Water Hydrogen Bond Structure and Dynamics in Ionic and Polymeric Aqueous Systems

Book excerpt: Water is a simple molecule with many unique physical properties that are critical to life on earth. Its properties arise from its extended hydrogen-bonded network, in which water-water hydrogen bonds are constantly breaking and forming. However, in many biological systems and materials, the water network is impacted by the presence of solutes and interfaces. In this thesis, the structure and dynamics of the hydrogen bond network are examined in technologically relevant materials where water plays a key role. The systems studied include fuel cell membranes, hydrogels and concentrated salt solutions. Nonlinear infrared spectroscopy can be used to experimentally observe ultrafast motions of water as well as its structural configurations within complex chemical systems. Polarization-selective pump-probe experiments on the OD stretch of dilute HOD in water provide information on both orientational and vibrational relaxation. Orientational relaxation describes the reorientation dynamics of water molecules in the hydrogen bond network. If angular diffusion is restricted, orientational relaxation also provides insight into how water may be sterically hindered within its environment. Vibrational relaxation describes coupling of vibrational energy absorbed by the HOD molecules to its surrounding media. The vibrational lifetime provides details on the local interactions of HOD and may allow separation of distinct dynamics near different species. Two-dimensional vibrational echo experiments on HOD molecules observe the time scales for structural evolution of the surrounding environment through ultrafast vibrational frequency fluctuations. With these experimental techniques, a holistic picture of the structure and motions of the water hydrogen bond network can be acquired.