Anisotropic dynamics of dipolar liquids in narrow slit pores

We report molecular dynamics simulation results for Stockmayer fluids confined to narrow slitlike pores with structureless, nonconducting walls. The translational and rotational dynamics of the dipolar particles have been investigated by calculating autocorrelation functions, diffusion coefficients, and relaxation times for various pore widths (five or less particle diameters) and directions parallel and perpendicular to the walls. The dynamic properties of the confined systems are compared to bulk properties, where corresponding bulk and pore states at the same temperature and chemical potential are determined in parallel grand canonical Monte Carlo simulations. We find that the dynamic behavior inside the pore depends on the distance from the walls and can be strongly anisotropic even in globally isotropic systems. This concerns especially the particles in the surface layers close to the walls, where the single particle and collective dipolar relaxation resemble that of true two-dimensional dipolar fluids with different in-plane and out-of-plane relaxations. On the other hand, bulklike relaxation is observed in the pore center of sufficiently wide pores.     

Does transparent nematic phase exist in 5CB∕DDAB∕water microemulsions? From the viewpoint of temperature dependent dielectric spectroscopy

Liquid crystal colloids have received tremendous attention because of its great potential both in the understanding of the liquid crystalline phase and in searching for new application of liquid crystals. Inverse microemulsion composed of 4-cyano-4-n-pentylbiphenyl (5CB), didodecyl dimethyl ammonium bromide, and water was investigated by means of broadband dielectric spectroscopy in this study. Based on the understanding of previous investigations on the same system, the isotropic phase was taken into account to quantitatively characterize the bulklike relaxations after the isotropic-to-nematic phase transition. Analogous results concerning the phase transition and phase composition to other investigations were obtained. In addition to bulklike relaxations, a new relaxation was observed at the frequency range about two orders lower than bulklike relaxations. This new relaxation shows abnormal temperature dependence, suggesting that superstructures composed of water droplets and confined 5CB molecules exist. This superstructure possibly possesses a confined nanoscaled liquid crystal ordering and may correspond to the notion of the transparent nematic phase.     

Liquid water confined in carbon nanochannels at high temperatures

Structure, hydrogen bonding, electrostatics, dielectric, and dynamical properties of liquid water confined in flat graphene nanochannels are investigated by molecular dynamics simulations. A wide range of temperatures (between 20 and 360 degrees C) have been considered. Molecular structure suffers substantial changes when the system is heated, with a significant loss of structure and hydrogen bonding. In such case, the interface between adsorbed and bulk-like water has a marked tendency to disappear, and the two preferential orientations of water nearby the graphite layers at room temperature are essentially merging above the boiling point. The general trend for the static dielectric constant is its reduction at high temperature states, as compared to ambient conditions. Similarly, residence times of water molecules in adsorbed and bulk-like regions are significantly influenced by temperature, as well. Finally, we observed relevant changes in water diffusion and spectroscopy along the range of temperatures analyzed.     

Structure of water nanoconfined between hydrophobic surfaces

We report the results of a series of molecular-dynamics simulations of liquid water confined between two graphite plates with separations ranging from 7 to 15 A. Energies and free energies are provided, indicating also the corresponding stability density span of confined water phases. The structure of the different liquid layers is also discussed for all the considered systems. In particular, we studied atomic density profiles, two-dimensional radial distribution functions, hydrogen bonding, and angular orientations near the carbon plates.     

Testing the core/shell model of nanoconfined water in reverse micelles using linear and nonlinear IR spectroscopy

A core/shell model has often been used to describe water confined to the interior of reverse micelles. The validity of this model for water encapsulated in AOT/isooctane reverse micelles ranging in diameter from 1.7 to 28 nm (w0 = 2-60) and bulk water is investigated using four experimental observables: the hydroxyl stretch absorption spectra, vibrational population relaxation times, orientational relaxation rates, and spectral diffusion dynamics. The time dependent observables are measured with ultrafast infrared spectrally resolved pump-probe and vibrational echo spectroscopies. Major progressive changes appear in all observables as the system moves from bulk water to the smallest water nanopool, w0 = 2. The dynamics are readily distinguishable for reverse micelle sizes smaller than 7 nm in diameter (w0 = 20) compared to the response of bulk water. The results also demonstrate that the size dependent absorption spectra and population relaxation times can be quantitatively predicted using a core-shell model in which the properties of the core (interior of the nanopool) are taken to be those of bulk water and the properties of the shell (water associated with the headgroups) are taken to be those of w0 = 2. A weighted sum of the core and shell components reproduces the size dependent spectra and the nonexponential population relaxation dynamics. However, the same model does not reproduce the spectral diffusion and the orientational relaxation experiments. It is proposed that, when hydrogen bond structural rearrangement is involved (orientational relaxation and spectral diffusion), dynamical coupling between the shell and the core cause the water nanopool to display more homogeneous dynamics. Therefore, the absorption spectra and vibrational lifetime decays can discern different hydrogen bonding environments whereas orientational and spectral diffusion correlation functions predict that the dynamics are size dependent but not as strongly spatially dependent within a reverse micelle.     

Specific ion effects in aqueous eletrolyte solutions confined within graphene sheets at the nanometric scale

The underlying mechanisms of specific ion effects on structure and dynamics of aqueous solutions have been long debated. On the other hand, the role of polarization at hydrophobic interfaces when aqueous electrolytes are present is of great importance, as it has been observed at the air-vapor interface. In this work, we have explored influence of ionic species on microscopical properties of aqueous sodium halide solutions constrained inside a double layer graphene channel, as a model for a realistic hydrophobic interface. Our systems have been simulated by molecular dynamics techniques, explicitly including polarization in water molecules and ions. Water and ionic density profiles showed the tendency of ionic species to occupy the whole space available, in good agreement with spectroscopic experimental data. The exception to this general behavior was fluoride, which preferred to stay away from interfaces. Two main regions were defined: interfaces and the central part of the slab, the bulklike region. Ionic hydration numbers at interfaces were lower than those at the bulklike area by about one to two units. We have also analyzed water-ion orientations and polarization distributions and obtained a marked dependence on ionic concentration. Residence time of anions suffered important fluctuations and tended to be largest at interfaces. Large variations of the static permittivity between interfacial and bulklike regions were observed. Ionic diffusion was found to be between 10(-5) and 10(-6) cm(2) s(-1) and showed to be mainly dependent on the concentration, whereas the type of anion considered and the polarizability had significantly less relevance. Conductivities were found to be dependent on ionic concentrations and the polarizabilities of anions, as well as on the spatial direction considered.     

Radiowave dielectric investigation of water confined in channels of carbon nanotubes

Structure and dynamics of water confined in channels of diameter of few nanometer in size strongly differ from the ones of water in the bulk phase. Here, we present radiowave dielectric relaxation measurements on water-filled single-walled carbon nanotubes, with the aim of highlighting some aspects on the molecular electric dipole organization of water responding to high spatial confinement in a hydrophobic environment. The observed dielectric spectra, resulting into two contiguous relaxation processes, allow us to separate the confined water in the interior of the nanotubes from external water, providing support for the existence in the confinement region of water domains held together by hydrogen bonds. Our results, based on the deconvolution of the dielectric spectra due to the presence of a bulk and a confined water phase, furnish a significantly higher Kirkwood correlation factor, larger than the one of water in bulk phase, indicating a strong correlation between water molecules inside nanotubes, not seen in bulk water.     

Dynamics of water confined within reverse micelles

We report structural and dynamical properties of water confined within reverse micelles (RMs) ranging in size from R = 10 A to R = 23 A as determined from molecular dynamics simulations. The low-frequency infrared spectra have been calculated using linear response theory and depend linearly on the fraction of bulklike water within the RMs. Furthermore, these spectra show nearly isosbestic behavior in the region near 660 cm(-1). Both of these characteristics are present in previously measured experimental spectra. The single dipole spectra for interfacial trapped, bound, and bulklike water within the RMs have also been calculated and show region-dependent frequency shifts. Specifically, the bound and trapped water spectra have a peak at lower frequencies than that for the inner core water. We therefore assign the low-frequency band in the IR spectra to bound and trapped interfacial water. Finally, region-dependent hydrogen bonding profiles and spatial distribution functions are also presented.     

Ultrafast hydration dynamics in the lipidic cubic phase: Discrete water structures in nanochannels

We report here our studies of hydration dynamics of confined water in aqueous nanochannels (approximately 50 A) of the lipidic cubic phase. By systematically anchoring the hydrocarbon tails of a series of tryptophan-alkyl ester probes into the lipid bilayer, we mapped out with femtosecond resolution the profile of water motions across the nanochannel. Three distinct time scales were observed, revealing discrete channel water structures. The interfacial water at the lipid surface is well-ordered, and the relaxation dynamics occurs in approximately 100-150 ps. These dynamically rigid water molecules are crucial for global structural stability of lipid bilayers and for stabilization of anchored biomolecules in membranes. The adjacent water layers near the lipid interface are hydrogen-bonded networks and the dynamical relaxation takes 10-15 ps. This quasi-bound water motion, similar to the typical protein surface hydration relaxation, facilitates conformation flexibility for biological recognition and function. The water near the channel center is bulklike, and the dynamics is ultrafast in less than 1 ps. These water molecules freely transport biomolecules near the channel center. The corresponding orientational relaxation at these three typical locations is well correlated with the hydration dynamics and local dynamic rigidity. These results reveal unique water structures and dynamical motions in nanoconfinements, which is critical to the understanding of nanoscopic biological activities and nanomaterial properties.     

Far-infrared absorption of water clusters by first-principles molecular dynamics

Based on first-principle molecular dynamic simulations, we calculate the far-infrared spectra of small water clusters (H(2)O)(n) (n = 2, 4, 6) at frequencies below 1000 cm(-1) and at 80 K and at atmospheric temperature (T>200 K). We find that cluster size and temperature affect the spectra significantly. The effect of the cluster size is similar to the one reported for confined water. Temperature changes not only the shape of the spectra but also the total strength of the absorption, a consequence of the complete anharmonic nature of the classical dynamics at high temperature. In particular, we find that in the frequency region up to 320 cm(-1), the absorption strength per molecule of the water dimer at 220 K is significantly larger than that of bulk liquid water, while tetramer and hexamer show bulklike strengths. However, the absorption strength of the dimer throughout the far-infrared region is too small to explain the measured vapor absorption continuum, which must therefore be dominated by other mechanisms.     

Dynamics of water trapped between hydrophobic solutes

We describe the model dynamical behavior of the solvent between two nanoscopic hydrophobic solutes. The dynamics of the vicinal water in various sized traps is found to be significantly different from bulk behavior. We consider the dynamics at normal temperature and pressure at three intersolute distances corresponding to the three solvent separated minima in the free energy profile between the solutes with attractions. These three states correspond to one, two, and three intervening layers of water molecules. Results are obtained from a molecular dynamics simulation at constant temperature and pressure (NPT) ensemble. Translational diffusion of water molecules trapped between the two solutes has been analyzed from the velocity correlation function as well as from the mean square displacement of the water molecules. The rotational behavior has been analyzed through the reorientational dynamics of the dipole moment vector of the water molecule by calculating both first and second rank dipole-dipole correlation functions. Both the translational and reorientational mobilities of water are found to be much slower at the smaller separation and increases as the separation between solutes becomes larger. The occupation time distribution functions calculated from the trajectories also show that the relaxation is much slower for the smallest intersolute separation as compared to other wider separations. The sublinear trend in mean square displacement and the stretched exponential decay of the relaxation of dipolar correlation and occupation distribution function indicate that the dynamical behavior of water in the confined region between two large hydrophobic solutes departs from usual Brownian behavior. This behavior is reminiscent of the behavior of water in the vicinity of protein surface clefts or trapped between two domains of a protein.     

Water dynamics in graphite oxide investigated with neutron scattering

Graphite oxide is an inorganic multilayer system that preserves the layered structure of graphite but not the conjugated bond structure. In the past few years, detailed studies of the static structure of graphite oxide were carried out. This was mainly done by NMR investigations and led to a new structural model of graphite oxide. The layer distance of graphite oxide increases with increasing humidity level, giving rise to different spacings of the carbon layers in the range from 6 to 12 A. As a consequence, different types of motions of water and functional groups appear. Information about the mobility of the water molecules is not yet complete but is crucial for the understanding of the structure of the carbon layers as well as the intercalation process. In this paper, the hydration- and temperature-dependent dynamic behavior of graphite oxide will be investigated by quasielastic neutron scattering using the time-of-flight spectrometer NEAT at the Hahn-Meitner-Institut Berlin. The character of the embedded water does not change over a wide range of hydration levels. Especially the interlayer water remains tightly bound and does not show any translational motion. In samples with excess water, however, the water is also distributed in noninterlayer voids, leading to the observation of additional motions of bulklike or confined water. The dynamic behavior of hydrated graphite oxide can be described by a consistent model that combines two two-site jump motions for the motions of the water molecules and the motions of OH groups.     

A Model Electron Transfer Reaction in Confined Aqueous Solution

Liquid water confined within nanometer-sized channels exhibits a strongly reduced local dielectric constant perpendicular to the wall, especially at the interface, and this has been suggested to induce faster electron transfer kinetics at the interface than in the bulk. We study a model electron transfer reaction in aqueous solution confined between graphene sheets with classical molecular dynamics. We show that the solvent reorganization energy is reduced at the interface compared to the bulk, which explains the larger rate constant. However, this facilitated solvent reorganization is due to the partial desolvation by the graphene sheet of the ions involved in the electron transfer and not to a local dielectric constant reduction effect.     

A first principles simulation of rigid water

We present the results of Car-Parrinello (CP) simulations of water at ambient conditions and under pressure, using a rigid molecule approximation. Throughout our calculations, water molecules were maintained at a fixed intramolecular geometry corresponding to the average structure obtained in fully unconstrained simulations. This allows us to use larger time steps than those adopted in ordinary CP simulations of water, and thus to access longer time scales. In the absence of chemical reactions or dissociation effects, these calculations open the way to ab initio simulations of aqueous solutions that require time scales substantially longer than presently feasible (e.g., simulations of hydrophobic solvation). Our results show that structural properties and diffusion coefficients obtained with a rigid model are in better agreement with experiment than those determined with fully flexible simulations. Possible reasons responsible for this improved agreement are discussed.     

Transport properties and distribution of water molecules confined in hydrophobic nanopores and nanoslits

The transport properties, including the diffusivity and viscosity, of water confined in hydrophobic nanopores and nanoslits were studied by molecular dynamics simulations. The results show that the diffusion coefficient in nanopores and nanoslits is markedly lower than that in the bulk. But the viscosity is much larger than that in bulk. The parallel diffusion coefficient is obviously larger than the perpendicular ones. The diffusion coefficient in the channel pore is ever less than that in the slit pore at the same pore width, but the viscosity is larger. The temperature and density affect significantly the diffusivity and viscosity in nanopores and nanoslits. Lower density water exhibits some special characteristics on density profiles in nanopores and nanoslits at lower temperatures, and the density profiles show a change from homogeneous to inhomogeneous as the pore width is reduced. Even clusters occurred in micropores.     

Water confined in reverse micelles--probe tool in biomedical informatics

Water pools caged in reverse micelles have sizes comparable to the typical dimensions of aqueous cavities in cells and tissues. Therefore, these models of confined water can be extremely helpful in biomedical informatics. Here, we present a practical approach that facilitates the use of such models to interpreting data from measurements of the spectral density of water caged in cells and tissues. We start from the observation that water molecules confined in microscopic pools display both bulk-like and rotationally constrained dynamics. We show that the fraction of structured water molecules in a pool and the frequency of the orientational relaxation of these water molecules can be derived from basic molecular principles in terms of the geometrical dimension of the water pool. Then, we employ these equations to relate the dielectric and magnetic responses of confined water to the size of the water pool. The present study provides the basis of a mathematical model that can relate the magnetic and dielectric signals of water in cavities of cells and tissues to the dimensions of these cavities. The approach can be used to assess the degree of structural alteration of injured and pathological tissues from the patterns of the dielectric and magnetic relaxation of water in these tissues.     

Vibrational spectral diffusion and hydrogen bond dynamics in heavy water from first principles

We present a first-principles theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in heavy water without using any empirical model potentials. The calculations are based on ab initio molecular dynamics simulations for trajectory generation and a time series analysis using the wavelet method for frequency calculations. It is found that, in deuterated water, although a one-to-one relation does not exist between the instantaneous frequency of an OD bond and the distance of its associated hydrogen bond, such a relation does hold on average. The dynamics of spectral diffusion is investigated by means of frequency-time correlation and spectral hole dynamics calculations. Both of these functions are found to have a short-time decay with a time scale of approximately 100 fs corresponding to dynamics of intact hydrogen bonds and a slower long-time decay with a time constant of approximately 2 ps corresponding to lifetimes of hydrogen bonds. The connection of the slower time scale to the dynamics of local structural relaxation is also discussed. The dynamics of hydrogen bond making is shown to have a rather fast time scale of approximately 100 fs; hence, it can also contribute to the short-time dynamics of spectral diffusion. A damped oscillation is also found at around 150-200 fs, which is shown to have come from underdamped intermolecular vibrations of a hydrogen-bonded water pair. Such assignments are confirmed by independent calculations of power spectra of intermolecular motion and hydrogen bond kinetics using the population correlation function formalism. The details of the time constants of frequency correlations and spectral shifts are found to depend on the frequencies of chosen OD bonds and are analyzed in terms of the dynamics of hydrogen bonds of varying strengths and also of free non-hydrogen-bonded OD groups.     

疏水性微孔中水的结构和扩散性质的分子模拟

用分子动力学（MD）方法模拟了受限在疏水性微孔中的水的结构与动力学行为.分别考察了孔径、温度和压力对水在孔道方向的密度分布和自扩散系数的影响,计算了不同温度下水的径向分布函数.发现在小孔径的微孔中,随着温度的降低,水分子沿孔道的分布逐渐变得不均匀,最终导致气-液相分离,微孔孔道内有明显的分段现象.受限在小孔径微孔中水的自扩散系数大约为体相流体水的20％～30％,并且随着孔径的减小,自扩散系数也减小.同时还发现沿孔道方向的自扩散系数分量大约为孔径方向的4～5倍.提出了微孔中水自扩散系数的关联模型.     

Energy relaxation versus spectral diffusion of the OH-stretching vibration of HOD in liquid-to-supercritical deuterated water

The dynamics of vibrational energy relaxation (VER) of the OH-stretching vibration of HOD in liquid-to-supercritical heavy water is studied as a function of temperature and solvent density by femtosecond mid-infrared spectroscopy. Using the dielectric constant of the fluid both, the OH-stretching absorption frequency and the VER rate, can be correlated phenomenologically with the average hydrogen-bond connectivity within the random D2O network. This correlation enables the identification of thermodynamic conditions under which spectral diffusion due to hydrogen-bond breakage/formation is much faster than VER.     

Dynamics of water confined in the interdomain region of a multidomain protein

Molecular dynamics simulations are performed to study the dynamics of interfacial water confined in the interdomain region of a two-domain protein, BphC enzyme. The results show that near the protein surface the water diffusion constant is much smaller and the water-water hydrogen bond lifetime is much longer than that in bulk. The diffusion constant and hydrogen bond lifetime can vary by a factor of as much as 2 in going from the region near the hydrophobic domain surface to the bulk. Water molecules in the first solvation shell persist for a much longer time near local concave sites than near convex sites. Also, the water layer survival correlation time shows that on average water molecules near the extended hydrophilic surfaces have longer residence times than those near hydrophobic surfaces. These results indicate that local surface curvature and hydrophobicity have a significant influence on water dynamics.