Desorption Mechanism of a Pesticide from a Hydrated Calcium Montmorillonite Unraveled by Molecular Dynamics Simulations

Fenhexamid, N-(2,3-dichloro-4-hydroxyphenyl)-1-methylcyclohexanecarboxamide, a fungicide, is authorized in the European Union to be used in agriculture. Active molecule behavior in the environment and its transfer to environmental components (soil, water, and air) are primordial topics to study to limit environmental pollution. Following a step by step approach, we studied fenhexamid. In previous works, we worked on the fenhexamid pesticide isolated and then on its complexes with one or two Ca2+ and Na+, the most abundant cations in soils, in the gas phase. We also reported theoretical calculations about fenhexamid in interaction with a clay surface: Ca–montmorillonite. In the present paper, potentials of mean force describing overall fenhexamid desorption from a Ca–montmorillonite surface in the presence of water were computed from molecular dynamics simulations combined with umbrella sampling. Several mechanisms were envisaged. The results allow to quantitatively estimate the free energy of desorption and to provide a detailed insight into the hydration sphere of Ca2+ during the desorption process.