102
S. Roy et al.
the height of a liquid sample is adjusted with the reference quartz using high precision optical displacement sensor (resolution ∼ 100 nm) coupled with the motorized
sample stage (z-translational stage).
Classical- and HD-VSFG techniques can directly measure liquid surfaces without
much of a sample preparation. For instance, to measure water surface such as the pristine air-water or air-aqueous solution interfaces, ~5 mL Milli-Q water (18.2 M cm
resistivity, pH = 5.8) or aqueous solution of electrolytes/solutes of desired concentration is taken in a Petri dish of ~4 cm diameter and placed on the sample stage
as shown in Figs. 4 and 5. For long chain alcohol/lipid monolayer-water interfaces,
stock solution of the alcohol/lipid is prepared in chloroform; ~10 μL of the stock
is spread on the water in the Petri dish, while monitoring its surface tension. On
evaporation of the chloroform at ambient condition, the self-assembled monolayer
of alcohol/lipid is formed on water surface. The phase of the monolayer or the degree
of adsorption of a solute is reflected by the surface tension values measured in situ.
5 Applications
5.1 Air-Water Interface: Does “Ice-like” Water Exist
at the Air-Water Interface?
The pristine air-water interface is one of the most extensively investigated soft interface due to the omnipresence of water and its vital roles in innumerable physicochemical processes. Often, the pristine air-water interface is considered as the bench
mark in understanding the effect of ions, molecules and surfactants at aqueous interface. The OH stretch spectrum of water at the air-water interface, first measured by
Shen and coworkers [19] using a narrowband classical-VSFG spectrometer (Fig. 7a),
shows three distinct features of interfacial water: The red region of the SFG-intensity
spectrum shows a prominent band around 3250 cm
−1 followed by a stronger band
around 3450 cm
−1 ; while the extreme blue region shows a sharp band centered around
3700 cm
−1 . These spectral features have been confirmed by numerous groups subsequently [20–24]. The sharp band at 3700 cm
−1 and the strong shoulder at 3250 cm
−1
are interface-selective features of water. Based on the frequency position of these
bands, the 3250 cm
−1 band was assigned to tetrahedrally coordinated “ice-like” structure of water in analogy with the tetrahedrally coordinated OH stretch band of ice,
which appears dominantly around 3200 cm
−1 . The 3700 cm
−1 band is assigned to
the non-hydrogen bonded water O–H or free O–H protruding towards the air, specifically known as ‘dangling OH’ [19]. The 3450 cm
−1 band in analogy with liquid
water is attributed to the “liquid-like” structure of water at the air-water interface.
Later, the Imχ
(2) spectrum of the air-water interface was measured by Shen and
coworkers [3, 25] with narrowband phase-sensitive-VSFG as well as by Tahara and
coworkers [4] with broadband HD-VSFG spectroscopy. The Imχ
(2) spectrum showed
a sharp positive band around 3700 cm
−1 and a negative band within 3250–3600 cm
−1 ,
S. Roy et al.
the height of a liquid sample is adjusted with the reference quartz using high precision optical displacement sensor (resolution ∼ 100 nm) coupled with the motorized
sample stage (z-translational stage).
Classical- and HD-VSFG techniques can directly measure liquid surfaces without
much of a sample preparation. For instance, to measure water surface such as the pristine air-water or air-aqueous solution interfaces, ~5 mL Milli-Q water (18.2 M cm
resistivity, pH = 5.8) or aqueous solution of electrolytes/solutes of desired concentration is taken in a Petri dish of ~4 cm diameter and placed on the sample stage
as shown in Figs. 4 and 5. For long chain alcohol/lipid monolayer-water interfaces,
stock solution of the alcohol/lipid is prepared in chloroform; ~10 μL of the stock
is spread on the water in the Petri dish, while monitoring its surface tension. On
evaporation of the chloroform at ambient condition, the self-assembled monolayer
of alcohol/lipid is formed on water surface. The phase of the monolayer or the degree
of adsorption of a solute is reflected by the surface tension values measured in situ.
5 Applications
5.1 Air-Water Interface: Does “Ice-like” Water Exist
at the Air-Water Interface?
The pristine air-water interface is one of the most extensively investigated soft interface due to the omnipresence of water and its vital roles in innumerable physicochemical processes. Often, the pristine air-water interface is considered as the bench
mark in understanding the effect of ions, molecules and surfactants at aqueous interface. The OH stretch spectrum of water at the air-water interface, first measured by
Shen and coworkers [19] using a narrowband classical-VSFG spectrometer (Fig. 7a),
shows three distinct features of interfacial water: The red region of the SFG-intensity
spectrum shows a prominent band around 3250 cm
−1 followed by a stronger band
around 3450 cm
−1 ; while the extreme blue region shows a sharp band centered around
3700 cm
−1 . These spectral features have been confirmed by numerous groups subsequently [20–24]. The sharp band at 3700 cm
−1 and the strong shoulder at 3250 cm
−1
are interface-selective features of water. Based on the frequency position of these
bands, the 3250 cm
−1 band was assigned to tetrahedrally coordinated “ice-like” structure of water in analogy with the tetrahedrally coordinated OH stretch band of ice,
which appears dominantly around 3200 cm
−1 . The 3700 cm
−1 band is assigned to
the non-hydrogen bonded water O–H or free O–H protruding towards the air, specifically known as ‘dangling OH’ [19]. The 3450 cm
−1 band in analogy with liquid
water is attributed to the “liquid-like” structure of water at the air-water interface.
Later, the Imχ
(2) spectrum of the air-water interface was measured by Shen and
coworkers [3, 25] with narrowband phase-sensitive-VSFG as well as by Tahara and
coworkers [4] with broadband HD-VSFG spectroscopy. The Imχ
(2) spectrum showed
a sharp positive band around 3700 cm
−1 and a negative band within 3250–3600 cm
−1 ,