## 3 Results and Discussion  
Figure 2 shows an optical image of the cross section of a FOAM60 sample. The cell structure is regular with interconnected bubbles [22]. However, some local heterogeneities on the bubble size can be observed. At the foam borders some larger bubbles are clearly identifiable. The composite foam, although characterized by these local defects, can be considered almost homogeneous. Analyzing the details of Figure 2 (left), it is possible to highlight the channel interconnection among bubbles. These channels act as preferential pathways for the water vapor diffusion toward the inner parts of the foams [21], avoiding limiting mass diffusion phenomena usually observed on composite sorbent materials [25]. Furthermore, the cell walls were thick enough to lead to a structurally stable morphology. Furthermore, the salt, 60 wt.%, is well embedded in the silicone matrix and no evident heterogeneities were observed. Figure 1: Schematic representation of environmental scanning electron microscopy (ESEM) analysis set-up. XRD patterns are shown in Figure 3. XRD analysis was carried out on the salt, considering that the analysis on the composite foam is not possible without the necessary condition (flat surface) to carry out this type of structural analysis. To investigate the structural features of MgSO\({}_{4}\cdot\)7H\({}_{2}\)O salt in various application conditions, XRD analysis was performed both in air, at different heating rates, and in vacuum which represents the condition applied during the following ESEM analysis. At 25 \({}^{\circ}\)C (Figure 3a), the peaks of orthorhombic magnesium sulfate heptahydrate MgSO\({}_{4}\cdot\)7H\({}_{2}\)O (20: 14.75\({}^{\circ}\), 16.54\({}^{\circ}\), 19.76\({}^{\circ}\), 21.03\({}^{\circ}\)), in agreement with standard data (JCPDS: 04-009-8711), are present. The peaks appear sharp and narrow because of the high degree of crystallization of the MgSO\({}_{4}\cdot\)7H\({}_{2}\)O. Increasing the temperature up to 40 \({}^{\circ}\)C, the peaks of hexahydrate monoclinic MgSO\({}_{4}\cdot\)6H\({}_{2}\)O (20: 16.25\({}^{\circ}\), 17.36\({}^{\circ}\), 20.20\({}^{\circ}\), 22.01\({}^{\circ}\)) in agreement with JCPDS 04-009-8713, appear. At 60 \({}^{\circ}\)C, only the latter phase is visible and an amorphous state forms, accordingly with [PERSON] et al. [26]. Above 60 \({}^{\circ}\)C, no peaks can be observed, indicating that the amorphous phase remains stable also when the room temperature is reached again after the dehydration process. It is worth noticing that by increasing the dehydration temperature rate from 1 \({}^{\circ}\)C/min to 10 \({}^{\circ}\)C/min, the structure of the salt loses its crystallinity later; in fact, in Figure 3b at 80 \({}^{\circ}\)C, MgSO\({}_{4}\cdot\)6H\({}_{2}\)O peaks are still visible, contrary to what is reported in Figure 3a. The effect of the variable heating rate is the shift of the dehydration peaks to higher temperatures, suggesting that dehydration kinetics represent a critical issue to be assessed. High temperature increase rates result in a higher dehydration temperatures, owing to the poor salt thermal conductivity: at high temperature increase rates, there is insufficient time for the heat to be transmitted from the boundary surfaces to the bulk, leading to a delay in the dehydration step. The low heating rate leads to a sequence of equilibrium steps, favoring the dehydration phase achievement at lower temperatures [27]. For these reasons, the first dehydration step (MgSO\({}_{4}\cdot\)7H\({}_{2}\)O \(\rightarrow\) MgSO\({}_{4}-\)6H\({}_{2}\)O + H\({}_{2}\)O) is completed at 80 \({}^{\circ}\)C in Figure 3a and it is not in Figure 3b, where the amorphous phase, due to the second dehydration step (MgSO\({}_{4}-\)6H\({}_{2}\)O \(\rightarrow\) MgSO\({}_{4}-\)0.1H\({}_{2}\)O + 5.9H\({}_{2}\)O, according to [28]), appears only at 100 \({}^{\circ}\)C. Indeed, as reported by [PERSON] et al. [28], initially, the MgSO\({}_{4}\cdot\)7H\({}_{2}\)O is a heptahydrate crystal without cracks.