*3.2. Dehydration Behavior of DIC-Na 4.75H*

The dehydration of DIC-Na 4.75H was examined by simultaneous PXRD-DSC measurements, and the results are presented in Figure 10. The preliminary DSC measurement was performed up to 300 ◦C to show there was no change from the dehydration temperature (108.4 ◦C) to the melting point of DIC-Na (292.4 ◦C) (Figure S1). Therefore, the PXRD-DSC and the TG-DTA data (Figure 11) were collected up to 120 ◦C and 150 ◦C, respectively. The DSC curve showed two endothermic peaks corresponding to multi-step dehydration transitions. After the first transition, the PXRD pattern changed slightly, indicating the emergence of a hemi-heptahydrate phase (i.e., 3.5H). The second transition led to a distinct pattern derived from an anhydrous phase. Because the second peak bifurcated, the second dehydration may be a multi-step process, and 3.5H may transform into AH via intermediate phases.

**Figure 9.** Experimental and calculated powder X-ray diffraction (PXRD) patterns for DIC-Na 4.75H, 3.5H, and AH forms.

**Figure 10.** Simultaneously measured variable-temperature powder X-ray diffraction (PXRD) (**left**) and differential scanning calorimetry (DSC) (**right**) diagrams. The colored PXRD patterns and segments in the DSC diagram correspond to the emergence of new phases.

**Figure 11.** Thermogravimetric (TG) (left axis) and differential thermal analysis (DTA) (right axis) diagrams of DIC-Na 4.75H.

The TG-DTA curves in Figure 11 exhibit the dehydration behavior in further detail. Two DTA endothermic peaks appear. The first peak (42.2 ◦C) is accompanied by a 5.32% weight decrease, indicating partial dehydration (4.75 to 3.5, calculation: 5.58%), which is followed by the second peak (56.7 ◦C) with a 15.07% weight decrease, which corresponds to complete dehydration (3.5 to 0, calculation: 16.54%).

The DVS experiments performed at 25 ◦C revealed hysteresis between the water sorption and desorption processes (Figure 12). During the sorption process, AH started to absorb water at 60% RH and transformed into 4.75H at 65% RH. On the other hand, during the desorption process, 4.75H was almost stable down to 35% RH but started to desorb water at 30% RH and completely changed into AH at 30% RH. The intermediate 3.5H form (simulation: 19.8% mass change) was not observed during the desorption process.

**Figure 12.** Dynamic vapor sorption (DVS) isotherm plot at 25 ◦C of DIC-Na anhydrate (AH).
