## 2 Materials and Methods  
The foam synthesis was conducted using a mixture of poly(methylhydrosiloxane) (PMHS--Gelest Inc., Morrisville, PA, USA) and a silanol terminated polydimethylsiloxane (PDMS--Gelest Inc., Morrisville, PA, USA). The foam represents the porous hosting matrix for magnesium sulfate heptahydrate (MgSO4.7H2O) salt (Aldrich Chemical Co., St. Louis, MO, USA), characterized by an excellent sorption capacity. The chosen polymers present a high permeability that does not affect the water vapor diffusion during the hydration operating conditions. During the foam preparation, the polymers precursors were mixed with MgSO4.7H2O in a suitable percentage (range 50-70 wt.%). In [22], the effective salt content was calculated by analyzing the water desorption process through a thermo-gravimetric (TG) analysis, demonstrating that it is lower than expected, as reported in Table 1. The effective salt content is the result of an average calculated on 9 replicas. The foaming process was performed at 60 \({}^{\circ}\)C for 24 h [22] using a catalyst (Sn2+ compound--Aldrich Chemical Co., St. Louis, MO, USA) as a reaction promoter. The crosslinking between the siloxane compounds leads to a rubber-like silicone network. The chemical blowing due to hydrogen evolution following the reaction among siloxane compounds and a physical blowing of the solvent evaporation cause the foaming expansion. The product is a well-structured cellular foam [23]. The composite foams were identified with the code \"FOAM\" followed by a number indicating the theoretical MgSO4.7H2O salt percentage added to the silicone matrix; e.g., the code \"FOAM60\" refers to a foam with 60 wt.% of theoretical salt hydrate filler. MgSO4.7H2O pure salt was analyzed by means of X-ray diffraction (XRD, Bruker D8 Advance, Bruker, Billerica, MA, USA) to determine crystal structure changes during the dehydration conditions, and the synthesized foam morphology was investigated by environmental scanning electron microscopy (ESEM, Quanta 450, FEI, Hillsboro, OR, USA). To study the stability of the salt crystalline structure during dehydration, the diffractograms were acquired on the hydrated MgSO4.7H2O in a 2\(\theta\) range between 10 and 80, in vacuum (\(6\times 10^{-2}\) mbar) from room temperature up to 140 \({}^{\circ}\)C. Each step was kept at the particular temperature for 60 min; then the material was suddenly reported at room temperature and a last diffractogram was immediately recorded. The comparison of the crystal stability in air was performed at two different heating rates (1 \({}^{\circ}\)C/min and 10 \({}^{\circ}\)C/min) to investigate the influence of the heating rate on the dehydration kinetics. ESEM analysis was performed on produced foam, operating with an accelerating voltage of 8 kV, using isobaric conditions (10 Pa) from 25 to 40 \({}^{\circ}\)C, and at 40 \({}^{\circ}\)C, in controlled water vapor atmosphere, from 10 Pa to 800 Pa (0.1% to 10.9% of relative humidity), repeating the same conditions of dehydration \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Sample** & **MgSO4** **7H2O Bulk** & **FOAM50** & **FOAM60** & **FOAM70** \\ \hline Nominal salt content [wt.%] & 100 & 50 & 60 & 70 \\ Actual salt content [wt.%] & 100 & \(41.08\pm 0.76\) & \(47.33\pm 0.62\) & \(58.3\pm 0.53\) \\ Sorption enthalpy (30–180 \({}^{\circ}\)C & 48.2 & 786.2 & 821.3 & 1012.8 \\ range) []/[g] & & & & \\ Sorption enthalpy—Peak & & & & \\ reaction 65 \({}^{\circ}\)C []/g] & 48.2 & 725.5 & 756.4 & 934.6 \\ Sorption enthalpy—Peak & & & & \\ reaction 111 \({}^{\circ}\)C []/g] & - & 35.8 & 37.3 & 46.2 \\ Sorption enthalpy—Peak & - & 24.9 & 28.2 & 32.0 \\ reaction 146 \({}^{\circ}\)C []/g] & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Obtained sorption enthalpy for the tested materials in the TG/DSC apparatus. (P = 10 Pa, T = 40 \({}^{\circ}\)C) and vice-versa for the sample hydration (P = 800 Pa, T = 5 \({}^{\circ}\)C).