Biomedical Engineering Reference
In-Depth Information
molecules, those commonly used are carbon monoxide and water, because of their
selectivity towards cations, the ease of interpretation of their spectra, and the high
sensitivity of IR instrumentation in the region where their vibrational frequencies fall
(medium IR). The (010) stoichiometric surface has already been fully characterized by Corno
et al.
in relation to the adsorption of these molecules (Corno et al., 2009), whereas the non-
stoichiometric surfaces are the subject of this work. Calculations provide binding energies
and vibrational frequencies of the probe molecules which can be used as a future reference
to be compared with experimental measurements. In Fig. 4, the optimized structures of the
adducts are reported for water and CO.
The adsorptions of water upon the most exposed cations of the non-stoichiometric surfaces
are characterized by BE values of 131 and 125 kJ/mol (BSSE ≈ 35 %), for the Ca-rich and P-
rich surfaces, respectively. These values also take into account the formation of two
hydrogen bonds between the water molecule and exposed anions, either phosphate only, or
also hydroxyl anions in the case of the (010) Ca-rich surface. If the thermal and vibrational
contributions are taken into account, these values decrease to 117 and 110 kJ/mol,
respectively, allowing a consistent comparison with the experimental differential heat of
adsorption of water of 110 kJ/mol (Corno et al., 2009). The latter value has been obtained
from experimental micro-calorimetric studies of water adsorption on microcrystalline HA,
for a loading of water comparable with our models.
The adsorption of CO upon the most exposed Ca ions gives results highly representative of
the strength of the cationic site as no hydrogen bond can be formed: the BE values upon the
Ca-rich and P-rich surfaces are, respectively, 38 and 40 kJ/mol (BSSE ≈ 20 %), showing an
almost equivalence for the two Ca ions for the two surfaces.
When the vibrational features are considered, a crucial point is the comparison between the
frequencies of the free and the adsorbate molecule. While the stretching mode of CO is
easily identified in both cases, the modes of the adsorbed water molecule are no longer
easily referable to those of the free molecule. The free water molecule is characterized by
two stretching modes (the symmetric and, at higher frequencies, the anti-symmetric) and
one bending mode. When the molecule interacts with the surface, these two kinds of
stretching modes are no longer classifiable on a symmetry ground. The hydrogen bonds
which are formed with the most exposed anions of the surface cause the loss of the C
2v
symmetry of the molecule: each OH bond oscillates independently. As in previous work, we
decided to compare the lower OH stretching frequency of the adsorbed molecule to the
symmetric mode and
vice versa
(Corno et al., 2009).
With these remarks, the calculated symmetric stretching shifts are -761 and -329 cm
-1
for the
(010) Ca-rich and (010) P-rich surfaces, respectively, while the anti-symmetric shifts are -310
and -328 cm
-1
. The shifts of the bending frequencies are 113 and 88 cm
-1
. When a hydrogen
bond pattern occurs, the stretching mode shifts are negative due to the electronic density
transfer and the resulting decrease of the bond strength. Instead, the bending frequencies
increase because of the restraint caused by the hydrogen bond itself. The larger shift of the
symmetric stretching of the H
2
O adsorbed on the (010) Ca-rich surface, in combination with
a larger bending shift, indicates the formation of a very strong hydrogen bond with an
exposed anion of the (010) Ca-rich surface. Experimentally, the shift of the stretching is -400
cm
-1
while the bending shift is 40 cm
-1
. The calculated values are, then, in agreement with
the experimental ones as long as the signs and the trends are considered (Bertinetti et al.,
2007).
