Diclofenac sodium drug adsorption isotherm on carbon xerogels via an advanced two layers model with two energies

Abstract

In this work, four carbon xerogels (XCs) adsorbents, which were prepared by the polymerization of formaldehyde (F) and resorcinol (R) using cesium carbonate (Cs) as catalyst at 4 M ratios R/Cs = XCs-100, XCs-500, XCs-1000 and XCs-2000, were utilized to interpret and understand the adsorption mechanism at a molecular level of non steroidal anti inflammatory drug (NSAID), namely diclofenac sodium (DCF). Hence, Four adsorption isotherms were utilized to present the evolution of the adsorbed quantity (Qa) as a function of drug concentration describing the DCF adsorption on the four carbons xerogels at T = 298 K and at pH = 7. In addition, two other adsorption systems, describing the adsorption of DCF on XCs-500, were plotted by varying two experimental conditions: temperature and solution pH. All these experimental data were adjusted and analyzed via the advanced two layers model with two adsorption energies (A2LM2E), which was chosen as the most adequate statistical physics model. Furthermore, stereographic and energetic parameters of A2LM2E namely: the number of DCF molecules per XCs sites (n), the adsorbed quantity at saturation (Qas) and the concentrations at half saturation (C1 and C2) were estimated by a multi-variable non-linear regression. The fitting results indicated that the DCF drug molecules were docked with mixed and non-parallel orientations on the different XCs adsorbents. The investigation of the adsorbed quantity at saturation indicated that Qas increased as follows: Qas (XCs-2000) < Qas (XCs-100) < Qas (XCs-1000) < Qas (XCs-500), showing that the XCs-500 was more efficient for the adsorption of DCF drug than the other adsorbents. The calculated molar adsorption energies ΔE1 and ΔE2 for the studied adsorption systems affirmed the exothermic and the physical characters of the adsorption mechanism since ΔE1 and ΔE2 values were positive and inferior to 40 kJ/mol. Lastly, the XCs-100, XCs-500, XCs-1000 and XCs-2000 adsorbents presented unimodal pore size distributions (PSDs) centered at a pore radius of 8.38 nm, 7.17 nm, 6.05 nm and 5 nm, respectively. Finally, the estimated thermodynamic parameters such as the adsorption entropy (Sa), the Gibbs free enthalpy (G) and the internal energy (Eint) indicated that DCF adsorption on XCs-500 was spontaneous and exothermic process.