## 3 Results and Discussion
### Adsorption and Release of Diclefonac Sodium
The K\({}_{\rm L}\) constant values for all samples indicate that the bonding between diclofenac sodium and MIL-88A-4 is the strongest [100]. The value of 1/n determined from the Freundlich isotherm was less than 1 for all samples, confirming that the adsorption of diclofenac sodium is thermodynamically favorable [18]. The maximum theoretical values (Q\({}_{\rm max}\)) of materials' adsorption capacities toward diclofenac sodium are comparable to those obtained experimentally. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  
The release studies of diclofenac sodium from MIL-88A materials were performed in a phosphate buffer of pH 6.8, simulating intestinal fluid. The release profiles of the drug from MIL-88A carriers are shown in Figure 10. The percentage of the drug desorbed from the metal-organic frameworks ranges from 58 to 97%, depending on their synthesis conditions. Considering that the molecular size of diclofenac sodium is 0.97 nm \(\times\) 0.71 nm \(\times\) 0.47 nm, it should be assumed that the drug diffusion rate is affected by samples' textural parameters and functional groups present on the surface [77; 101]. The drug release profiles obtained for MIL-88A-1, MIL-88A-2, and MIL-88A-3 materials showed a two-stage diffusion process. It indicates diclofenac sodium rapid release during the initial stage of the experiment due to the presence of its molecules mainly on the carriers' external surface, followed by a subsequent stage of slower release of the drug from the pores over an extended period [102]. At pH 6.8, which simulated intestinal condition, the highest drug amount was liberated from rod-like MIL-88A-1, synthesized in water at 70 \({}^{\circ}\)C. The release profile of the diclofenac sodium from the sample presented a high burst liberation of the drug (70%) during the first six hours and then slower desorption of 27% after 30 h. This material was characterized by the largest pore diameter, which resulted in weaker interactions between material pore walls and diclofenac sodium molecules. The extended drug release from the MIL-88A-2 sample in simulated intestinal fluid finished within 32 h. Regarding MIL-88A-2, MIL-88A-3, and MIL-88A-4 materials, larger specific surface area and porosity \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multicolumn{3}{c}{**Langmuir**} & \multicolumn{3}{c}{**Freundlich**} \\ \cline{2-7}  & **Q\({}_{\text{max}}\)** & **K\({}_{\text{L}}\)** & **R\({}^{\text{2}}\)** & **K\({}_{\text{F}}\)** & **1/n** & **R\({}^{\text{2}}\)** \\ \hline MIL-88A-1 & 2070 & 0.049 & 0.793 & 1523 & 0.04 & 0.798 \\ MIL-88A-2 & 1502 & 0.058 & 0.995 & 349 & 0.22 & 0.867 \\ MIL-88A-3 & 905 & 0.027 & 0.967 & 162 & 0.25 & 0.758 \\ MIL-88A-4 & 1255 & 0.075 & 0.773 & 728 & 0.08 & 0.951 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The parameters of non-linear Langmuir and Freundlich isotherm models fitted to equilibrium data of diclofenac sodium adsorption on the surface of MIL-88A samples. Figure 9: Non-linear fitting of diclofenac sodium adsorption isotherms to Langmuir and Freundlich models for (**A**) MIL-88A-1, (**B**) MIL-88A-2, (**C**) MIL-88A-3, (**D**) MIL-88A-4. promote higher accessibility of adsorption sites, leading to stronger interactions between the drug molecules and the materials' surface [103]. Drug desorption from MIL-88A-3 and MIL-88A-4 reached about 58% after 30 h, extending the therapeutic effect of this drug in the human body. The high percentage of drug release (from 58% for samples synthesized in DMF to 97% for material obtained in water at 70 \({}^{\circ}\)C) at pH 6.8 could be associated with the repulsion between the drug's anionic form and the negatively charged surface of carriers [104; 105; 106]. Various materials have been presented in the literature as advanced diclofenac sodium delivery systems. [PERSON] and co-workers [107] utilized SBA-15 silica functionalized with polythevenimine polymers as a drug carrier.