Polyethylene and polyvinyl chloride microplastics as environmental carriers of 2,4-D, thiamethoxam, and clothianidin: a DFT, QTAIM, and NCI study of interaction mechanisms and binding energetics

Microplastics (MPs), plastic fragments smaller than 5 mm, have been recognized as potential carriers of contaminants in the environment. In this study, computational simulations based on the Density Functional Theory (DFT) were used to evaluate the interactions of pure (unoxidized) polyethylene (PE) and polyvinyl chloride (PVC) MPs with three widely used pesticides: 2,4-D acid, thiamethoxam, and clothianidin. The calculations included geometric optimizations, interaction energies, and analyses of Molecular Electrostatic Potential (MEP), Frontier Molecular Orbitals (FMOs), Quantum Theory of Atoms in Molecules (QTAIM) and noncovalent interactions Analysis (NCI). The MEP showed sites concentrated in functional groups such as nitro and carboxyl, while the FMOs revealed moderate chemical reactivity for the pesticides (E gap 4.1-5.4 eV) and high stability for the polymers (E gap 7.5−8.1 eV). Binding energies (ΔE bind < 0) confirmed favorable adsorption, and enthalpy values Δ r H < 0 indicated exothermic processes; however, Gibbs energies (Δ r G > 0) demonstrated that adsorption between the pure polymers and pesticides was not spontaneous under standard conditions. The Quantum Theory of Atoms in Molecules (QTAIM) showed that the interactions are electrostatic character and the analysis of noncovalent interactions (NCI) showed that confirmed that adsorption occurs through interactions dominated by van der Waals forces. According to these results, PVC and PE showed similar mechanisms in the adsorption of the three pesticides and the results demonstrate how pesticides can interact with microplastics, giving a theoretical insight to understanding the intrinsic interactions between microplastics no-aged and contaminants