Structure of diclofenac in an aqueous medium and its adsorption onto carbons: Molecular insights through simulation

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Vol. 686; p. 133373
• Main Authors: Richard, Axel, Camara, Fatokhoma A., Ramézani, Hamidréza, Mathieu, Nathalie, Delpeux, Sandrine, Bhatia, Suresh K.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05.04.2024
• Subjects: Aqueous medium; Carbon-based adsorbents; Density functional theory (DFT); Hybrid reverse Monte Carlo (HRMC); Molecular dynamics (MD); Molecule polarity; Organic molecule; Organic molecule; Aqueous medium; Carbon-based adsorbents; Molecular dynamics (MD); Hybrid reverse Monte Carlo (HRMC); Molecule polarity; Density functional theory (DFT)
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/j.colsurfa.2024.133373

Diclofenac adsorption in aqueous media was investigated via molecular dynamics (MD), using protonated and deprotonated forms of diclofenac, optimized using density functional theory (DFT). The study focuses on the impact of the relative amount of protonated and deprotonated diclofenac in solution on molecular interactions in the bulk and on the adsorption mechanism on different carbon surfaces. The parameters under investigation were solution pH, diclofenac concentration in the bulk, and the presence or absence of ionic forms of water molecules. Solvent accessible surface area (SASA) calculations clearly illustrate that under pH ⩽ 2, the dominant protonated form of diclofenac tends to form aggregates, resulting in very low solubility in water. In contrast, these diclofenac aggregates do not form under pH ⩾ 6, leading to significantly higher solubility of diclofenac in water. The interactions between protonated and deprotonated forms of diclofenac are not significantly affected by the ionic forms of water molecules. Therefore, the primary parameter affecting aggregate size is the form of diclofenac molecules and their relative amount in aqueous solutions. The adsorption of diclofenac near a graphene sheet, single nanotube, and activated carbon, constructed using hybrid reverse Monte Carlo (HRMC), was examined using MD. For graphene sheets and single nanotubes, diclofenac molecules exhibit stronger adsorption capacities under strongly acidic conditions. For activated carbon, adsorption is improved at a pH equal to the compound’s pKa. This indicates that both the porous texture of activated carbon (curvature, pore diameters, defects) and the ability of diclofenac molecules to adopt various conformations of protonated and deprotonated forms enhance diclofenac adsorption. The flexibility of the diclofenac molecule arises from possible rotation along the C-N-C axis. This pivot axis plays a critical role in enabling the formation of various conformers in bulk and near the adsorbents, ultimately leading to the creation of aggregates and double layers adjacent to the adsorbents. This phenomenon was analyzed using the Non-Covalent Index method based on the electronic density of diclofenac atoms. [Display omitted] •Protonated and deprotonated forms of diclofenac molecules only impact their behavior in water.•Flexibility of diclofenac and conformer shapes dominate its adsorption on carbon-based adsorbents.•Diclofenac adsorption onto activated carbon exhibits isotropic behavior across various pH ranges.•Low aggregation of diclofenac at acidic pH leads to high solubility and removal capacity.