## 3 Results and Discussion
### Adsorption and Release of Diclefonac Sodium
The average pore diameters of the MIL-88A materials, in the range of 2.83-20.3 nm, were higher than the molecular dimensions of the diclofenac sodium (0.97 nm \(\times\) 0.71 nm \(\times\) 0.47 nm). The adsorption process was enhanced by allowing unrestricted drug transport within the pores, as steric obstacles do not impose limitations [77]. Taking into account the morphology of the synthesized materials, it can be observed that rod-like particles significantly increase the amount of adsorbed drug. High MIL-88A/drug affinity was also the result of unique interactions between diclofenac sodium and samples. The adsorption of drugs onto the surface of MIL-88A samples is primarily based on hydrogen bonding, facilitated by the presence of proton donors and acceptors in both the drug and the samples. The amine groups in diclofenac sodium could act as proton donors, and the electronegative parts of materials as proton acceptors. On the other side, the formation of hydrogen bonds may occur between \(\mu\)3-OH([Fe(OH)]) in MIL-88A and functional groups containing oxygen or nitrogen in the diclofenac sodium molecules [26; 78]. Secondly, the metal coordination effect enhances the effective adsorption of drug molecules via the interaction between the MIL-88A central iron trimers and the nitrogen and oxygen functional groups of the drug [57; 76]. Finally, in addition to these two types of interplays, electrostatic interactions may affect the adsorption of diclofenac onto the MIL-88A samples. The isoelectric point of MIL-88A is 6.0, and the pKa value for diclofenac sodium is 4.0 [79; 80]. This means that the samples maintain a positive surface charge until the pH reaches 6, while diclofenac sodium takes on an anionic form when the pH exceeds 4 (Figure 7). The pH of the drug solution was 5.5; hence, the drug adsorption at this pH value can be based on the electrostatic interactions occurring between the anions of diclofenac sodium and the positively charged surface of MIL-88A [57]. The possible mechanism of drug adsorption onto MIL-88A materials is depicted in Figure 8. Table 3 compares the maximum sorption capacity of the MIL-88A-1 sample synthesized in distilled water at lower temperatures with other adsorbents described in the literature. The MIL-88A-1 material adsorbed the highest amount of diclofenac sodium. In contrast to other MOF materials prepared in DMF (i.e., UIO-66, MOR-1, MIL-100, [Cu(BTTA)]n\(\cdot\)2 DMF), synthesis of MIL-88A-1 conducted in distilled water under mild conditions makes it more environmentally friendly. The high adsorption ability of MIL-88A-1 can be attributed to its flexible structure and breathing effect, as well as different types of interactions with diclofenac sodium: hydrogen bonding, metal coordination, and electrostatic interactions. Figure 8: Proposed mechanism of the interactions between diclofenac sodium and MIL-88A samples. Figure 6: Adsorption isotherms of diclofenac sodium on the surface of MIL-88A carriers. Figure 7: Different forms of diclofenac sodium at various pH conditions. To understand the adsorption mechanism, diclofenac sodium adsorption isotherms were fitted to the non-linear form of Langmuir and Freundlich models (Figure 9). It was found that the correlation coefficients R\({}^{2}\) for the Freundlich model are higher than for Langmuir in the case of drug adsorption onto MIL-88A-1 (distilled water, 70 \({}^{\circ}\)C) and MIL-88A-4 (DMF, 120 \({}^{\circ}\)C) (Table 4). It implies multilayer adsorption, which assumes the interaction between adsorbed molecules on a non-uniform surface [26]. The calculations indicate that the Langmuir model can best represent the adsorption of diclofenac sodium onto MIL-88A-2 (distilled water, 120 \({}^{\circ}\)C) and MIL-88A-3 (DMF, 70 \({}^{\circ}\)C) (Table 4). It suggests that drug molecules form a monolayer on the surface of the MIL-88A materials, and the adsorption process is mainly controlled by host-guest interactions [93; 98]. There is no lateral interaction between the adsorbed drug molecules [99].