**4. Conclusions**

This study has shown the influence of the chemical nature of particles interacting with lipid layers. The results provided evidence that CB and SiO2 particles alter the lateral packing of the DPPC monolayers, modifying the cohesion between the lipid molecules within the interface and worsening the mechanical performance of the lipid film. However, the mechanisms driving such modifications are strongly dependent on the specific chemistry of the considered particles, and, in particular, on the interactions involved in their incorporation. This recalls a framework needing the consideration of the complex balance between the contribution of the hydrophobic and electrostatic interactions and the steric hindrance in the behavior of the lipid layer. Therefore, even though particles' most fundamental physical properties can appear to be similar, their specific chemistry, and possibly their geometry and aggregation at the interface, determine their impact on their incorporation into lipid layers, which may even result in the modification of the lipid ability to form ordered phases. This distortion of the lateral order of the lipid monolayers is also associated with the modification of the relaxation mechanism driving the re-equilibration of the interfacial layers after periodic dilational deformations, i.e., the reorganization of molecules within the interface. Despite the simplicity of the considered model, the results have shown that the study of the changes of the equilibrium properties and the rheological response of lipid layers due to their interaction can be used as a powerful for evaluating the impact of pollutants in the functioning of biological layers.

**Author Contributions:** Conceptualization, E.G., L.L., and F.R.; methodology, E.G., E.S., and M.F.; software, E.G.; validation, L.L. and F.R.; formal analysis, E.G.; investigation, E.G., E.S., M.F., L.L., and F.R.; resources, L.L. and F.R.; data curation, E.G.; writing—original draft preparation, E.G.; writing—review and editing, E.G., E.S., and M.F., L.L., and F.R.; visualization, E.G.; supervision, L.L. and F.R.; project administration, L.L. and F.R.; funding acquisition, E.G., L.L., and F.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by IIT-Istituto Italiano di Tecnologia within the Project SEED 2009 "Nanoparticle Impact of Pulmonary Surfactant Interfacial Properties-NIPS", by MINECO under grant CTQ2016-78895-R and by Banco Santander-Universidad Complutense grant PR87/19-22513.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
