Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

arrangement. However, the hydrogen bonded network is not static. The network evolves constantly on a
picosecond time scale by the concerted dissociation and formation of hydrogen bonds. This rapid
evolution of water’s hydrogen bond network enables processes ranging from proton diffusion to protein
folding. While a great deal is known about the dynamics of bulk water, much less is known about the
dynamics of water in nanoconfinement, at interfaces and when it is interacting with ionic species. The
dynamics of water are changed a great deal when water is in nanoscopic environments, interacting with
interfaces or ions. In addition, important processes such as proton transport, which is necessary in fuel
cells, are dramatically changed when they do not occur in bulk pure water.
The infinite miscibility of dimethyl sulfoxide (DMSO) with water at room temperature makes
water/DMSO solutions useful for gaining insight into changes in water hydrogen bond dynamics as water
goes from a minor component to a major component of the solutions. While water is both a hydrogen
bond donor and acceptor, DMSO is only a hydrogen bond acceptor. This difference gives rise to a variety
of important properties of water/DMSO binary solutions. Solutions of water/DMSO form a eutectic in
molar ratios of 2:1 and at low DMSO concentrations serves as a cryoprotectant with a depressed freezing
point of 203 K. At mole fractions of 0.3-0.4 DMSO, water/DMSO solutions deviate strongly from ideality,
with positive deviations in viscosity and density, and negative deviations in the heat of mixing. We have
performed a comprehensive examination of the dynamics and interactions of water in mixtures of water
and DMSO. Solutions were studied that range from very low water content to high water content. Five
experimental observables were employed. The techniques, which were all applied as a function of water
concentration, are infrared (IR) absorption spectra of the water hydroxyl stretch, IR pump-probe
experiments that measure the hydroxyl stretch vibrational lifetimes, polarization selective IR pump-probe
experiments that measure the water orientational relaxation times, two dimensional IR (2D IR) vibrational
echo experiments that measure spectral diffusion of the hydroxyl stretch, and optical heterodyne detected
optical Kerr effect (OHD-OKE) experiments that measure the orientational relaxation of DMSO. By
combining the results of these methods, a detailed picture of the nature of water/DMSO dynamics and
interactions was obtained, which in turn increased our understanding of how water behaves in complex
environments.
Vibrational population relaxation of the hydroxyl stretch displayed two vibrational lifetimes even at
very low water concentrations. The two lifetimes are associated with water-water and water-DMSO
hydrogen bonds. The IR absorption spectra also showed characteristics of both water-DMSO and waterwater
hydrogen bonding. Although two populations were observed, water anisotropy decays
(orientational relaxation) exhibited single ensemble behavior, indicative of concerted reorientation
involving water and DMSO molecules. OHD-OKE experiments, which measure the orientational
relaxation of DMSO, revealed that the DMSO orientational relaxation times are the same as orientational
relaxation times found for water over a wide range of water concentrations within experimental error. The
fact that the reorientation times of water and DMSO are basically the same shows that the reorientation of
water is coupled to the reorientation of DMSO itself. These observations were described in terms of a
jump reorientation model. Frequency-frequency correlation functions determined from the 2D IR
experiments on the OD stretch show both fast and slow spectral diffusion. In analogy to bulk water, the
fast component was assigned to very local hydrogen bond fluctuations. The slow component, which is
similar to the slow water reorientation time at each water concentration, was associated with global
hydrogen bond structural randomization.
Room temperature ionic liquids (RTIL) have been the object of extensive study in the past decade
due to their unique properties including very low vapor pressure and non-flammability under ambient
conditions, selective reaction medium, etc. One particular area of interest is the possible use of ionic
liquids as alternative solvents in lithium ion batteries. The current generation of such batteries uses
organic compounds as solvents. These solvents present safety issues due to properties such as volatility
and high flammability. Thus RTILs could become greener and safer alternative.
Water dynamics are strongly influenced by their interactions with ions. RTILs are extremely
hygroscopic. In practical applications, RTILs will always contain some water. We are studying water in
RTILs using a variety of methods. RTILs provide a medium in which water can be studied at all
concentrations. In normal salt solutions, such as NaBr, at high concentration the salt crystallizes out of
the solution. Therefore, it is not possible to go to low enough water concentration to study the nature of
water interacting with a sea of ions but not interacting with other water molecules. Therefore, RTILs
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