University of Groningen
Femtosecond vibrational dynamics in water nano-droplets
Cringus, Gheorghe Dan
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2008
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Cringus, G. D. (2008). Femtosecond vibrational dynamics in water nano-droplets s.n.
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Summary
Summary
Water is probably the most researched substance on Earth. The interest in water, and
predominantly in liquid water, is due to its importance on both macro- and microscopic
scales. Although people have been trying to understand water for centuries, this ubiquitous
liquid is still surrounded by mystery and has never stopped intriguing us. Even nowadays,
we are sometimes powerless in front of large floods provoked by tsunamis or hurricanes.
Water is even more mysterious on a microscopic scale. What differentiates water from
other substances with a similar molecular structure is its three-dimensional network of
intermolecular hydrogen bonds. In one way or another, the hydrogen bond network is
responsible for most of the unusual properties of water, such as a maximum density at
~ 4 OC, a surprisingly high specific heat, a high melting point and a high boiling point, etc.
In biological systems, water often occupies tiny volumes and consequently its hydrogen
bond network is spatially limited. As a result, water that is spatially confined on a
nanometer scale is expected to display different properties than bulk water and has
therefore generated a lot of scientific interest.
One of the most important attributes of the hydrogen bond network of water is its
highly dynamic character, with bonds being made and broken in less then one millionth of
a millionth of a second. To date, the only technique which allows experimental access to
such short time scales is ultrafast laser spectroscopy. State-of-the-art laser systems can
produce extremely short pulses of infrared light which are used to reveal the dynamics of
this hydrogen bond network. In the simplest experiment, known as pump-probe, a laser
pulse initially excites the sample and subsequently a delayed pulse monitors the evolution
of the system towards the equilibrium.
A home-built laser system providing sub-100 fs pulses (1 fs = 10-15 sec) with a central
wavelength tunable around 3 microns (mid-infrared) was the primary tool used for the
experiments presented in this thesis. Our focus was on the ultrafast dynamics of liquid
water, and particularly on the properties of water in spatially restricted (confined)
environments. Since real biological systems are at the moment too complex for such a
study, we focused on several model systems which mimic geometrically confined water.
Another simplification employed in some of our experiments is the use of deuterated water
HDO, where D stands for deuterium, as a substitute for the common H2O molecule. In this
case, the coupling between the two OH oscillators of the same molecule is suppressed,
while the hydrogen bonding properties remain practically identical to the H2O. Each model
system presents certain advantages and allows understanding different aspects of water.
The introductory chapter of this thesis presents an overview of the most relevant
properties of water. The consequences of confining water to nanometer-size droplets are
also briefly discussed. Subsequently, our experimental approach is presented and the
benefits of employing the modern techniques of ultrafast infrared spectroscopy are
outlined. The principle of the method used for generating ultrashort laser pulses is then
discussed, together with a few practical details. Finally, the model systems used throughout
the thesis are listed and the advantages and limitations of each of them are explained.
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Summary
In Chapter 2 we investigate liquid water dynamics from the single molecule level to
the bulk water hydrogen bond network by mixing water with acetonitrile. The water-water
hydrogen bonds begin to play an important role even for a molar water fraction as low as
10 %. Already at this concentration, the water molecules begin to group together and the
solution becomes heterogeneous on a microscopic scale, consisting of water nanoclusters
surrounded by solvent. These clusters grow at higher concentrations, and finally the
situation reverses and the aqueous phase becomes dominant, with acetonitrile only filling
the voids in the hydrogen bond network. In order to avoid the complications related to the
intramolecular coupling of the two OH groups of a H2O molecule, we performed these
experiments on isotopically substituted water (HDO in D2O). The water clusters appeared
to consist of two distinct regions, which display different dynamics and energy transfer
properties. On the one hand, the water layer at the interface with the solvent shows the
same spectro-temporal signatures as isolated molecules in acetonitrile. This indicates that
water hydrogen bond network is severely disturbed in the interfacial layer. The core of the
water clusters on the other hand closely resembles bulk water, suggesting that in the
interior of the clusters the hydrogen bond network is quite well established. Still, we
observed some variations with concentration, which hint at longer range confinement
effects, as opposed to purely interfacial effects at the outer shell. Quite surprisingly, the
rotational diffusion times of interfacial and core water molecules are practically identical,
although the hydrogen bonding environment is completely different in the two cases. This
suggests that, although the hydrogen bonds are stronger in the droplet core, they do not
determine the rotational diffusion because each bond exists only for a short time.
Based on the knowledge accumulated by studying this HDO-based model system, we
then studied samples containing isotopically pure H2O. The next step in our investigation
was aimed at providing a better understanding of the ultrafast phenomena that occur on a
molecular level in H2O. In order to insure that the H2O molecules do not interact with each
other, our samples consisted of H2O dissolved in acetonitrile at very low concentration. The
same pump-probe technique used in Chapter 2 was also employed here. These experiments
revealed new sub-picosecond dynamics, which are specific to H2O and had not been
observed for isolated HDO molecules. In particular, the surprisingly fast anisotropy decay
suggests that the two OH oscillators of the H2O molecule are anharmonically coupled.
For a better understanding of this system, we also performed two dimensional
correlation experiments on H2O molecules dissolved in acetonitrile. This technique is more
powerful then the pump-probe and yields a time-dependent correlation map between the
excitation frequency and the system response. However, such experiments are more
difficult to execute, as they involve double the number of laser pulses (i.e. two pairs)
compared to pump-probe. In addition, interferometric stability between the pulses from
each pair is required. This means that the optical paths of the laser beams should be
controlled at all the times with a precision of less then 10 microns, which compares to a
tenth of the diameter of a human hair. The evolution of the correlation maps obtained in
this way confirmed the anharmonic coupling between the OH oscillators and directly
revealed an ultrafast vibrational energy exchange within a single H2O molecule.
Reverse micelles represent another convenient model system, which mimics the water
nano-pools encountered in some biological settings such as, for instance, protein pockets.
142
Summary
Easily made by mixing water, oil and soap, reverse micelles consist of quasi-spherical
water droplets covered by a soap membrane and floating in the oil phase. An important
advantage offered by this system is that the diameter of the enclosed water droplets can be
adjusted by varying the relative water/soap ratio. Chapter 5 presents a thorough
investigation of the ultrafast dynamics of H2O trapped in reverse micelles. Time and
frequency resolved data were collected for different water droplet sizes, ranging from 1 to
10 microns diameter. The formed droplets were shown to contain two types of water, with
different vibrational relaxation pathways and structure of the hydrogen bond network.
Thus, while the core of the water droplet is very similar to bulk water, the water layer at the
interface with the soap membrane has distinctly different properties. First, the rotational
dynamics of the interfacial molecules are much slower than in bulk water. Second, in
contrast to bulk water, where the vibrational energy is quickly delocalized over several
molecules, in interfacial water the excess energy deposited with the pump laser pulse
appears to remain localized for much longer times.
To recapitulate: In this thesis we have studied the ultrafast dynamics of liquid water
confined on a nanometer scale. Two model systems have been used, namely mixtures of
water and acetonitrile and reverse micelles. We have shown that the water clusters consist
of two distinct regions, which are characterized by different dynamics, structure of the
hydrogen bond network and pathways for vibrational energy relaxation. Water from the
center of the droplet is almost identical to bulk water, especially in the case of H2O in
reverse micelles. Still, for the HDO in D2O/acetonitrile samples, the observed variation of
the vibrational lifetime of the core water suggests the presence of nanoconfinement effects
beyond the interfacial layer. In the outer 1-2 molecular layers of a water cluster, the
dynamics and energy transfer processes are mainly determined by the interaction of the
interfacial water molecules with the neighboring non-aqueous phase (acetonitrile and
respectively micelle membrane). Consequently, the excess vibrational energy is dissipated
through a different channel then in bulk water. In addition, the water-water interactions are
considerably weaker in the interfacial layer and the intermolecular transfer of vibrational
energy is blocked. This result opens a new perspective on the role of water in biological
environments, as it suggests that in the vicinity of a biological interface, for instance near a
protein, water cannot transfer the energy as efficiently as in bulk phase.
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