A combined experimental and computational modeling study on adsorption of propionic acid onto silica-embedded NiO/MgO nanoparticles

Abstract

Adsorption of propionic acid (PA) onto silica-embedded NiO/MgO nanoparticles (i.e., SiO2-NiO, SiO2-MgO, and SiO2-(Ni0.5Mg0.5)O) was investigated experimentally and theoretically by carrying out computational modeling through molecular mechanics, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. The experimental adsorption isotherm fit well to the Sips model with a heterogeneity factor between 0.2 and 0.5 in most cases, indicating a heterogeneous adsorption system. SiO2-NiO nanoparticles showed the highest uptake on a normalized surface area basis due to its stability in aqueous solutions. Moreover, the results of thermodynamic studies, namely, changes in Gibbs free energy (ΔGads∘) and standard enthalpy (ΔHads∘), confirmed that the adsorption is spontaneous and exothermic in nature, respectively. Furthermore, computational modeling of the molecular interaction between the PA molecule and the nanoparticle surfaces of both NiO and MgO were implemented to address the adsorption behavior comprehensively. Interestingly, in vacuum media, the computational modeling and DFT calculations showed that MgO favored the PA molecule adsorption stronger than the NiO, contrary to what observed experimentally. MD simulations counted the presence of water molecules and provided more linkable results to the ones observed experimentally. Eventually, by having a meticulous eye of the equilibrated structures of PA molecules at the NiO-water interfaces according to the MD simulation, we could confirm theoretically the maximum adsorption capacity for complete monolayer coverage with the one obtained experimentally which was around 2.8(molecules/nm2).