Fayer Lab: Dynamics of Hydrogen Bonded Systems

Water Dynamics in Nanoscopic Environments
David Ben Spry , David Moilanen , and Ivan Piletic

Water and its properties have been studied extensively during this and the previous century. Peculiarities in the temperature dependence of the density of water as well as its boiling and melting points (among many other properties) have caused chemists to postulate the existence of weak interactions between these molecules that were eventually called hydrogen bonds. These interactions are weak enough to be able to dissociate and reform at room temperature in hydrogen bonded liquids. The timescale of these fluctuations are of fundamental importance since many physical and chemical properties of water can be attributed to the existence of a dynamic hydrogen bonded network. Only recently have these timescales been accessed using multidimensional ultrafast spectroscopic techniques. Many experiments have elucidated the behavior of bulk water and have revealed the complexity of the dynamics occurring on picosecond and femtosecond timescales.

A question that arises is to what extent are the dynamics of water affected by interactions with other molecules or functional groups. In particular, in many instances water molecules are found trapped in cavities so the influence of confinement on a nanometer length scale is an important issue that can be addressed using ultrafast infrared nonlinear spectroscopic techniques.
A model system for examining water in a confined medium is a reverse micelle. Figure 1 displays a schematic of a reverse micelle along with the surfactant molecules that are used (Aerosol OT or AOT). When appropriate amounts of water and surfactant are mixed with a nonpolar solvent (isooctane), an inverted micelle forms, encapsulating a nanoscopic pool of water in the middle. Many different experimental techniques predict that the size of a reverse micelle may be adjusted by varying wo = [H2O]/[surfactant]. This ratio (wo) is roughly proportional to the size of the water pool in a reverse micelle. As a result, water pools may be made to vary in size from ~1 nm to approximately 30 nm’s in diameter.

We have used ultrafast pump-probe and vibrational echo spectroscopies to examine the behavior of water confined in AOT reverse micelles. Polarization resolved pump-probe spectroscopy (see Figure 2) allows us to probe rotational correlation functions that give us an indication of how quickly water molecules reorient in a sample.

The anisotropic (orientational) response shown in Figure 3 reveals a dramatic decrease in waters mobility for the smallest reverse micelles. Compared to bulk water, the rate of orientational relaxation, which is intimately related to the global structural evolution of the hydrogen bond network, becomes increasingly small as the water nanopool decreases in size. For w0 = 10, 5, and 2, the diameters of the nanopools are 4 nm (~1000 water molecules), 2.5 nm (~300 water molecules) and 1.7 nm (~40 water molecules), respectively. As the nanopools increase in size, it is found that the bulk limit is obtained at ~28 nm. In these pump-probe experiments, the theoretical value of the initial anisotropy is 0.4. The data indicate that there is a relatively large very fast component (~40 fs) to the orientational relaxation in the reverse micelles relative to bulk water. This may be due to less restricted librational motion of water molecules at the interface where the hydrogen bond network is disrupted by the head groups.

Ultrafast vibrational echo spectroscopy (see Figure 4) has provided detailed insights into the hydrogen bond network dynamics of bulk water. The hydroxyl stretch is an ideal probe of hydrogen bond network dynamics since spectral diffusion is caused by the hydrogen bond fluctuations. The timescale of these fluctuations are observable due to the strong coupling between the hydroxyl stretch and hydrogen bond modes. We have conducted stimulated vibrational echoes of water in AOT reverse micelles in order to observe the effects of confinement on the hydrogen bond network dynamics. Figure 5 displays vibrational echo decays for bulk water and wo = 10 and 2. The data illustrate the slower spectral diffusion dynamics in the smaller reverse micelles. The hydrogen bond network is observed to be more rigid here due to confinement (water cannot translate freely) as well as the presence of strong interactions between water molecules and the charged surfactant head groups.

It will be interesting to observe the effects of confinement on water in different systems. Are the effects described above a general phenomenon or is the nature of the confining environment a more significant determinant of the water dynamics? These questions are currently being pursued by exploring water in other confined systems.

For a more thorough summary of our studies of hydrogen bond dynamics in liquid and glasses, see the links listed below:

Population relaxation dynamics

Orientational relaxation dynamics

Spectral relaxation dynamics

Vibrational Echo Experiments

Recent Publications

1) Piletic, Ivan R.; Tan, Howe-Siang; Fayer, M. D. Dynamics of Nanoscopic Water: Vibrational Echo and Infrared Pump-Probe Studies of Reverse Micelles.    Journal of Physical Chemistry (in press).

2) Tan, Howe-Siang; Piletic, Ivan R.; Fayer, M. D. Orientational dynamics of water confined on a nanometer length scale in reverse micelles.    Journal of Chemical Physics (2005), 122(17), 174501/1-174501/9. pdf

3) Tan, Howe-Siang; Piletic, Ivan R.; Riter, Ruth E.; Levinger, Nancy E.; Fayer, M. D. Dynamics of water confined on a nanometer length scale in reverse micelles: Ultrafast infrared vibrational echo spectroscopy.    Physical Review Letters (2005), 94(5), 057405/1-057405/4. pdf