9.3 Electrolyte Solution Surfaces
231
0
3000 3200 3400 3600 3800
SFG Intensity [arb. unit]
Frequency [cm -1 ]
0
(MD)
pure water
NaF
(Exp.)
0
3000 3200 3400 3600 3800
Frequency [cm -1 ]
0
(MD)
pure water
Na 2 SO 4
(Exp.)
−15
−10
−5
0
5
10
0
0.5
1
1.5
z [Å]
(z) / bulk
H 2 O
Na
+
F
−
−15
−10
−5
0
5
10
0
0.5
1
1.5
(z) / bulk
H 2 O
Na
+
SO 4
2−
−15
−10
−5
0
5
10
−0.1
0
0.1
< cos >
pure water
Na 2 SO 4
NaF
(A) SFG
(B) Density profiles
(C) Water orientation
NaF
NaF
Na SO
2
4
Na SO
2
4
^
Fig. 9.6 (a) SFG spectra of 0.016 mole fraction (≈0.9 M) NaF (blue) and 1M Na 2 SO 4 (red)
solutions in comparison with that of pure water (black). Upper panels are calculated results [23],
while lower panels are experimental [17, 49]. (b) Density profiles of ions and water along the depth
coordinate ˆ
z. The density of each species is normalized with that in the bulk. (c) cos θ profiles
of water orientation for pure water (gray), Na 2 SO 4 (red) and NaF solutions (blue). The definition
of the tilt angle θ is illustrated in the inset. (Reprinted with the permission from Refs. [17, 23, 49].
Copyright 2012, 2004, 2005 American Chemical Society)
solution spectra are noticeably different. NaF little perturbs the SFG spectrum of
neat water (black), whereas Na 2 SO 4 obviously enhances the SFG intensity. These
differences illustrate that specific effects on the surface are present among such
buried ions. The reason of the spectral difference is understood as follows.