## 3 Results
### Adsorption of Pluronic-F-127-Decorated Silica Nanoparticles at the Water-Vapor Interface
#### 3.3.1 Interfacial Tension of the Water-Vapor Interface  
Before discussing the adsorption of Pluronic F-127 and copolymer-decorated silica nanoparticles at the water-vapor interface, it is important to highlight that the bare silica nanoparticles exhibit minimal interfacial activity when considering their interaction with water-vapor interfaces. This observation is consistent with previous results reported in the literature [14; 42; 43]. The lack of significant interfacial activity may be attributed to reluctance of the nanoparticles to spontaneously adsorb at the water-vapor interface. However, it is also possible that they do adsorb to some extent but exert weak repulsive forces at the interfaces [44]. In contrast, the interfacial tension of the bare water-vapor interface is significantly affected by both Pluronic F-127 and copolymer-decorated nanoparticles. Figure 4 shows the relationship between Pluronic F-127 concentration and interfacial tension observed at the water-vapor interface after adsorption of either Pluronic F-127 or copolymer-coated nanoparticles. In the case of the interaction of Pluronic F-127 with the liquid-vapor interface, the behavior is consistent with that expected for the adsorption of an amphiphilic copolymer with surfactant characteristics. In fact, Pluronic F-127 is able to to reorganize at the interface with its most hydrophobic blocks, i.e., the poly(propylene oxide) ones, protruding towards the vapor phase, while the poly(ethylene oxide) blocks ensure the attachment of the copolymer to the liquid interface. Considering the investigated concentration range of Pluronic F-127, it is expected that the Pluronic F-127 is attached to the interface in a mushroom or brush conformation [45; 46; 47; 48]. It is more interesting to evaluate the change in water-vapor interfacial tension with the Pluronic F-127 concentration when dispersions of  
Figure 4: Pluronic F-127 concentration dependence of the interfacial tension of the bare water–vapor interface upon the adsorption of Pluronic F-127 or copolymer-decorated silica nanoparticles. (\(\blacktriangle\)) \(\alpha_{\text{NP}}=0\%\)\(w/w\) (Pluronic F-127 solutions), (\(\blacksquare\)) \(\alpha_{\text{NP}}=0.1\%\)\(w/w\) and (\(\bullet\)) \(\alpha_{\text{NP}}=1\%\)\(w/w\). The lines are guides for the eyes, and the error bars were obtained as the standard deviation of three replicates for each sample. Pluronic-F-127-decorated silica nanoparticles are considered. The results show that for a fixed concentration of Pluronic F-127, the interfacial tension increases with the concentration of silica nanoparticles. This is similar to the effect observed by [PERSON] et al. [PERSON] et al. (2018) in their study of the adsorption of surfactant-decorated particles at the water-vapor interface. This type of phenomenon is different from that observed in other colloidal systems, such as polymer-surfactant mixtures, where the association process is accompanied by an enhanced decrease in the surface tension of the fluid-fluid interface [PERSON] et al. (2018). For the case of Pluronic F-127 solutions in the investigated copolymer concentration range, the interfacial tension is on a plateau region related to the maximum coverage of the interface, or closer to such a situation (at the lowest Pluronic F-127 concentrations); for copolymer-decorated particles the results show higher values of interfacial tension than for the copolymer solutions at low Pluronic F-127 concentrations, and then as the Pluronic F-127 concentration increases, the interfacial tension decreases to values similar to those found for bare Pluronic F-127 solutions. These results can be explained by considering two different frameworks: The first assumes that the introduction of silica nanoparticles causes a depletion of Pluronic F-127 molecules from the aqueous phase to the surface of the silica nanoparticles, thereby reducing the effective concentration of free copolymer in the solution.