9.5 Water at Monolayers
239
The MD simulation of the SFG spectra was performed [34], which well
reproduced the distinct SFG spectra of CCl 4 /water and DCE/water interfaces.
The MD simulation allows for direction observation of interfacial water structure.
However, the MD showed rather similar structure of interfacial water at CCl 4 and
DCE in terms of the density profile or orientation of water molecules, irrespective
of the apparently distinct SFG spectra reproduced. The mechanism of the noticeable
spectral differences becomes clearer in the calculated Im[χ (2) ] spectra in Fig. 9.11b.
Comparing the Im[χ (2) ] spectra of the two oil/water interfaces, we find that the
spectra in the H-bonding region below 3600 cm −1 not much different, which is
consistent to the direct MD observation that the density and orientation of surface
water are analogous at CCl 4 /water and DCE/water interfaces. However, we find that
the free O–H band in 3600–3700 cm −1 is particularly suppressed in the DCE/water
interface.
The perturbation on the free O–H band is understood in the following manner.
The free O–H of water at oil/water interface actually interacts with adjacent oil
molecules, as illustrated in Fig. 9.11c. The “free” O–H at DCE/water interface is
more perturbed than that at CCl 4 /water because of larger polarity of DCE than
CCl 4 . As a consequence, the positive Im[χ (2) ] band of “free” O–H is substantially
red shifted and broadened for the DCE/water interface, as evidenced in Fig. 9.11d.
The red-shifted free O–H band of DCE/water interface overlaps with the negative
Im[χ (2) ] band of the H-bonding O–H, and cancel the intensity. The apparent spectral
difference between CCl 4 /water and DCE/water interfaces is attributed to the local
interaction of water and oil molecules at the interfaces, rather than qualitatively
distinct structure of molecular orientation.
9.5 Water at Monolayers
Amphiphilic molecules tend to form various self assembled structures in/on water,
such as Langmuir monolayer, micelle and lipid bilayers. Such structures generally
include interfaces of water and amphiphilic molecules, and their interfaces govern
the stability of these structures. The interfaces of phospholipid membranes have
been drawing particular attention by SFG spectroscopy [41], as the lipid membranes
define the boundary of cells, control mass transport, and thereby play vital roles of
living functions [7, 63]. A number of MD studies in relation to the SFG spectroscopy
have been performed to aim at selective detection and understanding of water structure in contact with those amphiphilic monolayers [37, 38, 59, 69, 86, 87, 90, 91].
One of the basic concepts of the water structure is the flip-flop model of water
orientation in Fig. 9.12. The orientational structure of water molecules is determined
by the net charges of the monolayer molecules. When the monolayer molecules are
negatively charged, such as sodium dodecyl sulfate (SDS, C 12 H 25 SO
−
4 · Na
+ ), the
water molecules take upward orientation and leads to positive Im[χ (2) ] band. On the
other hand, if the monolayer is positively charged, such as cetyltrimethylammonium
bromide (CTAB, C 16 H 33 N + (CH 3 ) 3 · Br
− ), the water takes downward orientation
239
The MD simulation of the SFG spectra was performed [34], which well
reproduced the distinct SFG spectra of CCl 4 /water and DCE/water interfaces.
The MD simulation allows for direction observation of interfacial water structure.
However, the MD showed rather similar structure of interfacial water at CCl 4 and
DCE in terms of the density profile or orientation of water molecules, irrespective
of the apparently distinct SFG spectra reproduced. The mechanism of the noticeable
spectral differences becomes clearer in the calculated Im[χ (2) ] spectra in Fig. 9.11b.
Comparing the Im[χ (2) ] spectra of the two oil/water interfaces, we find that the
spectra in the H-bonding region below 3600 cm −1 not much different, which is
consistent to the direct MD observation that the density and orientation of surface
water are analogous at CCl 4 /water and DCE/water interfaces. However, we find that
the free O–H band in 3600–3700 cm −1 is particularly suppressed in the DCE/water
interface.
The perturbation on the free O–H band is understood in the following manner.
The free O–H of water at oil/water interface actually interacts with adjacent oil
molecules, as illustrated in Fig. 9.11c. The “free” O–H at DCE/water interface is
more perturbed than that at CCl 4 /water because of larger polarity of DCE than
CCl 4 . As a consequence, the positive Im[χ (2) ] band of “free” O–H is substantially
red shifted and broadened for the DCE/water interface, as evidenced in Fig. 9.11d.
The red-shifted free O–H band of DCE/water interface overlaps with the negative
Im[χ (2) ] band of the H-bonding O–H, and cancel the intensity. The apparent spectral
difference between CCl 4 /water and DCE/water interfaces is attributed to the local
interaction of water and oil molecules at the interfaces, rather than qualitatively
distinct structure of molecular orientation.
9.5 Water at Monolayers
Amphiphilic molecules tend to form various self assembled structures in/on water,
such as Langmuir monolayer, micelle and lipid bilayers. Such structures generally
include interfaces of water and amphiphilic molecules, and their interfaces govern
the stability of these structures. The interfaces of phospholipid membranes have
been drawing particular attention by SFG spectroscopy [41], as the lipid membranes
define the boundary of cells, control mass transport, and thereby play vital roles of
living functions [7, 63]. A number of MD studies in relation to the SFG spectroscopy
have been performed to aim at selective detection and understanding of water structure in contact with those amphiphilic monolayers [37, 38, 59, 69, 86, 87, 90, 91].
One of the basic concepts of the water structure is the flip-flop model of water
orientation in Fig. 9.12. The orientational structure of water molecules is determined
by the net charges of the monolayer molecules. When the monolayer molecules are
negatively charged, such as sodium dodecyl sulfate (SDS, C 12 H 25 SO
−
4 · Na
+ ), the
water molecules take upward orientation and leads to positive Im[χ (2) ] band. On the
other hand, if the monolayer is positively charged, such as cetyltrimethylammonium
bromide (CTAB, C 16 H 33 N + (CH 3 ) 3 · Br
− ), the water takes downward orientation