Research Topics
Experimental Methods
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Dynamics of Water and Nanoscopic Water
Water is ubiquitous in nature and plays an important role in chemical, physical, and biological processes. In the pure liquid, water molecules are hydrogen bonded to neighboring water molecules in an approximately tetrahedral geometry and form an extended hydrogen bond network. Hydrogen bond lengths (strengths) are continually changing and water molecules are constantly switching hydrogen bond partners. The structure of water fluctuates on femtosecond to picosecond time scales. The slowest component of the fluctuations is associated with the global structural rearrangement of the hydrogen bond network. Rapid structural evolution of the hydrogen bond network of water is responsible for water’s unique properties. The properties and dynamics of water are changed in the presence of charges and in nanoscopically confined environments. In aqueous solutions, water dissolves ionic compounds, charged chemical species, and macromolecules with charged surfaces including proteins, micelles, and lipid bilayers by forming hydration shells (layers) around them. In nanoscopic water environments, water molecules are in direct contact with different types of interfaces. At and near an interface, water’s hydrogen bond network is significantly modified because the network must make accommodation for the distinct topology of the interface. Water confined on nanometer length scales is found in a many of physical and biological environments. Near charges or interfaces, the properties and dynamics of water cannot be extrapolated from those of bulk water and need to be examined and compared to the dynamical properties of bulk water.
Figure 4 shows two examples of nanoconfined water systems. Reverse micelles are important systems for the study of the influence of nanoscopic distance scales on the properties of water. Reverse micelles are surfactant assemblies that encapsulate nanoscopic pools of water. For some well characterized reverse micelles, the size of the water pool is readily controlled on nanometer length scales by changing the concentration ratio of water and surfactant. We are studying a variety of reverse micelle systems. Important technological systems that involve nanoconfined water are polyelectrolyte fuel cell membranes such as Nafion. Nafion has nanoscopic water channels through which protons can move. The membrane separates the reactants in a fuel cell, but permits the transport of protons. We are also studying lamellar structures and other nanoscopic water environments.
Prior to our recent work, nanoscopic water was studied using fluorescent probe molecules and other types of indirect measurements to gain understanding of the dynamics of nanoscopic water. We are performing a variety of direct measurements on the water itself. The hydroxyl (OH and OD) stretch frequency of water in aqueous solutions is closely related to the local hydrogen bond structure. By performing ultrafast IR experiments on the hydroxyl stretch of water, we can extract very detailed information on the dynamics and interactions of water molecules as a function of the size and topology of the system as well as the influence of additives such as salts. We are using ultrafast 2D IR vibrational echo spectroscopy and ultrafast IR polarization selective pump-probe experiments to study water dynamics. These methods are discussed below in the Experimental Methods section.
Figure 5 shows IR absorption spectrum for water and water in AOT reverse micelles as a function of the size of the water nanopool. The water nanopool diameters range from 28 nm to 1.7 nm with a range of the number of water molecules from ~350,000 to ~40. As the nanopool becomes smaller, the spectrum shifts to the blue. This is an indication that the properties of the water are changing as the confinement becomes more extreme. However, the absorption spectrum cannot provide information of the dynamics of the water molecules.
Figure 6 shows experimental data on the orientational relaxation time of water in the AOT reverse micelles. The data reflects the time it takes for a water molecule to randomize its orientation relative to the orientation it had at t = 0. These measurements were made with polarization selective pump-probe experiments. For water to randomize its orientations, it is necessary for global hydrogen bond rearrangement to occur. That is, hydrogen bonds must break and new hydrogen bonds must form. The top panel shows that water in the largest reverse micelles behaves essentially identically to bulk water. However, by w0 = 20, there is just beginning to be some deviation from that of bulk water. The bottom panel shows that as the size becomes smaller than ~7 nm, the water dynamics slow dramatically. Nanoconfinement has a profound effect on the dynamics of the water hydrogen bond network. Detailed analysis of data like that shown in figure 6 along with vibrational echo experiments on reverse micelles, Nafion fuel cell membranes, water at the surface of biological model membranes, water in salt solutions, and water in other non-bulk environments is providing unique and detailed insights the dynamics, interactions, and structure of water in important systems.
Nanoscopic Water and Water Studied with Ultrafast Infrared Methods Publications322. “Dynamics of Water Probed with Vibrational Echo Correlation Spectroscopy,” John B. Asbury, Tobias Steinel, Kyungwon Kwak, S. A. Corcelli, C. P. Lawrence, J. L. Skinner, and M. D. Fayer, J. Chem. Phys. 121, 12431-12446 (2004). 323. “Dynamics of Water Confined on a Nanometer Length Scale in Reverse Micelles: Ultrafast Infrared Vibrational Echo Spectroscopy,” Howe-Siang Tan, Ivan R. Piletic, Ruth E. Riter, Nancy E. Levinger and M. D. Fayer, Phys. Rev. Lett. 94, 057405(4) (2005). 327. “The Dynamics of Nanoscopic Water: Vibrational Echo and IR Pump-probe Studies of Reverse Micelles,” Ivan R. Piletic, Howe-Siang Tan, and M. D. Fayer, J. Phys. Chem. B 109, 21273-21284 (2005). 339. “Testing the Core/Shell Model of Nanoconfined Water in Reverse Micelles Using Linear and Nonlinear IR Spectroscopy,” Ivan R. Piletic, David E. Moilanen, D. B. Spry, and Nancy E. Levinger, M. D. Fayer, J. Phys. Chem. A 110, 4985-4999 (2006). 342. “What Non-linear IR Experiments Can Tell You About Water That the IR Spectrum Can’t,” Ivan R. Piletic, David E. Moilanen, Nancy E. Levinger, Michael D. Fayer, J. Am. Chem. Soc. 128, 10366-10367 (2006).
343. “Tracking Water’s Response to Structural Changes in Nafion
Membranes,” David E. Moilanen, Ivan R. Piletic, and M. D. Fayer, J. Phys.
Chem. A 110, 9084-9088 (2006).
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