"Water Dynamics in Nanoconfined Systems and Interacting with Ions Studied with Ultrafast 2D IR Vibrational Echo and IR Pump-Probe Experiments"
Presented by Prof. Michael D. Fayer, Stanford University
In
chemical, biological, and materials systems, water is frequently found
in nanoscopically confined environments, interacting with interfaces and
ions, rather than as the pure bulk liquid. Water’s unique properties,
such as its ability to solvate ions or accommodate protein folding,
result form its extended hydrogen bonding network and hydrogen bond
dynamics, which are profoundly influenced by nanoconfinement,
interfaces, and ions. Ultrafast 2D IR vibrational echo experiments and
IR polarization and frequency selective pump-probe experiments on the
hydroxyl stretching mode are employed to investigate water dynamics.
Reverse micelles are used to control the size of water nanopools down to
as few as 40 water molecules. Lamellar structures are also
investigated. Comparisons are made between charged and uncharged
interfaces. Large reverse micelles have a bulk-like water core and
interfacial water. Bulk water undergoes orientational relaxation in 2.6
ps. In contrast, water at the interface of large reverse micelles has
an orientational relaxation time of 18 ps. In small reverse micelles,
the interface influences all of the water molecules, and there is no
division into core and interfacial water. The 2D IR vibrational echo
experiments determine the frequency-frequency correlation function
(FFCF) for the hydroxyl stretch (spectral diffusion). The FFCF is
controlled by the structural evolution of the water system. In small
reverse micelles, it is found that the decay of the FFCF is much faster
than the orientational relaxation measured with IR pump-probe
experiments. It is suggested that shape and size fluctuations of the
water nanodroplets contribute to the structural dynamics observed by the
vibrational echoes. To obtain a detailed understanding of the affect
of ions, 2D IR vibrational echo chemical exchange experiments are used
to directly observe hydrogen bond switching between water bound to ions
and water bound to other water molecules.
Hosted by Prof. Ken Eisenthal
Thursday, October 7, 2010
Meet the Speaker at 1:00pm in The Miller Seminar Room, 328 Havemeyer