  In order to understand why carbon dioxide (CO$_2$) and methane (CH$_4$)
molecules interact differently with surfaces, we investigate the Casimir-Polder
energy of a linearly polarizable CO$_2$ molecule and an isotropically
polarizable CH$_4$ molecule in front of an atomically thin gold film and an
amorphous silica slab. We quantitatively analyze how the anisotropy in the
polarizability of the molecule influences the van der Waals contribution to the
binding energy of the molecule.
