An investigation is reported on the interfacial properties of nanometric colloidal silica dispersions in the presence of a cationic surfactant. These properties are the result of different phenomena such as the particle attachment at the interface and the surfactant adsorption at the liquid and at the particle interfaces. Since the latter strongly influences the hydrophobicity/lipophilicity of the particle, i.e., the particle affinity for the fluid interfacial environment, all those phenomena are closely correlated. 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. The interfacial rheology of the same systems has been also investigated by measuring the dilational viscoelasticity as a function of the area perturbation frequency. These results are then crossed with the values of the surfactant adsorption on the silica particles, indirectly estimated through experiments based on the centrifugation of the dispersions. In this way it has been possible to point out the mechanisms determining the observed kinetic and equilibrium features. In particular, an important role in the mixed particle-surfactant layer reorganization is played by the Brownian transport of particles from the bulk to the interface and by the surfactant redistribution between the particle and fluid interface.

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The study of the interaction between Langmuir monolayers of 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), as the major component of lung surfactant (LS), and nanoparticles of different nature, hydrophilic silica (SiO 2 ) and hydrophobic carbon black (CB), has been carried out by measuring the compression ΠÀA isotherms and the response of the surface pressure to harmonic variations of the interfacial area simulating respiratory cycles in a Langmuir trough. The change of the monolayer interfacial structure induced by nanoparticles was monitored by Brewster angle microscopy. The results point out that nanoparticles incorporating into the monolayers influence the interfacial organization of the molecules and induce important modifications in both the phase behavior and the mechanical properties. Silica has stronger effects on DPPC phase behavior, compared to carbon black, while both affect the monolayer elasticity, the collapse conditions, and the nonlinearity of the surface pressure response to area expansionÀcompression simulating the respiratory cycles. With DPPC being the major component of pulmonary surfactant, the results here obtained are relevant in the framework of wider studies on the effect of nanoparticles on the pulmonary surfactant interfacial properties.