Interfacial Properties of Nanoparticle-Surfactant Systems

Abstract

The utilisation of solid micro/nanoparticles particles in association with surfactants represents an attractive route to control drop/bubble coalescence or liquid film stability in foam and emulsion technology. In fact, the transfer/attachment of nanoparticles to liquid interfaces, driven by their partial wettability, strongly modifies the mechanical properties of the interfacial layers. At large surface coverage, particles can build solid-like barriers to coalescence, such as particle networks, while at low surface coverage, small particles segregating at the interface may play a role more similar to surfactants, affecting equilibrium and dynamic interfacial tension. Surfactant adsorption at the particle surface can be utilised to control such situations, by tuning the hydrophilic/hydrophobic character of the particles and then their ability to form interfacial layers. For these mixed particle-surfactant systems interfacial properties, such as dynamic interfacial tension are still poorly investigated. These studies, however, can bring important information on the undergoing dynamic processes associated with particles and surfactant transfer and organisation at the interface and are essential to understand how to achieve film stabilisation by the smallest amount of particles or surfactant. Previous investigations [1.2] have shown the effectiveness of dilational rheology in providing a better comprehension of dynamic mechanisms occurring in the interfacial layer. In some cases, rheological studies allow in fact the equilibrium and kinetic parameters related to the structure of the particle layer and to the particle interaction to be accessed. Here, we summarise results obtained for interfacial layers containing silica or alumina nanoparticles, which can be utilized as stabilisers for foams and/or emulsions. Different degree of hydrophobicity and, consequently, different affinity with the water/oil interfaces is obtained by using specific anionic or cationic surfactants. The results for a particular arrangement with the surfactant soluble only in the oil phase and hydrophilic particles dispersed in the aqueous phase, where the surfactant-nanoparticle interaction occurs only at the interface, is also reported. [1]. F. Ravera, E. Santini, G. Loglio, M. Ferrari, L. Liggieri, J. of Physical Chemistry B, /10,, 19543. 2006. [2]. F. Ravera, M. Ferrari, L. Liggieri , G. Loglio, E. Santini, A. Zanobini, Colloids Surf A. 323,, 99-108, 2006 1