Water Rotational Dynamics Research Articles

Nuclear magnetic resonance study on rotational dynamics of water and benzene in a series of ionic liquids: Anion and cation effects

The rotational correlation times (τ(2R)) for polar water (D(2)O) molecule and apolar benzene (C(6)D(6)) molecule were determined in ionic liquids (ILs) by means of the (2)H (D) NMR spin-lattice relaxation time (T(1)) measurements. The solvent IL was systematically varied to elucidate the anion and cation effects separately. Five species, bis(trifluoromethylsulfonyl)imide (TFSI(-)), trifluoromethylsulfonate (TfO(-)), hexafluorophosphate (PF(6)(-)), chloride (Cl(-)), and formate (HCOO(-)), were examined for the anion effect against a fixed cation species of 1-butyl-3-methyl-imidazolium (bmim(+)). Four species, bmim(+), N-methyl-N-butylpyrrolidinium (bmpy(+)), N,N,N-trimethyl-N-propylammonium (N(1,1,1,3)(+)), and P,P,P-trihexyl-P-tetradecylphosphonium (P(6,6,6,14)(+)), were employed for the cation effect against a fixed anion species of TFSI(-). The τ(2R) ratio of water to benzene, expressed as τ(W/B), was used as a probe to characterize the strength of Coulombic solute-solvent interaction in ILs beyond the hydrodynamic limit based on the excluded-volume effect. The τ(W/B) value was found to strongly depend on the anion species, and the solute dynamics are sensitive not only to the size but also to the chemical structure of the component anion. The cation effect was rather weak, in contrast. The largest and most hydrophobic P(6,6,6,14)(+) cation was exceptional and a large τ(W/B) was observed, indicating a unique solvation structure in [P(6,6,6,14)(+)]-based ILs.

Hydration Dynamics of Hyaluronan and Dextran

Hyaluronan is a polysaccharide, which is ubiquitous in vertebrates and has been reported to be strongly hydrated in a biological environment. We study the hydration of hyaluronan in solution using the rotational dynamics of water as a probe. We measure these dynamics with polarization-resolved femtosecond-infrared and terahertz time-domain spectroscopies. Both experiments reveal that a subensemble of water molecules is slowed down in aqueous solutions of hyaluronan amounting to ∼15 water molecules per disaccharide unit. This quantity is consistent with what would be expected for the first hydration shell. Comparison of these results to the water dynamics in aqueous dextran solution, a structurally similar polysaccharide, yields remarkably similar results. This suggests that the observed interaction with water is a common feature for hydrophilic polysaccharides and is not specific to hyaluronan.

Vibrational Spectroscopy of Water at Interfaces

Understanding liquid water's behavior at the molecular level is essential to progress in fields as disparate as biology and atmospheric sciences. Moreover, the properties of water in bulk and water at interfaces can be very different, making the study of the hydrogen-bonding networks therein very important. With recent experimental advances in vibrational spectroscopy, such as ultrafast pulses and heterodyne detection, it is now possible to probe the structure and dynamics of bulk and interfacial water in unprecedented detail. We consider here three aqueous interfaces: the water liquid-vapor interface, the interface between water and the surfactant headgroups of reverse micelles, and the interface between water and the lipid headgroups of aligned multi-bilayers. In the first case, sum-frequency spectroscopy is used to probe the interface. In the second and third cases, the confined water pools are sufficiently small that techniques of bulk spectroscopy (such as FTIR, pump-probe, two-dimensional IR, and the like) can be used to probe the interfacial water. In this Account, we discuss our attempts to model these three systems and interpret the existing experiments. For the water liquid-vapor interface, we find that three-body interactions are essential for reproducing the experimental sum-frequency spectrum, and presumably for the structure of the interface as well. The observed spectrum is interpreted as arising from overlapping and canceling positive and negative contributions from molecules in different hydrogen-bonding environments. For the reverse micelles, our theoretical models confirm that the experimentally observed blue shift of the water OD stretch (for dilute HOD in H(2)O) arises from weaker hydrogen bonding to sulfonate oxygens. We interpret the observed slow-down in water rotational dynamics as arising from curvature-induced frustration. For the water confined between lipid bilayers, our theoretical models confirm that the experimentally observed red shift of the water OD stretch arises from stronger hydrogen bonding to phosphate oxygens. We develop a model for heterogeneous vibrational lifetime distributions, and we implement the model to calculate isotropic and anisotropic pump-probe decays. We then compare these results with experimental data. Clearly, recent experimental advances in vibrational spectroscopy have led to beautiful new results, providing information about the structure and dynamics of water at interfaces. These experimental and concomitant theoretical advances (particularly the unified theoretical framework of non-linear response functions) have greatly contributed to our understanding of this unique and important substance.

Specific cellular water dynamics observed in vivo by neutron scattering and NMR

Neutron scattering, by using deuterium labelling, revealed how intracellular water dynamics, measured in vivo in E. coli, human red blood cells and the extreme halophile, Haloarcula marismortui, depends on the cell type and nature of the cytoplasm. The method uniquely permits the determination of motions on the molecular length (approximately ångstrøm) and time (pico- to nanosecond) scales. In the bacterial and human cells, intracellular water beyond the hydration shells of cytoplasmic macromolecules and membrane faces flows as freely as liquid water. It is not "tamed" by confinement. In contrast, in the extreme halophile archaeon, in addition to free and hydration water an intracellular water component was observed with significantly slowed down translational diffusion. The results are discussed and compared to observations in E. coli and Haloarcula marismortui by deuteron spin relaxation in NMR--a method that is sensitive to water rotational dynamics on a wide range of time scales.

Evolution from Surface-Influenced to Bulk-Like Dynamics in Nanoscopically Confined Water

We use molecular dynamics simulations to study the influence of confinement on the dynamics of a nanoscopic water film at T = 300 K and rho = 1.0 g cm(-3). We consider two infinite hydrophilic (beta-cristobalite) silica surfaces separated by distances between 0.6 and 5.0 nm. The width of the region characterized by surface-dominated slowing down of water rotational dynamics is approximately 0.5 nm, while the corresponding width for translational dynamics is approximately 1.0 nm. The different extent of perturbation undergone by the in-plane dynamic properties is evidence of rotational-translational decoupling. The local in-plane rotational relaxation time and translational diffusion coefficient collapse onto confinement-independent "master" profiles as long as the separation d >or= 1.0 nm. Long-time tails in the perpendicular component of the dipole moment autocorrelation function are indicative of anisotropic behavior in the rotational relaxation.

Distribution, Structure, and Dynamics of Cesium and Iodide Ions at the H2O−CCl4and H2O−Vapor Interfaces

Molecular dynamics simulations utilizing many-body potentials of H2O-CCl4 and H2O-vapor interfaces were carried out at different cesium and iodide ion concentrations to compare ion distribution, interfacial orientational and structural properties, and dynamics. It was found that cesium was repelled by both interfaces, and iodide was active at both interfaces, but to a much greater degree at the H2O-vapor interface. At the interface, the iodide dipole was strongly induced, orienting perpendicular to the interface for both systems, leading to stronger hydrogen bonds with water. For the H2O-CCl4 interface, though, there was a compensation between these strong hydrogen bonds and short to moderate ranged repulsion between iodide and CCl4. Hydrogen bond distance and angular distributions showed weaker water-water hydrogen bonds at both interfaces, but generally stronger water-iodide hydrogen bonds. Both translational and rotational dynamics of water were faster at the interface, while for CCl4, its translational dynamics was slower, but rotational dynamics faster at the interface. For many of the studied systems and species, translational diffusion was found to be anisotropic at both interfacial and bulk regions.

Dynamics and Thermodynamics of Water in PAMAM Dendrimers at Subnanosecond Time Scales

Atomistic molecular dynamics simulations are used to study generation 5 polyamidoamine (PAMAM) dendrimers immersed in a bath of water. We interpret the results in terms of three classes of water: buried water well inside of the dendrimer surface, surface water associated with the dendrimer-water interface, and bulk water well outside of the dendrimer. We studied the dynamic and thermodynamic properties of the water at three pH values: high pH with none of the primary or tertiary amines protonated, intermediate pH with only the primary amines protonated, and low pH with all amines protonated. For all pH values we find that both buried and surface water exhibit two relaxation times: a fast relaxation ( approximately 1 ps) corresponding to the libration motion of the water and a slow ( approximately 20 ps) diffusional component related to the escaping of water from one domain to another. In contrast for bulk water the fast relaxation is approximately 0.4 ps while the slow relaxation is approximately 14 ps. These results are similar to those found in biological systems, where the fast relaxation is found to be approximately 1 ps while the slow relaxation ranges from 20 to 1000 ps. We used the 2PT MD method to extract the vibrational (power) spectrum and found substantial differences for the three classes of water. The translational diffusion coefficient for buried water is 11-33% (depending on pH) of the bulk value while the surface water is about 80%. The change in rotational diffusion is quite similar: 21-45% of the bulk value for buried water and 80% for surface water. This shows that translational and rotational dynamics of water are affected by the PAMAM-water interactions as well as due to the confinement in the interior of the dendrimer. We find that the reduction of translational or rotational diffusion is accompanied by a blue shift of the corresponding libration motions ( approximately 10 cm(-1) for translation, approximately 35 cm(-1) for rotation), indicating higher local force constants for these motions. These effects are most pronounced for the lowest pH, probably because of the increased rigidity caused by the internal charges. From the vibrational density of states we also calculate the enthalpies and entropies of the various waters. We find that water molecules are enthalpically favored near the PAMAM dendrimer: energy for surface water is approximately 0.1 kcal/mol lower to that in the bulk, and approximately 0.5-0.9 kcal/mol lower for buried water. In contrast, we find that both the buried and surface water are entropically unfavored: buried water is 0.9-2.2 kcal/mol lower than the bulk while the surface water is 0.1-0.2 kcal/mol lower. The net result is a thermodynamically unfavored state of the water surrounding the PAMAM dendrimer: 0.4-1.3 kcal/mol higher for buried water and 0.1-0.2 kcal/mol for surface water. This excess free energy of the surface and buried waters is released when the PAMAM dendrimer binds to DNA or metal ions, providing an extra driving force.

Dynamic of solitary water in hydrophobic solvents

Structural and dynamical properties of D 2O infinitely dilute in supercritical CO 2 have been studied by molecular dynamics simulations. A new intermolecular potential model taking explicitly into account electron donor acceptor (EDA) interactions between water and CO 2 has been developed from ab initio calculations. The analysis of the local ordering between water and CO 2 from simulated radial distribution functions put in evidence specific short-ranged EDA complexes. Taking xenon as a reference inert solvent in which the re-orientational dynamics of D 2O is quasi-free, we comparatively showed that the rotational dynamics of water is weakly anisotropic due to the EDA interactions, which affect more specifically the re-orientational motions of the main symmetry axis. These results have been used to assess the contribution of the vibrational relaxation in the experimental mid-infrared (MIR) profiles associated with the ν 1 symmetric and ν 3 anti-symmetric stretching modes of D 2O. It is found that only the rotational dynamics contribute to the broadening of the IR profiles. However, vibrational processes contribute to the red-shifts of the band centres and the relative intensity enhancements of the ν 1 and ν 3 modes of D 2O. Comparison with another existing model reveals that EDA interactions have to be taken into account to reproduce MIR and FIR measurements. We argue that vibrational relaxation plays an increasing role in the genesis of the MIR profiles as the hydrophilic character of the solvent increases. From this point of view, CO 2 can be classified as an intermediate solvent between xenon and carbon tetrachloride on a hydrophilic solvent scale based upon the solubility criterion.

Dynamics of supercooled water in mesoporous silica matrix MCM-48-S.

Using three different quasielastic neutron spectrometers with widely different resolutions, we have been able to study the microscopic translational and rotational dynamics of water, in a mesoporous silica matrix MCM-48-S, from T=300 K to 220 K, with a single consistent model. We formulated our fitting routine using the relaxing cage model. Thus, from the fit of the experimental data, we extracted the fraction of water bound to the surface of the pore, the characteristic relaxation times of the long-time translational and rotational decays, the stretch exponent describing the shape of the relaxation processes, and the power exponent determining the Q-dependence of the translational relaxation time. A tremendous slowing down of the rotational relaxation time, as compared to the translational one, has been observed.

Water structure and dynamics in phosphate fluorosurfactant based reverse micelle: A computer simulation study

We performed a molecular dynamics simulation on a system containing a water pool inside the reverse micelle made up of an assembly of phosphate fluorosurfactant molecules dissolved in supercritical carbon dioxide. The water molecules in the first solvation shell of the headgroup lose the water to water tetrahedral hydrogen bonded network but are strongly bonded to the surfactant headgroups. This change in inter-water hydrogen bonding in connection with the confined geometry of the reverse micelle slows down the translational and especially the rotational dynamics of water.

Structural study of supercritical water. III. Rotational dynamics

The rotational dynamics of water in super- and subcritical conditions is investigated by measuring the spin-lattice relaxation time T1 of heavy water (D2O). The experimentally determined T1 is shown to be governed by the quadrupolar relaxation mechanism even in the supercritical conditions and to provide the second-order reorientational correlation time τ2R of the O–D axis of a single water molecule. It is then found that while τ2R decreases rapidly with the temperature on the liquid branch of the saturation curve, it remains on the order of several tens of femtoseconds when the density is varied up to twice the critical at a fixed supercritical temperature of 400 °C. The comparison of τ2R with the angular momentum correlation time shows that the rotational dynamics is not diffusive in supercritical water. The dependence of τ2R on the hydrogen bonding state is also examined in combination with molecular dynamics simulations, and the effect of the hydrogen bonding on the rotational dynamics in supercritical water is found to be weaker than but to be on the same order of magnitude as that in ambient water on the relative scale. Actually, although τ2R is divergent in the limit of zero density, it is observed to increase with the density when the density is above ∼1/3 of the critical.

流体のダイナミクス 超臨界水の動的構造解析

The rotational dynamics of water in super- and subcritical conditions is investigated by measuring the spin-lattice relaxation time T1 of heavy water (D2O). The experimentally determined T1 is shown to be governed by the quadrupolar mechanism even in the supercritical conditions and to provide the second-order reorientational correlation time τ2R of the O-D axis. It is then found that while τ2R decreases rapidly with the temperature on the saturation curve, it remains on the order of several tens of femtoseconds when the density is varied at a temperature above the critical. The comparison of τ2R with the angular momentum correlation time shows that the inertial effect is operative in the rotational dynamics of supercritical water. The dependence of τ2R on the hydrogen bonding state is also examined in combination with computer simulations, and the effect of the hydrogen bonding on the rotational dynamics in supercritical water is found to be weaker than but to be on the same order of magnitude as that in ambient water on the relative scale. Actually, although τ2R is divergent in the limit of zero density, it is observed to increase with the density when the density is above ∼1/3 of the critical.

Curvature effects on hydrophobic solvation

Molecular dynamics simulations were performed on four systems: pure SPC/E water, and three solutes with different surface curvatures in water. The same intermolecular potential was employed throughout, but the radius of curvature was altered by inserting a hard ‘inner core’ into the solute. The radial distribution function, angular distribution, a bond-order parameter, a hydrogen bond analysis and a near-neighbour analysis were used to assess the static properties of water molecules in the first solvation shell and in the bulk phase, while their dynamic behaviour was examined using various velocity and reorientational autocorrelation functions. Water molecules in the solvation shell exhibit enhanced structuring, but this structuring is decreased when the curvature is decreased. The translational and rotational dynamics of water are slowed down, but the effect is independent of curvature.

Dynamics of water confined in non-ionic amphiphiles supramolecular structures

Light scattering studies of water properties in supramolecular structures of the aqueous solutions of polyoxyethylene non-ionic amphiphiles C 10E 5 are presented. Raman and Depolarized Rayleigh spectra are studied to obtain the OH stretching vibrational contribution and the rotational relaxational time of water along an isothermal path, crossing the isotropic one-phase region from 0 to 1 amphiphile volume fraction φ, and at the structural phase transitions at relatively large amphiphiles concentrations. Data analysis of the vibrational mode points out that the structure of bound water presents a local fourfold-coordinated environment, whereas the water rotational dynamics evidences a behavior that depends on the packing effects of the amphiphile aggregates.

Dynamics of Hydrophobic Hydration of Benzene

The dynamics of hydrophobic hydration of benzene was examined by NMR for both the solute and the solvent water. The rotational dynamics of water in the benzene hydration shell was observed to be sl...

Light-scattering studies on water-nonionic-amphiphile solutions.

The structure of water in aqueous solutions of the polyoxyethylene nonionic amphiphile ${\mathrm{C}}_{10}$${\mathrm{H}}_{21}$(${\mathrm{OCH}}_{2}$${\mathrm{CH}}_{2}$${)}_{5}$OH is studied by depolarized Rayleigh-wing and Brillouin light scattering along an isothermal path crossing the isotropic one-phase region from 0 to 1 amphiphile volume fraction \ensuremath{\varphi}. Depolarized Rayleigh-wing light scattering shows a well defined slowing down in the water rotational dynamics just where the structure of the system changes from the micellar structure to a block copolymeric melt. The interpretation of the obtained data gives the following structural picture for water: for \ensuremath{\varphi} lower than 0.75, water is partially bound to the oxyethylene groups of the amphiphile; above \ensuremath{\varphi}=0.75 all the water present in the system is bound. Brillouin scattering, probing thermodynamical quantities as the bulk modulus, gives information about the collective properties of the system. Two distinct behaviors in the bulk compressibility of the system are observed in two different concentration ranges. In the micellar region, measurements of the dispersion of the sound velocity as a function of the droplet volume fraction show an increased rigidity of the system at high frequencies. This increase in the bulk modulus is consistent with a percolation phenomenon. Micelles build up transient networks due to the attractive interaction. Above this region Brillouin scattering probes the evolution of the system towards the pure amphiphile phase for which a decrease of sound dispersion with \ensuremath{\varphi} is observed. Strong correlations between amphiphile molecules are present at large amphiphile concentrations.

Neutron scattering of supercooled water in silica gels

The effects of temperature on the incoherent quasielastic and inelastic neutron scattering of water in two silica gels (56 and 73 % w/w SiO2) have been measured by time-of-flight spectroscopy from 300 down to 233 K, where the water is in the frozen state. Quasielastic scattering (QENS) data have been analysed using a model applied by Teixeira et al. in a recent study of the diffusive motions in supercooled water. This model involves a fast and a slow component, corresponding to the rotational and translational motions of the water molecule. The temperature dependence of the rotational dynamics of water in the gels is similar to that in the bulk, as is evident from the activation energy (ca. 10 kJ mol–1). At higher temperatures (≳278K), translational diffusion is more restricted in the gels, and possibly reflects the effect of long-range surface interactions. However, in the supercooled region where hydrogen bonding is extensive in the bulk, and leads to marked reductions in the rate of diffusion, the translational mobility of water in the gels is similar to that reported for the bulk. Changes in the inelastic scattering, due to intermolecular modes, reflect the increase in hydrogen bonding which occurs on supercooling and also the structural change on freezing. It is also shown that partial freezing can occur at intermediate temperatures (ca. 260 K), where the water content of the gel is high and thus possibly less influenced by the silica surface.