Grand Canonical Monte Carlo Simulation Study of Water Adsorption in Silicalite at 300 K
Résumé
This molecular simulation work focuses on the adsorption of water in a priori hydrophobic
silicalite-1, a microporous ordered silica. The water-water interactions are described with the SPC
model while water-silica interactions are calculated in the framework of the PN-TrAZ model. The water
adsorption isotherm at 300 K, the configurational energies, and the isosteric heat of adsorption are
calculated by the Grand Canonical Monte Carlo (GCMC) simulation method. The thermodynamic
integration scheme allows to calculate the grand potential along the adsorption isotherm. The adsorption
results are compared with experiments, showing only qualitative agreement. Indeed, the simulations do
not reproduce the expected hydrophobicity of silicalite (V. Eroshenko, R.-C. Regis, M. Soulard, J.
Patarin, C. R. Phys. 2002, 3, 111). This indicates that common models used to describe confined polar
molecules are far from being operative. In this work, it is shown, on the basis of periodic ab initio
calculations, that confined water molecules in silicalite have a dipole value roughly 10 percent smaller
than in the bulk liquid phase, indicating that the environment felt by a confined water molecule in
silicalite pores is not equivalent to that in the bulk liquid. This suggests that effective intermolecular
potentials parameterized for bulk water are inefficient to describe ultra confined water molecules.
Reducing the SPC water dipole moment by 5% (i.e. decreasing water partial charges in magnitude) in
GCMC calculations does allow reproducing the experimental water/silicalite isotherm at 300K.
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