orientations with H-atoms pointing towards the surface (up). While the frequency
and width of both features is similar, which indicates similar strengths of hydrogen
bonding, a close analysis of the CLS from 2D SFG spectra at a series of population
delays revealed that the hydrogen bond dynamics in the HOD molecules near the
charged interfaces drastically depend on the surface charge (Fig. 14b and c). That is,
initial CLS values (0.8 for DPPG and 0.3 for DPTAP) strongly depend on the
surface charge, but both exponentially decay with very similar time constants of
about 750 fs [198]. The strong difference in the initial CLS values was argued to
result from a hidden underlying process of spectral diffusion only in case of
positively charged DPTAP, which is too fast to be observed with the given temporal
resolution (* 200 fs). Such presumably sub-100 fs dynamics were argued to be
similar to dynamics that have been observed in the bulk phase of water [203]. As a
consequence, it was inferred that in case of negative surface-charge, the water
molecules form hydrogen bonds to the charged phosphate headgroup of DPPG,
which inhibit the bulk-like behavior in the ultrafast spectral diffusion. Such
hydrogen bond acceptor sites do not exist in the case of DPTAP, which exhibits a
positively charged ammonium headgroup. The observed orientation and dynamics
of water molecules can in principle have a strong impact in chemistry and biology.
Many lipid membranes are surrounded by water molecules and the hydrogen
bonding, as well as the orientation might have a profound influence on the
properties of e.g. cell membranes.
Ultrafast dynamics of molecules at liquid-gas interfaces is currently a strongly
active field of research, and 2D SFG spectroscopy has been used to reveal further
detailed understanding of the mechanisms, which dictate the underlying molecular
dynamics. Other negatively charged surfactants have been used at liquid-gas
interfaces to identify different types of water molecules at the interface by the
observation of distinct vibrational bands in the OD stretch region [199]. The isolated
bands have been argued to originate from localized vibrations near the surfactant
and delocalized modes, which are shared between different OD-bonds, respectively.
Resolving the energy transfer between the sub-ensembles via cross peak dynamics,
it could be proven that the different vibrations strongly interact. The prominence of
energy transfer at water-gas interfaces was highlighted even earlier with 2D SFG
studies that employed homodyne detection [182, 195]. Such experiments addressed
ultrafast energy transfer between water molecules at uncharged water-air interfaces.
Using isotope-dilution, it was determined that intermolecular energy transfer is the
process, which overall dominates the dynamics of spectral diffusion at the interface.
The slowdown of spectral diffusion at the interface as compared to water molecules
in bulk solution environments was discussed to stem from the reduced number of
acceptor molecules at the interface, which alters the probability of energy transfer.
The strong contributions of energy transfer that dominate the ultrafast dynamics
of small molecules at liquid-gas interfaces are remarkable if compared to the
missing energy transfer between small molecules [204, 205] (e.g. CO and CN
- ) at
ultrathin (few nanometers) metal layers and also to much bulkier and much more
flexible monolayer molecules at solid-liquid and solid-gas interfaces
[173, 177, 178, 180]. The observation of missing energy transfer in the latter
systems (e.g. Fig. 12) suggests that the distance between the headgroups of the
Top Curr Chem (Z) (2017) 375:86
123
151
Reprinted from the journal
and width of both features is similar, which indicates similar strengths of hydrogen
bonding, a close analysis of the CLS from 2D SFG spectra at a series of population
delays revealed that the hydrogen bond dynamics in the HOD molecules near the
charged interfaces drastically depend on the surface charge (Fig. 14b and c). That is,
initial CLS values (0.8 for DPPG and 0.3 for DPTAP) strongly depend on the
surface charge, but both exponentially decay with very similar time constants of
about 750 fs [198]. The strong difference in the initial CLS values was argued to
result from a hidden underlying process of spectral diffusion only in case of
positively charged DPTAP, which is too fast to be observed with the given temporal
resolution (* 200 fs). Such presumably sub-100 fs dynamics were argued to be
similar to dynamics that have been observed in the bulk phase of water [203]. As a
consequence, it was inferred that in case of negative surface-charge, the water
molecules form hydrogen bonds to the charged phosphate headgroup of DPPG,
which inhibit the bulk-like behavior in the ultrafast spectral diffusion. Such
hydrogen bond acceptor sites do not exist in the case of DPTAP, which exhibits a
positively charged ammonium headgroup. The observed orientation and dynamics
of water molecules can in principle have a strong impact in chemistry and biology.
Many lipid membranes are surrounded by water molecules and the hydrogen
bonding, as well as the orientation might have a profound influence on the
properties of e.g. cell membranes.
Ultrafast dynamics of molecules at liquid-gas interfaces is currently a strongly
active field of research, and 2D SFG spectroscopy has been used to reveal further
detailed understanding of the mechanisms, which dictate the underlying molecular
dynamics. Other negatively charged surfactants have been used at liquid-gas
interfaces to identify different types of water molecules at the interface by the
observation of distinct vibrational bands in the OD stretch region [199]. The isolated
bands have been argued to originate from localized vibrations near the surfactant
and delocalized modes, which are shared between different OD-bonds, respectively.
Resolving the energy transfer between the sub-ensembles via cross peak dynamics,
it could be proven that the different vibrations strongly interact. The prominence of
energy transfer at water-gas interfaces was highlighted even earlier with 2D SFG
studies that employed homodyne detection [182, 195]. Such experiments addressed
ultrafast energy transfer between water molecules at uncharged water-air interfaces.
Using isotope-dilution, it was determined that intermolecular energy transfer is the
process, which overall dominates the dynamics of spectral diffusion at the interface.
The slowdown of spectral diffusion at the interface as compared to water molecules
in bulk solution environments was discussed to stem from the reduced number of
acceptor molecules at the interface, which alters the probability of energy transfer.
The strong contributions of energy transfer that dominate the ultrafast dynamics
of small molecules at liquid-gas interfaces are remarkable if compared to the
missing energy transfer between small molecules [204, 205] (e.g. CO and CN
- ) at
ultrathin (few nanometers) metal layers and also to much bulkier and much more
flexible monolayer molecules at solid-liquid and solid-gas interfaces
[173, 177, 178, 180]. The observation of missing energy transfer in the latter
systems (e.g. Fig. 12) suggests that the distance between the headgroups of the
Top Curr Chem (Z) (2017) 375:86
123
151
Reprinted from the journal