232
9 Applications: Aqueous Interfaces
Panels (b) and (c) show the calculated density profiles and water orientation,
respectively, of the NaF and Na 2 SO 4 solution surfaces. The density profiles in
(b) confirm that all the present ions of Na + , F − and SO
2−
4 are repelled from
the topmost layer of the surface. By comparing the density profiles of NaF and
Na 2 SO 4 solutions in panel (b), we find that the NaF solution (left) exhibits almost
overlapping profiles of Na + and F − ions, whereas the Na 2 SO 4 solution (right)
shows slight but significant difference in the Na + and SO
2−
4 profiles. In the Na 2 SO 4
solution, SO
2−
4 is more repelled from the surface than Na + , arguably because SO
2−
4
is a divalent ion. Consequently, charge separation between Na + and SO
2−
4 generates
an electric double layer in a deep region by a few monolayers from the surface
(ˆ z ∼ −5 Å).
The effect of electric double layer in the Na 2 SO 4 solution is manifested in the
orientational profile of water in panel (c). This panel displays the cos θ profile as
a function of the depth coordinate ˆ
z, where θ is the tilt angle of the water dipole
from the surface normal. Near the Gibbs dividing surface of water (ˆ z ≈ 0 Å), all
the cos θ profiles of pure water (black), Na 2 SO 4 solution (red), and NaF solution
(blue) show negative and nearly identical shapes. This feature means that the water
orientation of the top layer is little perturbed by the buried ions. However, we see a
negative cos θ region in a deeper region ˆ
z ≈ −5 ∼ −10 Å for the Na 2 SO 4 solution
(red dashed). This feature is a consequence of the electric double layer of Na + and
SO
2−
4 formed in that region. Further MD analysis confirmed that the enhanced SFG
intensity in the Na 2 SO 4 solution originates from the perturbed water orientation of
the negative cos θ in that deep region, and the perturbed Im[χ (2) ] band of Na 2 SO 4
solution was confirmed by the heterodyne detected SFG measurement [107].
In summary, the SFG signals of water originate from the surface region where
the isotropic orientation is broken, and the electric double layer formation of
electrolytes is a typical cause to perturb the water orientation. The above cases
of NaF and Na 2 SO 4 solutions exemplify that the SFG spectroscopy is sensitive to
the perturbed water orientation induced by slight charge separation, even when the
charge separation arises from a somewhat deep region from the topmost layer. The
perturbed SFG spectra of electrolyte solutions may reflect the structural change in a
deeper region than the topmost layer of the water surface.
9.3.3 Acid
In the surface of acid solutions, the excess hydronium (H 3 O + ) cations preferentially
reside on the topmost surface of water. This microscopic behavior was predicted
by MD simulation [58, 78]. It has been long known experimentally that the
surface tension of acid solutions becomes smaller than that of neat water [84],
implying a positive surface excess from the thermodynamic view. The relation to
the microscopic surface structure of acid solutions and their SFG spectra is analyzed
in the following.
9 Applications: Aqueous Interfaces
Panels (b) and (c) show the calculated density profiles and water orientation,
respectively, of the NaF and Na 2 SO 4 solution surfaces. The density profiles in
(b) confirm that all the present ions of Na + , F − and SO
2−
4 are repelled from
the topmost layer of the surface. By comparing the density profiles of NaF and
Na 2 SO 4 solutions in panel (b), we find that the NaF solution (left) exhibits almost
overlapping profiles of Na + and F − ions, whereas the Na 2 SO 4 solution (right)
shows slight but significant difference in the Na + and SO
2−
4 profiles. In the Na 2 SO 4
solution, SO
2−
4 is more repelled from the surface than Na + , arguably because SO
2−
4
is a divalent ion. Consequently, charge separation between Na + and SO
2−
4 generates
an electric double layer in a deep region by a few monolayers from the surface
(ˆ z ∼ −5 Å).
The effect of electric double layer in the Na 2 SO 4 solution is manifested in the
orientational profile of water in panel (c). This panel displays the cos θ profile as
a function of the depth coordinate ˆ
z, where θ is the tilt angle of the water dipole
from the surface normal. Near the Gibbs dividing surface of water (ˆ z ≈ 0 Å), all
the cos θ profiles of pure water (black), Na 2 SO 4 solution (red), and NaF solution
(blue) show negative and nearly identical shapes. This feature means that the water
orientation of the top layer is little perturbed by the buried ions. However, we see a
negative cos θ region in a deeper region ˆ
z ≈ −5 ∼ −10 Å for the Na 2 SO 4 solution
(red dashed). This feature is a consequence of the electric double layer of Na + and
SO
2−
4 formed in that region. Further MD analysis confirmed that the enhanced SFG
intensity in the Na 2 SO 4 solution originates from the perturbed water orientation of
the negative cos θ in that deep region, and the perturbed Im[χ (2) ] band of Na 2 SO 4
solution was confirmed by the heterodyne detected SFG measurement [107].
In summary, the SFG signals of water originate from the surface region where
the isotropic orientation is broken, and the electric double layer formation of
electrolytes is a typical cause to perturb the water orientation. The above cases
of NaF and Na 2 SO 4 solutions exemplify that the SFG spectroscopy is sensitive to
the perturbed water orientation induced by slight charge separation, even when the
charge separation arises from a somewhat deep region from the topmost layer. The
perturbed SFG spectra of electrolyte solutions may reflect the structural change in a
deeper region than the topmost layer of the water surface.
9.3.3 Acid
In the surface of acid solutions, the excess hydronium (H 3 O + ) cations preferentially
reside on the topmost surface of water. This microscopic behavior was predicted
by MD simulation [58, 78]. It has been long known experimentally that the
surface tension of acid solutions becomes smaller than that of neat water [84],
implying a positive surface excess from the thermodynamic view. The relation to
the microscopic surface structure of acid solutions and their SFG spectra is analyzed
in the following.