Solvation Free Energies and Adsorption Energies at the Metal/Water Interface from Hybrid Quantum-Mechanical/Molecular Mechanics Simulations

Modeling adsorption at metal/water
interfaces is a cornerstone
toward an improved understanding in a variety of fields from heterogeneous
catalysis to corrosion. We propose and validate a hybrid scheme that
combines the adsorption free energies obtained in the gas phase at
the density functional theory level with the variation in solvation
from the bulk phase to the interface evaluated using a MM-based alchemical
transformation, denoted MMsolv. Using the GAL17 force field for the
platinum/water interaction, we retrieve a qualitatively correct interaction
energy of the water solvent at the interface. This interaction is
of near chemisorption character and thus challenging, both for the
alchemical transformation and also for the fixed point-charge electrostatics.
Our scheme passes through a state characterized by a well-behaved
physisorption potential for the Pt(111)/H2O interaction
to converge the free energy difference. The workflow is implemented
in the freely available SolvHybrid package. We first assess the adsorption
of a water molecule at the Pt/water interface, which turns out to
be a stringent test. The intrinsic error of our quantum-mechanical/molecular
mechanics (QM/MM) hybrid scheme is limited to 6 kcal mol–1 through the introduction of a correction term to attenuate the electrostatic
interaction between near-chemisorbed water molecules and the underlying
Pt atoms. Next, we show that the MMsolv solvation free energy of Pt
(−0.46 J m–2) is in good agreement with the
experimental estimate (−0.32 J m–2). Furthermore,
we show that the entropy contribution at room temperature is roughly
of equal magnitude as the free energy but with an opposite sign. Finally,
we compute the adsorption energy of benzene and phenol at the Pt(111)/water
interface, one of the rare systems for which experimental data are
available. In qualitative agreement with the experiment, but in stark
contrast with a standard implicit solvent model, the adsorption of
these aromatic molecules is strongly reduced (i.e., less exothermic
by ∼30 and 40 kcal mol–1 for our QM/MM hybrid
scheme and experiment, respectively, but ∼0 with the implicit
solvent) at the solid/liquid interface compared to the solid/gas interface.
This reduction occurs mainly because of the competition between the
organic adsorbate and the solvent for adsorption on the metallic surface.
The semiquantitative agreement with experimental estimates for the
adsorption energy of aromatic molecules thus validates the soundness
of our hybrid QM/MM scheme.