Dilational Rheology, Structure and Phase Behavior of Mixed Lipid-Nanoparticle Monolayers

Lipid monolayers spread on aqueous phases have been extensively investigated in recent years because of their well recognized appropriatness as models for studying biologicacally relevant systems sucg as cellmembrane and lung surfactant. Moreover, studying the interaction of these layers with nanoparticles (NP) is of great importance as a first approach to understand the interaction of NP with living systems and more generally to develop innovative biomaterials. The here presented work focuses on the interaction between NP of different chemical nature (silica, carbon, titania) and mixtures of lipids mimicking the surface behaviour of the pulmonary surfactants (DPPC, DOPC, Palmic acid). The effects of the NP on the interfacial properties of such layers were investigated by using a set of complementary techniques. Langmuir trough was utilized to investigate the modifications induced by NPs on the phase behavior of spread lipid layers. Such information were complemented by those on the lateral and vertical structure of the mixed layers accessed by Brewster Angle Microscopy and Ellipsometry. Moreover, the mechanical features of these mixed layers were investigated by the Oscillating Barrier method in the Langmuir trough and by the Oscillating Bubble method in a drop shape tensiometer. These latter rheological studies were especially effective to evaluate surface re-organization processes involving lipid molecules and NP. Depending on the NP characteristics and concentration, the results have evidenced a variety of complex interactions between particles and lipids, such as lipid sequestration via adsorption at the particle surface or particle incorporation in the monolayer. In particular, the formation of NP-lipid complexes incorporated in the monolayers induces changes in their phase behaviour and structure as well as in the dynamical response to area perturbation. Regards the effects of NP on the simulated pulmonary surfactant, these studies provided the definition of parameters which could be suitably utilised to score NPs for their potential adverse impact on the respiratory physiology. References: 1. E. Guzmán, L. Liggieri, E. Santini, M. Ferrari, F. Ravera, Soft Matter, 8 (2012), 3938-3948 2. E. Guzmán, L. Liggieri, E. Santini, M. Ferrari, F. Ravera, Effect of Hydrophilic and Hydrophobic Nanoparticles on the Surface Pressure Response of DPPC Monolayers. J. Phys. Chem. C 115 (2011), 21715- 21722. 3. E. Guzmán, L. Liggieri, E. Santini, M. Ferrari, F. Ravera, Influence of silica nanoparticles on phase behavior and structural properties of DPPC--Palmitic acid Langmuir monolayers. Colloids Surf. A, 2012, doi: 10.1016/j.colsurfa.2011.11.023.