**1. Introduction**

The continuous ejection of pollutants into the atmosphere as a result of the industrial activity and combustion processes has raised many questions related to the potential impact of pollution on human health [1–4]. This is even more important analyzing the World Health Organization (WHO) statistics which ascribe one third of the deaths caused by strokes, lung cancer, or cardiac diseases to the air pollution [5]. Therefore, the severity of this problem makes it necessary to deepen the study of the impact of pollutants on biological systems [6].

Lipid layers, e.g., skin, tear film, or lung surfactant, provide one of the first protective barriers of the human body against environmental pollution. Therefore, it should be expected that lipid layers should be considered among the most important biological structures where pollution may impact negatively [7–9]. This creates the careful examination of the impact of different chemical species, such as nanoparticles, on the physico-chemical properties of lipid layers that may be used as a tool

for a preliminary understanding of the most fundamental bases governing the alteration of their physiological response as a result of the incorporation of pollutants [9]. However, the direct in vivo evaluation of the impact of pollutants in the behavior of lipid films is difficult in most of the cases, which makes the use of model systems necessary [10]. The use of such models provides the bases for establishing preliminary assays for evaluating the alterations induced by pollutant species on the physico-chemical properties of lipid layers with potential biological relevance.

Langmuir monolayers of lipids at the water/vapor interface are probably among the most widespread models used as tools on the evaluation of the effects of different chemicals on the behavior of biologically relevant systems [11–16]. This is because Langmuir monolayers allow for performing physico-chemical studies on ordered lipid films, which are reminiscent of different biological layers, e.g., a single cellular membrane leaflet or the lung surfactant film [11,17,18]. However, the use of Langmuir monolayers only allows for mimicking some specific aspects of the physico-chemical behavior of biological relevant systems that are relatively complex. Thus, the use of Langmuir monolayers helps with the study of minimal systems, i.e., the model including a limited number of chemical species, which allows for exploring the potential role of each single species in the interaction with pollutants, and in the modification of the physico-chemical properties of the whole system. The most common lipid used as a model for studying the interaction of biological relevant layers and pollutants is the 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) [17,19–26]. This is because this lipid is one of the main components of many biological membranes and fluids, e.g., DPPC accounts for 40 wt % of the total weight of lung surfactant in mammals [27]. Despite the simplicity of the models based only on DPPC, they are useful tools for a preliminary evaluation of the worsening of the physico-chemical properties of lipid layers as a result of the incorporation of solid particles. However, the extrapolation of the results obtained from such studies to real biophysical situations studies may require some cautions and additional considerations [28,29]. This is especially important because the specific characteristic of the method used for evaluating the incorporation of particles into the lipid layer impacts the modifications of the interfacial behavior of DPPC layers strongly due to the incorporation of colloidal particle [9,30]. The effect of the methodology used for the incorporation of particles has been recently explored in relation to the interaction of ceria particles with DPPC at the water/vapor interface, using for such evaluation up to three different methodologies for preparing the monolayers containing both DPPC and particles: (i) particles deposited from dispersions in chloroform onto preformed DPPC monolayers, (ii) mixed monolayers prepared from simultaneous co-spreading of the particles and the DPPC at the interface, and (iii) aerosolized particles deposited onto the preformed DPPC monolayer [30]. The obtained results in such study provided evidence that the use of different delivery methods leads to different modifications of the surface tension behavior. Furthermore, special caution must be taken with the use of techniques based on the dispersion of the particles from dispersions because they can impact the agglomeration of the particles and the distribution of such agglomerated within the lipid layer, which may lead to a situation different to that occurring upon the in vivo interaction of pollutants with lipid layers.

The investigation of the impact of particles on lipid layers takes particular importance because several studies have provided evidence that the interaction of colloidal particles with surfactant layers modifies both the lateral organization and the mechanical of such surfactant layers [31–33]. Therefore, a strong modification of the physico-chemical behavior of biologically-relevant lipid layers as results of the particles incorporation may be expected [34,35]. Our previous study showed the different impact of particles with different hydrophobicity on the interfacial properties of DPPC monolayers, which is ascribed to the nature of the interactions involved in the incorporation of the particles [36]. Furthermore, previous studies have shown that hydrophobic particles present a strong impact on the lateral packing and mechanical properties of DPPC layers [21,23,37,38]. Despite this impact, there is not any systematic study comparing the impact of carbonaceous and silica particles on the interfacial properties of biologically-relevant lipid layers, with such comparative evaluation being of particular interest due to the widespread of nanomaterials based in carbon and silica in different technologies

and industries. This work tries to shed light on the potential effect of the above-mentioned particles on the behavior of DPPC Langmuir monolayers to perform a preliminary evaluation of the potential risks and hazards associated with their incorporation into biologically-relevant systems. This has been performed analyzing the modifications in the 2D lateral packing of the lipid molecules at the interface, and the cohesion interactions within the interfacial layers. It is expected that the picture obtained from this work may be useful as foundations for a broader study aimed to elucidate impact of pollutants on the physiological response of biological relevant layers.
