A ‘snapshot’ from a molecular dynamic simulation reveals that water molecules align at air–water interfaces as coordinated pairs linked by hydrogen bonds. Image: RIKEN |
Water
(H2O) has a simple composition, but its dizzyingly interconnected
hydrogen-bonded networks make structural characterizations challenging.
In particular, the organization of water surfaces—a region critical to
processes in cell biology and atmospheric chemistry—has caused profound
disagreements among scientists. Now, Tahei Tahara and colleagues from
the RIKEN Advanced Science Institute in Wako, in collaboration with
researchers in Japan and Europe, have uncovered the presence of strongly
bonded water pairs at the air–water interface, rather than previously
hypothesized ‘ice-like’ surface structures.
Observing
surface water molecules, just a few monolayers thick, requires special
experimental techniques that prevent interference by more plentiful bulk
particles. One such approach is called vibrational sum frequency
generation (VSFG), a laser-based method that selectively vibrates
interfacial molecules. Previous VSFG measurements of surface water
showed two vibrations that resemble signals recorded from bulk ice and
liquid water states. Some scientists have proposed that these vibrations
correspond to a partially disordered mix of liquid and four-coordinated
ice-like surface structures—a theory at odds with thermodynamic
evidence.
Other
VSFG experiments, however, have suggested that the two vibrations arise
from one structure undergoing coupling interactions. To resolve this
dispute, Tahara and colleagues turned to heterodyne-detected VSFG
(HD-VSFG), a high-level spectroscopic method that detects how the phase
of the vibrational signals shifts with respect to the incident laser
beam—information that can pinpoint molecular orientation at interfaces.
The
researchers then employed a trick using isotopes to account for
coupling effects of water molecules: they added the deuterium
(D)-bearing compounds HOD and D2O to pure water. By gradually diluting
the number of oxygen–hydrogen (OH) bonds in the liquid, these isotopes
suppress the interactions between the vibrational modes that normally
occur. The remaining ‘stretching’ vibrations that extend and contract OH
bonds then provide clear information about the interfacial water
structure.
The
team’s experiments revealed that as the isotopic dilution progressed,
the two OH bands merged into a single peak, which is clear evidence of
vibrational coupling within a single structure. After performing
molecular dynamic simulations and comparing the results to the HD-VSFG
data, a new picture emerged of the air–water interface: the
low-frequency OH vibrations were due to tightly joined pairs of liquid
water molecules.
“We
were wondering what kind of structure can have strong hydrogen-bonds
other than ice at water surfaces,” says Tahara. “When our experiments
and [co-author] Morita’s simulation answered the question, rather than
surprise I felt that ‘This is it!’ because its structure is quite
reasonable.”
