Interfacial Properties of Mixed Palmitic Acid-Silica Nanoparticles Systems

The interaction between nanoparticles and surfactants is a topic of fundamental importance in the physical-chemistry of colloidal systems with special attention in the study of water soluble/dispersable systems [1]. In this communication, we analyze the interfacial properties of the systems formed by hydrophilic silica nanoparticles and a lipophilic surfactant, Palmitic Acid (PA). As shown in a previous study [2] the compression isotherm of Palmitic Acid spread on an aqueous dispersion of hydrophilic silica nanoparticles showed that the interaction with silica nanoparticles may lead to the removing of PA from the interfacial layer. This is due to the formation of water dispersable nanoparticle-surfactant complexes. However, when the concentration of Palmitic Acid is increased, these complexes increase their hydrophobicity and can re-adsorb onto the interface. This behaviour is confirmed by the results of the present tensiometric study, where the combination of a Drop Analysis Tensiometer and the Oscillatory Barrier Technique allowed the rheological properties of the interfacial film to be accessed. These properties were found to be strongly depended on the Palmitic Acid concentration and the Brewster Angle Microscopy (BAM) images showed that, under certain conditions, the interfacial layers may be considered as high density solid-like film. Further insight in the interaction between Palmitic Acid and Silica Nanoparticles was obtained investigating the adsorption process of these complexes onto the water/hexane interface by measuring the dynamic interfacial tension. This study leads to the conclusion that the interactions responsible of the complexes formation are the result of a complex balance where the affinity of the Palmitic Acid for the nanoparticle surface has to be taken into account together with the hydrophilic-lipophilic balance of PA [3]. These two parameters, in fact, control the surfactant adsorption/desorption onto the nanoparticle surface and, consequently, the incorporation of the nanoparticles to the fluid interface. This combined study, based on interfacial tension and dilational rheology measurements and structural characterization, lead to the conclusion that the formation of interfacial particular structures may be tuned controlling the affinity between the different components of the nanoparticle-surfactant mixed systems. These results suggest the application of these studies on nanoparticle-surfactant interaction in technological fields concerning detergency and surface coatings. [1] F. Ravera, E. Santini, G. Loglio, M. Ferrari, L. Liggieri, J. Phys. Chem. B 110 (2006) 19543. [2] E. Guzmán, L. Liggieri, E. Santini, M. Ferrari, F, Ravera. Influence of Silica Nanoparticles on Thermodynamic and Rheological Properties of DPPC - Palmitic Acid Langmuir Monolayers, Langmuir submitted (2011). [3] A. Honciuc, A.L. Howard, D.K. Schwartz, J. Phys. Chem. C 113 (2009) 2078.