  CO is one of the most abundant ice components on interstellar dust grains.
When it is mixed with amorphous solid water (ASW) or located on its surface, an
absorption band of CO at 2152 cm$^{-1}$ is always present in laboratory
measurements. This spectral feature is attributed to the interaction of CO with
dangling-OH bonds (dOH) in ASW. However, this band is absent in observational
spectra of interstellar ices. This raises the question whether CO forms a
relatively pure layer on top of ASW or is in close contact with ASW, but not
via dangling bonds. We aim to determine whether the incorporation of NH$_3$ or
CO$_2$ into ASW blocks the dOH and therefore reduces the 2152 cm$^{-1}$ band.
We performed laboratory experiments to simulate the layered structure of the
ice mantle, that is, we grew CO ice on top of 1) pure ASW, 2)
NH$_3$:H$_2$O=10:100 mixed ice, and 3) CO$_2$:H$_2$O=20:100 mixed ice. Infrared
spectra were measured to quantify the strength of the 2152 cm$^{-1}$ band. In
addition, a second set of experiments were performed to determine how the
incorporation of NH$_3$ into ASW affects the dOH band. We found that annealing
the ice reduces the 2152 cm$^{-1}$ band and that NH$_3$ blocks the dOH on ASW
surface and therefore reduces the 2152 cm$^{-1}$ band more effectively than
CO$_2$. We suggest that this difference between NH$_3$ and CO$_2$ can be
ascribed to the polarity of the guest molecule (NH$_3$ is a polar species,
whereas CO$_2$ is apolar). The polarity implies that the formation of an H-bond
between the N atom of ammonia and the dOH is a barrier-less reaction. We also
determined the pore surface area of the ice mixtures as a function of the
annealing temperature, and found that the nondetection of 2152 cm$^{-1}$ band
does not necessarily exclude the possibility of a porous ice mantle.
