*2.1. Materials*

Pangel S9 (PS9) and Cimsil G30 (G30) were kindly gifted by the TOLSA Group (Madrid, Spain). PS9 (d90 23.9 μm) and G30 (d90 49.3 μm) were mainly composed by sepiolite and palygorskite, respectively. According to their composition and properties, previously characterized by García-Villén et al. [32], they could be classified as "pharmaceutical-grade" excipients. Their corresponding pharmacopoeial denominations are "magnesium trisilicate" (PS9) and "Attapulgite" (G30) [33–35]. Sepiolite was present in PS9 in >92%, muscovite being the main mineralogical impurity detected. Palygorskite was present in G30 in 58%, accompanied by quartz (26%), fluorapatite (7%), smectites, and sepiolite (6%) and carbonates (3%) as associated minerals. Both PS9 and G30 excipients were dried in an oven at 40 ◦C for at least 48 h prior to being used in the preparation of the hydrogels. Spring water from Alicún thermal station (ALI), located in Granada (Spain) was used. ALI water is classified as hypothermal with strong mineralization [36,37].

Nanoclay/spring water hydrogels were prepared according to a process previously studied and optimized [32]. Briefly, clay minerals were mixed with ALI by means of a turbine high-speed agitator (Silverson LT, Chesham, UK), equipped with a high-traction stirrer head of square mesh, at 8000 rpm for 5 min. Samples were preserved in closed polyethylene containers, from which aliquots were sampled in order to monitor further analysis. Nanoclay/spring water hydrogels prepared had a 10% w/w of PS9 nanoclay concentration (ALIPS9) and 20% *w*/*w* of G30 nanoclay (ALIG30). Both of them were preserved and characterized in the same way.
