The dynamics of water in Aerosol-OT reverse micelles are investigated with ultrafast infrared spectroscopy of the hydroxyl stretch. In large reverse micelles, the dynamics of water are separable into two ensembles: slow interfacial water and bulklike core water. As the reverse micelle size decreases, the slowing effect of the interface and the collective nature of water reorientation begin to slow the dynamics of the core water molecules. In the smallest reverse micelles, these effects dominate and all water molecules have the same long time reorientational dynamics. To understand and characterize the transition in the water dynamics from two ensembles to collective reorientation, polarization and frequency selective infrared pump-probe experiments are conducted on the complete range of reverse micelle sizes from a diameter of 1.6-20 nm. The crossover between two ensemble and collective reorientation occurs near a reverse micelle diameter of 4 nm. Below this size, the small number of confined water molecules and structural changes in the reverse micelle interface leads to homogeneous long time reorientation.

The short-time orientational relaxation of water is studied by ultrafast infrared pump-probe spectroscopy of the hydroxyl stretching mode (OD of dilute HOD in H2O). The anisotropy decay displays a sharp drop at very short times caused by inertial orientational motion, followed by a much slower decay that fully randomizes the orientation. Investigation of temperatures from 1°C to 65°C shows that the amplitude of the inertial component (extent of inertial angular displacement) depends strongly on the stretching frequency of the OD oscillator at higher temperatures, although the slow component is frequency-independent. The inertial component becomes frequency-independent at low temperatures. At high temperatures there is a correlation between the amplitude of the inertial decay and the strength of the O-DOO hydrogen bond, but at low temperatures the correlation disappears, showing that a single hydrogen bond (ODOO) is no longer a significant determinant of the inertial angular motion. It is suggested that the loss of correlation at lower temperatures is caused by the increased importance of collective effects of the extended hydrogen bonding network. By using a new harmonic cone model, the experimentally measured amplitudes of the inertial decays yield estimates of the characteristic frequencies of the intermolecular angular potential for various strengths of hydrogen bonds. The frequencies are in the range of Ϸ400 cm ؊1 . A comparison with recent molecular dynamics simulations employing the simple point charge-extended water model at room temperature shows that the simulations qualitatively reflect the correlation between the inertial decay and the OD stretching frequency.ultrafast IR experiments ͉ dynamics ͉ motion ͉ MD simulations ͉ harmonic model A lthough a great deal is known about water's bulk thermodynamic properties, the complex intermolecular forces that govern its nanoscopic structural arrangements and dynamics have made a detailed picture of the instantaneous local structures of water and their evolution elusive. Numerous models for the structure of water have been proposed to account for its anomalous behavior at different temperatures and pressures (1-6). These models all have some degree of success in reproducing the observed radial distribution function and other properties of water.Here, we present results on the fast, local (inertial) angular motions of dilute HOD molecules in water within their initial hydrogen-bonding structure through a study of how the orientational motions of water depend on the OD stretching frequency and temperature. The short-time orientational motions of the HOD molecule depend on the potential energy surface established by neighboring water molecules. Measurements of the orientational relaxation as a function of OD stretching frequency and temperature provide insight into how the motions of the HOD molecule depend on the strength of the O-DOO hydrogen bond (nature of the potential energy surface) and how the local structure of water changes with temperature. The measurem...

The exchange of water hydroxyl hydrogen bonds between anions and water oxygens is observed directly with ultrafast 2D IR vibrational echo chemical exchange spectroscopy (CES). The OD hydroxyl stretch of dilute HOD in H 2O in concentrated (5.5 M) aqueous solutions of sodium tetrafluoroborate (NaBF 4) displays a spectrum with a broad water-like band (hydroxyl bound to water oxygen) and a resolved, blue shifted band (hydroxyl bound to BF 4 ؊ ).At short time (200 fs), the 2D IR vibrational echo spectrum has 4 peaks, 2 on the diagonal and 2 off-diagonal. The 2 diagonal peaks are the 0 -1 transitions of the water-like band and the hydroxylanion band. Vibrational echo emissions at the 1-2 transition frequencies give rise to 2 off-diagonal peaks. On a picosecond time scale, additional off-diagonal peaks grow in. These new peaks arise from chemical exchange between water hydroxyls bound to anions and hydroxyls bound to water oxygens. The growth of the chemical exchange peaks yields the time dependence of anionwater hydroxyl hydrogen bond switching under thermal equilibrium conditions as T aw ‫؍‬ 7 ؎ 1 ps. Pump-probe measurements of the orientational relaxation rates and vibrational lifetimes are used in the CES data analysis. The pump-probe measurements are shown to have the correct functional form for a system undergoing exchange.2D IR spectroscopy ͉ hydration of ions ͉ hydrogen bond dynamics ͉ ion hydrogen bonds chemical exchange ͉ ionic solutions W ater interacting with ions occurs in a wide variety of systems ranging from ocean salt water to water interacting with charged amino acids at the surfaces of proteins (1). The properties of pure liquid water are determined by the nature of its hydrogen bond network. A water molecule can have as many as 4 hydrogen bonds with other water molecules, forming an approximately tetrahedral structure. The pure water hydrogen bond network is constantly evolving with a range of time scales from tens of femtoseconds to picoseconds (2-5). Hydrogen bonds are continually forming and breaking through concerted hydrogen bond rearrangements (6). These dynamical processes can be observed on the time scale they occur in considerable detail by using ultrafast infrared spectroscopy. Measurements of spectral diffusion, described in terms of the frequencyfrequency correlation function (FFCF), by using ultrafast 2D IR vibrational echo spectroscopy (3,7,8) as well as other ultrafast IR techniques (4, 5) have determined the multiple time scales for the hydrogen bond dynamics. The slowest time component of the FFCF (1.7 ps) is associated with the randomization of the hydrogen bond network through concerted hydrogen bond rearrangements. The orientational relaxation time of pure water (2.6 ps) (2, 5) is also assigned to concerted hydrogen bond rearrangement via jump reorientation (6).In aqueous salt solutions the structure of water is modified in the vicinity of the ions as the water oxygens preferentially solvate the cations and the water hydroxyls solvate the anions (9, 10). The structures of the hyd...

The dynamics of water at the surface of artificial membranes composed of aligned multibilayers of the phospholipid dilauroyl phosphatidylcholine (DLPC) are probed with ultrafast polarization selective vibrational pump-probe spectroscopy. The experiments are performed at various hydration levels, x = 2 -16 water molecules per lipid at 37 °C. The water molecules are ~1 nm above or below the membrane surface. The experiments are conducted on the OD stretching mode of dilute HOD in H 2 O to eliminate vibrational excitation transfer. The FT-IR absorption spectra of the OD stretch in the DLPC bilayer system at low hydration levels shows a red-shift in frequency relative to bulk water, which is in contrast to the blue shift often observed in systems such as water nanopools in reverse micelles. The spectra for x = 4 -16 can be reproduced by a superposition of the spectra for x = 2 and bulk water. IR Pump-probe measurements reveal that the vibrational population decays (lifetimes) become longer as the hydration level is decreased. The population decays are fit well by biexponential functions. The population decays, measured as a function of the OD stretch frequency, suggest the existence of two major types of water molecules in the interfacial region of the lipid bilayers. One component may be a clathrate-like water cluster near the hydrophobic choline group and the other may be related to the hydration water molecules mainly associated with the phosphate group. As the hydration level increases, the vibrational lifetimes of these two components decrease, suggesting a continuous evolution of the hydration structures in the two components associated with the swelling of the bilayers. The agreement of the magnitudes of the two components obtained from IR spectra with those from vibrational lifetime measurements further supports the two component model. The vibrational population decay fitting also gives an estimation of the number of phosphate-associated water molecules and choline-associated water molecules from, which range from 1 to 4, and 1 to 12, respectively as x increases from 2 to 16. Time dependent anisotropy measurements yield the rate of orientational relaxation as a function of x. The anisotropy decay is biexponential. The fast component is almost independent of x, and is interpreted as small orientational fluctuations that occur without hydrogen bond rearrangement. The slower component becomes very long as the hydration level decreases. This component is a measure of the rate of complete orientational randomization, which requires hydrogen bond rearrangement and is discussed in terms of a jump reorientation model.

The orientational dynamics of water molecules at the interface in large Aerosol-OT (AOT) reverse micelles are investigated using ultrafast infrared spectroscopy of the OD stretch of dilute HOD in H2O. In large reverse micelles(~9 nm diameter or larger), a significant amount of the nanoscopic water is sufficiently distant from the interface that it displays bulk-like characteristics. However, some water molecules interact with the interface and have vibrational absorption spectra and dynamics distinct from bulk water. The different characteristics of these interfacial waters allow their contribution to the data to be separated from the bulk. The infrared absorption spectrum of the OD stretch is analyzed to show that the interfacial water molecules have a spectrum that peaks near 2565 cm−1 in contrast to 2509 cm−1 in bulk water. A two component model is developed that simultaneously describes the population relaxation and orientational dynamics of the OD stretch in the spectral region of the interfacial water. The model provides a consistent description of both observables and demonstrates that water interacting with the interface has slower vibrational relaxation and orientational dynamics. The orientational relaxation of interfacial water molecules occurs in 18 ± 3 ps in contrast to the bulk water value of 2.6 ps.

The orientational dynamics of water at a neutral surfactant reverse micelle interface are measured with ultrafast infrared spectroscopy of the hydroxyl stretch, and the results are compared to orientational relaxation of water interacting with an ionic interface. The comparison provides insights into the influence of a neutral vs. ionic interface on hydrogen bond dynamics. Measurements are made and analyzed for large nonionic surfactant Igepal CO-520re-verse micelles (water nanopool with a 9-nm diameter). The results are compared with those from a previous study of reverse micelles of the same size formed with the ionic surfactant Aerosol-OT (AOT). The results demonstrate that the orientational relaxation times for interfacial water molecules in the two types of reverse micelles are very similar (13 ps for Igepal and 18 ps for AOT) and are significantly slower than that of bulk water (2.6 ps). The comparison of water orientational relaxation at neutral and ionic interfaces shows that the presence of an interface plays the dominant role in determining the hydrogen bond dynamics, whereas the chemical nature of the interface plays a secondary role.reverse micelles ͉ ultrafast IR experiments ͉ interfacial water ͉ hydrogen bond dynamics W ater molecules at interfaces are involved in many processes. In biology, water is found in crowded environments, such as cells, where it hydrates membranes and large biomolecules. In geology, interfacial water molecules can control ion adsorption and mineral dissolution. Embedded water molecules can change the structure of zeolites. In chemistry, water plays an important role as a polar solvent often in contact with interfaces, for example, in ion exchange resin systems.When water interacts with an interface, its hydrogen bonding properties are distinct from those of bulk water. Interactions with an interface influence water's ability to undergo hydrogen bond network rearrangements, which is a concerted process that involves a water molecule and its two water solvation shells (1, 2). For a water molecule to switch hydrogen bonding partners, other waters must also break and form new hydrogen bonds (1, 2). Such hydrogen bond rearrangements are necessary for both orientational and translational motions. An interface eliminates many of the pathways for hydrogen bond rearrangement that are available in bulk water. A fundamental question is whether the composition of or solely the presence of an interface plays the dominant role in affecting the hydrogen bond dynamics of interfacial water.Ultrafast infrared (IR) spectroscopy is a valuable technique for probing the dynamics of both bulk water and water at interfaces using the hydroxyl stretching mode of water as a reporter (3-17). Because processes in water involving hydrogen bond rearrangements occur on the picosecond time scale, the femtosecond time resolution of the IR techniques makes it possible to resolve the motions of water molecules on the time scale on which they are occurring. Examples of confined or restricted environments that...

The dynamics of dimethyl sulfoxide (DMSO)/water solutions with a wide range of water concentrations are studied using polarization selective infrared pump–probe experiments, two-dimensional infrared (2D IR) vibrational echo spectroscopy, optical heterodyne detected optical Kerr effect (OHD-OKE) experiments, and IR absorption spectroscopy. Vibrational population relaxation of the OD stretch of dilute HOD in H2O displays two vibrational lifetimes even at very low water concentrations that are associated with water–water and water–DMSO hydrogen bonds. The IR absorption spectra also show characteristics of both water–DMSO and water–water hydrogen bonding. Although two populations are observed, water anisotropy decays (orientational relaxation) exhibit single ensemble behavior, indicative of concerted reorientation involving water and DMSO molecules. OHD-OKE experiments, which measure the orientational relaxation of DMSO, reveal 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 are discussed 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 is assigned to very local hydrogen bond fluctuations. The slow component, which is similar to the slow water reorientation time at each water concentration, is associated with global hydrogen bond structural randomization.

To determine the relative importance of the confining geometry and nanoscopic length scale versus water/interface interactions, the dynamic interactions between water and interfaces are studied with ultrafast infrared spectroscopy. Aerosol OT (AOT) is a surfactant that can form two-dimensional lamellar structures with known water layer thickness as well as well-defined monodispersed spherical reverse micelles of known water nanopool diameter. Lamellar structures and reverse micelles are compared based on two criteria: surface-to-surface dimensions to study the effect of confining length scales, and water-to-surfactant ratio to study water/interface interactions. We show that the water-to-surfactant ratio is the dominant factor governing the nature of water interacting with an interface, not the characteristic nanoscopic distance. The detailed structure of the interface and the specific interactions between water and the interface also play a critical role in the fraction of water molecules influenced by the surface. A two-component model in which water is separated into bulk-like water in the center of the lamellar structure or reverse micelle and interfacial water is used to quantitatively extract the interfacial dynamics. A greater number of perturbed water molecules are present in the lamellar structures as compared to the reverse micelles due to the larger surface area per AOT molecule and the greater penetration of water molecules past the sulfonate head groups in the lamellar structures.