*3.2. Apparent Solubility Studies*

The solubilization rate of KET from the crystalline form (acid form) from the hybrid (ZnAl-KET) was evaluated in vitro method in order to evaluate if the intercalation procedure allows enhancing this property. The obtained results showed that crystalline KET (acid form) had a limited solubilization rate, as testified by the concentration value measured (12 ± 0.15 μg/mL). This result was in accordance with the literature data [25]. On the other hand, the results obtained from ZnAl-KET were very different; in fact, a concentration of 106.00 ± 0.20 μg/mL was measured. This result can be explained by taking into account that KET is intercalated into HTlc lamellae as an anion in the molecular form. Thus, when ZnAl-KET makes contact with the water, KET- anions are exchanged and replaced by OH<sup>−</sup> (deriving from water molecules dissociation) in the interlayer space. Once in the medium, KET- anions are protonated obtaining a free dissolved molecules.
