104
S. Roy et al.
5.2 Air-Electrolyte Solution Interface: Surface Prevalence
of Structure-Making Anion
Surface prevalence and interfacial distribution of ions are of paramount importance
to interfacial chemistry which encompasses a wide range of fields including atmospheric aerosol to cellular process in biological systems. Previous studies, mostly
surface potential, surface tension, classical-VSFG, NAP-XPS measurements as well
as MD simulation revealed that weakly hydrated and highly polarizable anions such
as I
− and Br
− are adsorbed at the water surface and the counter-ion, e.g. Na
+ for NaI,
follows the anion [40–46]. Such non-uniform distribution of oppositely charged ions
is considered to create an electric double layer (EDL) which increase the orientational
order of water at the interface. On the other hand, strongly hydrated ions (structure
makers) are believed to be repelled from the water surface. However, the question
remains: how does an electrolyte with structure making ions (e.g., Na 2 SO 4 ) affects
the interfacial water? If a structure making anion (e.g., SO
2−
4 ) is repelled form the
surface, is it the counter ion (say, Na
+ ) that resides closer to the surface? When the
counter ion is also strongly hydrated (e.g., Mg
2+ ), what happens to the relative positioning of the cation and anion at the interface and how do they affect the H-bonding
and orientation of the interfacial water? Classical-VSFG measurement inadequately
answered these questions, because even if there is an increase in squared-χ
(2) intensity (OH stretch), it only reveals a non-uniform distribution of cation and anion at
the interface which may lead to increased ordering of the interfacial water. However,
it does not disclose whether the cation or the anion is closer to the water surface,
and whether the net orientation of the interfacial water is H-up or H-down. Recently,
(phase-sensitive) HD-VSFG measurements of structure making electrolytes solutions seem to suggest that the structure making anions such as SO
2−
4 and CO
2−
3 are
more strongly repelled from the interface than the mono and bivalent metal ions (Na
+
and Mg
2+ ) [47, 48]. Apart from the overall structure making/breaking nature of an
anion, the role of atomicity of the anion (e.g., monatomic F
− versus polyatomic IO
−
3 ;
both are structure maker but their atomicity are different) on its surface prevalence
and perturbation of interfacial water are important aspects of specific ion effect at
the interface.
Figure 8a shows the OH stretch Imχ
(2) spectra of the air-water interface in the
presence of IO
−
3 (0.3 M KIO 3 ) along with the spectrum of the pristine air-water interface. IO 3
– increases the negative Imχ
(2) signal in the OH stretch region, particularly
in the red region, 3000–3400 cm
−1 . The structure breaking monatomic anion, e.g.,
I
– (0.3 M CsI; effect of Cs
+ and K
+ ions are comparable), which is preferentially
adsorbed at the water surface shows an opposite change in the OH stretch band: the
negative signal around 3200 cm
−1 is reduced compared to neat air-water interface
with a negligible increase in negative signal around 3450 cm
−1 (Fig. 8b). Thus, at
low concentration of I
– (~0.3 M), the OH stretch band of interfacial water becomes
narrower (especially by the suppression around 3200 cm
−1 ) rather than an increase
in the OH stretch signal even at the peak. This observation suggests insignificant
change of the orientational order of interfacial water, though the structure (i.e., band
S. Roy et al.
5.2 Air-Electrolyte Solution Interface: Surface Prevalence
of Structure-Making Anion
Surface prevalence and interfacial distribution of ions are of paramount importance
to interfacial chemistry which encompasses a wide range of fields including atmospheric aerosol to cellular process in biological systems. Previous studies, mostly
surface potential, surface tension, classical-VSFG, NAP-XPS measurements as well
as MD simulation revealed that weakly hydrated and highly polarizable anions such
as I
− and Br
− are adsorbed at the water surface and the counter-ion, e.g. Na
+ for NaI,
follows the anion [40–46]. Such non-uniform distribution of oppositely charged ions
is considered to create an electric double layer (EDL) which increase the orientational
order of water at the interface. On the other hand, strongly hydrated ions (structure
makers) are believed to be repelled from the water surface. However, the question
remains: how does an electrolyte with structure making ions (e.g., Na 2 SO 4 ) affects
the interfacial water? If a structure making anion (e.g., SO
2−
4 ) is repelled form the
surface, is it the counter ion (say, Na
+ ) that resides closer to the surface? When the
counter ion is also strongly hydrated (e.g., Mg
2+ ), what happens to the relative positioning of the cation and anion at the interface and how do they affect the H-bonding
and orientation of the interfacial water? Classical-VSFG measurement inadequately
answered these questions, because even if there is an increase in squared-χ
(2) intensity (OH stretch), it only reveals a non-uniform distribution of cation and anion at
the interface which may lead to increased ordering of the interfacial water. However,
it does not disclose whether the cation or the anion is closer to the water surface,
and whether the net orientation of the interfacial water is H-up or H-down. Recently,
(phase-sensitive) HD-VSFG measurements of structure making electrolytes solutions seem to suggest that the structure making anions such as SO
2−
4 and CO
2−
3 are
more strongly repelled from the interface than the mono and bivalent metal ions (Na
+
and Mg
2+ ) [47, 48]. Apart from the overall structure making/breaking nature of an
anion, the role of atomicity of the anion (e.g., monatomic F
− versus polyatomic IO
−
3 ;
both are structure maker but their atomicity are different) on its surface prevalence
and perturbation of interfacial water are important aspects of specific ion effect at
the interface.
Figure 8a shows the OH stretch Imχ
(2) spectra of the air-water interface in the
presence of IO
−
3 (0.3 M KIO 3 ) along with the spectrum of the pristine air-water interface. IO 3
– increases the negative Imχ
(2) signal in the OH stretch region, particularly
in the red region, 3000–3400 cm
−1 . The structure breaking monatomic anion, e.g.,
I
– (0.3 M CsI; effect of Cs
+ and K
+ ions are comparable), which is preferentially
adsorbed at the water surface shows an opposite change in the OH stretch band: the
negative signal around 3200 cm
−1 is reduced compared to neat air-water interface
with a negligible increase in negative signal around 3450 cm
−1 (Fig. 8b). Thus, at
low concentration of I
– (~0.3 M), the OH stretch band of interfacial water becomes
narrower (especially by the suppression around 3200 cm
−1 ) rather than an increase
in the OH stretch signal even at the peak. This observation suggests insignificant
change of the orientational order of interfacial water, though the structure (i.e., band