has also been investigated by measuring the dilational visco-elasticity as a function of the area perturbation frequency. These results are then crossed with the estimation of the surfactant adsorption on the silica particles. By this way it has been possible to point out and analyse the key mechanisms determining the observed kinetic and equilibrium features. In particular, besides the surfactant adsorption on the silica particles, an important role is played by the Brownian transport of particles from the bulk to the interface, by the internal reorganisation of the mixed particle-surfactant layer at the interface and by the surfactant redistribution between particle and fluid interface. The reported results are critically discussed and compared with the available models in order to address future investigations