234
9 Applications: Aqueous Interfaces
solutions (mole fraction x 0.2) and the whole O–H band eventually vanishes with
increasing x. This behavior posed confusing interpretations for the surface structure.
This system poses an additional challenge in interpretation besides the strong
acid solutions discussed above. Sulfuric acid undergoes two-step ionization:
H 2 SO 4
pKa 1
← − → HSO
−
4
pKa 2
← − → SO
2−
4 .
(9.1)
The first ionization is considered to be strong enough (pKa 1 0), whereas the
second ionization (pKa 2 1.9 [20]) is not as strong as the first one. In such cases
that the acid dissociation is not complete, the local composition of the ion species
at the interface could be different from that in the bulk, since the interface is a less
polar environment than the bulk. The ion dissociation at interface may be sensitive
to the depth or other conditions, and coupled to the interface structure and SFG
spectra. These uncertainties make such surfaces challenging to understand. The
collaboration of SFG spectra and MD analysis can provide valuable information
on the surface structure and ion composition.
Since we could not determine the local ion composition a priori, we assumed
typical cases (a–c) about acid dissociation (9.1), where
(a) Same pKa 1,2 in Eq. (9.1) holds for surface as well, (HSO
−
4 dominant, SO
2−
4
present)
(b) Second ionization is suppressed at surface (pKa 2 0), (no SO
2−
4 )
(c) First ionization is also suppressed (pKa 1 0), (only neutral H 2 SO 4 )
and predicted the SFG spectra in each case by MD simulation. Figure 9.8 displays
the MD results in comparison with the experiments [2, 33] to see whether the
experimental features in O–H and S–O bands are consistently reproduced by MD
predictions. We first find that case (a) would lead to excessively strong hydrogen
bonding O–H signal, arguably because the presence of divalent anions (SO
2−
4 )
too much augments the electric double layer. Therefore, case (a) is not likely to
be the proper case. On the other hand, case (c) shows the main S–O band at
1150 cm −1 in the SSP spectrum, which is assigned to the neutral H 2 SO 4 , while
the experimental main S–O band as well as cases (a) and (b) arises at 1050 cm −1
from HSO
−
4 . Therefore, case (c) is not the proper case either. The combination of
SFG measurement and MD simulation thereby conclude that the first dissociation is
facile at the surface, whereas the second dissociation to form SO
2−
4 is substantially
more suppressed at the surface than in the bulk [30, 33, 53].
9.3.4 Base
The surface preference of hydroxide (OH
− ) anion remains a controversial and active
issue both theoretically and experimentally [1]. Previous MD simulation studies
reported different conclusions about its surface preference, which arguably depend
on the methods and conditions. Direct experimental measurement of surface OH
−
9 Applications: Aqueous Interfaces
solutions (mole fraction x 0.2) and the whole O–H band eventually vanishes with
increasing x. This behavior posed confusing interpretations for the surface structure.
This system poses an additional challenge in interpretation besides the strong
acid solutions discussed above. Sulfuric acid undergoes two-step ionization:
H 2 SO 4
pKa 1
← − → HSO
−
4
pKa 2
← − → SO
2−
4 .
(9.1)
The first ionization is considered to be strong enough (pKa 1 0), whereas the
second ionization (pKa 2 1.9 [20]) is not as strong as the first one. In such cases
that the acid dissociation is not complete, the local composition of the ion species
at the interface could be different from that in the bulk, since the interface is a less
polar environment than the bulk. The ion dissociation at interface may be sensitive
to the depth or other conditions, and coupled to the interface structure and SFG
spectra. These uncertainties make such surfaces challenging to understand. The
collaboration of SFG spectra and MD analysis can provide valuable information
on the surface structure and ion composition.
Since we could not determine the local ion composition a priori, we assumed
typical cases (a–c) about acid dissociation (9.1), where
(a) Same pKa 1,2 in Eq. (9.1) holds for surface as well, (HSO
−
4 dominant, SO
2−
4
present)
(b) Second ionization is suppressed at surface (pKa 2 0), (no SO
2−
4 )
(c) First ionization is also suppressed (pKa 1 0), (only neutral H 2 SO 4 )
and predicted the SFG spectra in each case by MD simulation. Figure 9.8 displays
the MD results in comparison with the experiments [2, 33] to see whether the
experimental features in O–H and S–O bands are consistently reproduced by MD
predictions. We first find that case (a) would lead to excessively strong hydrogen
bonding O–H signal, arguably because the presence of divalent anions (SO
2−
4 )
too much augments the electric double layer. Therefore, case (a) is not likely to
be the proper case. On the other hand, case (c) shows the main S–O band at
1150 cm −1 in the SSP spectrum, which is assigned to the neutral H 2 SO 4 , while
the experimental main S–O band as well as cases (a) and (b) arises at 1050 cm −1
from HSO
−
4 . Therefore, case (c) is not the proper case either. The combination of
SFG measurement and MD simulation thereby conclude that the first dissociation is
facile at the surface, whereas the second dissociation to form SO
2−
4 is substantially
more suppressed at the surface than in the bulk [30, 33, 53].
9.3.4 Base
The surface preference of hydroxide (OH
− ) anion remains a controversial and active
issue both theoretically and experimentally [1]. Previous MD simulation studies
reported different conclusions about its surface preference, which arguably depend
on the methods and conditions. Direct experimental measurement of surface OH
−