Bottom-Up Approach to the Coarse-Grained Surface Model: Effective Solid–Fluid Potentials for Adsorption on Heterogeneous Surfaces

Coarse-grained surface
models with a low-dimension positional dependence
have great advantages in simplifying the theoretical adsorption model
and speeding up molecular simulations. In this work, we present a
bottom-up strategy, developing a new two-dimensional (2D) coarse-grained
surface model from the “bottom-level” atomistic model,
for adsorption on highly heterogeneous surfaces with various types
of defects. The corresponding effective solid–fluid potential
consists of a 2D hard wall potential representing the structure of
the surface and a one-dimensional (1D) effective area-weighted free-energy-averaged
(AW-FEA) potential representing the energetic strength of the substrate–adsorbate
interaction. Within the conventional free-energy-averaged (FEA) framework,
an accessible-area-related parameter is introduced into the equation
of the 1D effective solid–fluid potential, which allows us
not only to obtain the energy information from the fully atomistic
system but also to get the structural dependence of the potential
on any geometric defect on the surface. Grand canonical Monte Carlo
simulations are carried out for argon adsorption at 87.3 K to test
the validity of the new 2D surface model against the fully atomistic
system. We test four graphitic substrates with different levels of
geometric roughness for the top layer, including the widely used reference
solid substrate Cabot BP-280. The simulation results show that adding
one more dimension to the traditional 1D surface model is essential
for adsorption on the geometrically heterogeneous surfaces. In particular,
the 2D surface model with the AW-FEA solid–fluid potential
significantly improves the adsorption isotherm and density profile
over the 1D surface model with the FEA solid–fluid potential
over a wide range of pressure. The method to construct an effective
solid–fluid potential for an energetically heterogeneous surface
is also discussed.