Influence of silica nanoparticles on dilational rheology of DPPC-palmitic acid Langmuir monolayers

The effect of silica nanoparticles on the dynamic behavior of monolayers composed by 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a mixture of DPPC with Palmitic acid (PA), has been investigated by comparing the dilational rheological response of these lipid layers, spread on water and on a silica nanoparticle dispersion. To this aim, the dilational viscoelasticity has been measured against the frequency of the surface area perturbation of the monolayer, according to the Oscillatory Barrier method in a Langmuir trough. These measurements were performed at different values of the surface pressure, corresponding to different degrees of compression of the monolayer. The results show that the incorporation of particles in the layer induces additional surface kinetic processes and, depending on the surface pressure, modifies both the quasi-equilibrium dilational elasticity and the high frequency limit of the viscoelastic modulus. Another important effect concerns the linearity of the dilational rheological response which is appreciably worsened by the presence of nanoparticles. With DPPC being the major component of pulmonary surfactant and PA used as a component in synthetic substitutes of it, the results here obtained are relevant in the framework of wider studies on the effect of nanoparticles on the pulmonary surfactant interfacial properties.