Dipoles Of Water Molecules Research Articles

The environment of graphene probed by electrostatic force microscopy

We employ electrostatic force microscopy to study the electrostatic environment of graphene sheets prepared with the micromechanical exfoliation technique. We detect the electric dipole of residues left from the adhesive tape during graphene preparation, as well as the dipole of water molecules adsorbed on top of graphene. Water molecules form a dipole layer that can generate an electric field as large as ∼109Vm−1. We expect that water molecules can significantly modify the electrical properties of graphene devices.

Effect of Surface Polarity on Water Contact Angle and Interfacial Hydration Structure

We perform molecular dynamics simulations of water in the presence of hydrophobic/hydrophilic walls at T = 300 K and P = 0 GPa. For the hydrophilic walls, we use a hydroxylated silica model introduced in previous simulations [Lee, S. H.; Rossky, P. J. J. Chem. Phys. 1994, 100, 3334. Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G.; Phys. Rev. E 2006, 73, 041604.]. By rescaling the physical partial atomic charges by a parameter 0 <or= k <or= 1, we can continuously transform the hydrophilic walls (hydroxylated silica, k = 1) into hydrophobic apolar surfaces (k = 0). From a physical point of view, k is the normalized magnitude of a surface dipole moment, and thus it quantifies the polarity of the surface. We calculate the contact angle of water for 0 <or= k <or= 1. We find that, at least for the present homogeneous, atomically flat, and defect-free surface model, the magnitude of the surface dipole correlates with the contact angle in a one-to-one correspondence. In particular, we find that polar surfaces with 0 < k <or= kc = 0.4 are macroscopically hydrophobic; that is, the contact angle is larger than 90 degrees . For the cutoff value k = kc, the magnitude of the dipole moment of the polar silica surface unit is 41% that of the water molecule dipole moment. We also study the water orientation distributions next to the walls (a microscopic property). We find that these distributions also correlate with the contact angle in a one-to-one correspondence. Thus, the structure of confined water, the surface polarity, and the contact angle are in a direct correspondence to each other, and therefore, each quantifies the hydrophobicity/hydrophilicity of the surface.

The structure and dynamics of water inside armchair carbon nanotube

In this paper we present some simulation results about the behaviour of water molecules inside a single wall carbon nanotube (SWNT). We find that the confinement of water in an SWNT can induce a wave-like pattern distribution along the channel axis, similar phenomena are also observed in biological water channels. Carbon nanotubes(CNTs) can serve as simple nonpolar water channels. Molecular transport through narrow CNTs is highly collective because of tight hydrogen bonds in the protective environment of the pore. The hydrogen bond net is important for proton and other signal transports. The average dipoles of water molecules inside CNTs (7,7), (8,8) and (9,9) are discussed in detail. Simulation results indicate that the states of dipole are affected by the diameter of SWNT. The number of hydrogen bonds, the water–water interaction and water–CNT interaction are also studied in this paper.

Multiple Time Scales in Solvation Dynamics of DNA in Aqueous Solution: The Role of Water, Counterions, and Cross-Correlations

Recent time domain experiments have explored solvation dynamics of a probe located inside a DNA duplex, in an effort to gain information, e.g., on the dynamics of water molecules in the DNA major and minor grooves and their environment. Multiple time constants in the range of a few picoseconds to several nanoseconds were obtained. We have carried out 15 ns long atomistic molecular dynamics simulations to study the solvation dynamics of bases of a 38 base-pair long DNA duplex in an aqueous solution containing counterions. We have computed the energy-energy time correlation function (TCF) of the four individual bases (A, T, G, and C) to characterize the solvation dynamics. All the TCFs display highly nonexponential decay with time. When the trajectories are analyzed with 100 fs time resolution, the TCF of each base shows initial ultrafast decay (with tau1 approximately equal 60-80 fs) followed by two intermediate components (tau2 approximately equal 1 ps, tau3 approximately equal 20-30 ps), in near complete agreement with a recent time domain experiment on DNA solvation. Interestingly, the solvation dynamics of each of the four different nucleotide bases exhibit rather similar time scales. To explore the existence of slow relaxation at longer times reported recently in a series of experiments, we also analyzed the solvation TCFs calculated with longer time trajectories and with a larger time resolution of 1 ps. In this case, an additional slow component with a time constant of the order of 250 ps is observed. Through an analysis of partial solvation TCFs, we find that the slow decay originates mainly from the interaction of the nucleotides with the dipolar water molecules and the counterions. An interesting negative cross-correlation between water and counterions is observed, which makes an important contribution to relaxation at intermediate to longer times.

Development of a Microwave System for Highly-Efficient Drying of Fish

Dried fish is an important product of Nakhon Si Thammarat province, located in southern Thailand. Fish are conventionally dried using heat from the sun or heat from burning wood as energy sources. These drying methods have problems such as low efficiency and environmental problems. Exploiting the strong electric dipole of the water molecules in the fish, which allows the fish to absorb microwave energy effectively, we have developed a novel microwave heating system for the efficient drying of fish. The system utilizes a high-voltage power supply so that the magnetron can generate a microwave field continuously, and its output power can be adjusted from 0 - 200 W making it very different to commercial microwave oven. The waveguide is designed for effective transmission of microwave fields into the multi-mode heating cavity. The experimental results reveal that heat produced by the microwave system causes evaporation of moisture from the fish making it possible to produce high quality dried fish. The drying process also shows a dependence of fish surface temperature and moisture content on the radiation time and microwave power.

Fluctuating Hydration Structure around Nanometer-Size Hydrophobic Solutes. I. Caging and Drying around C60 and C60H60 Spheres

The hydration structure around nanometer-size hydrophobic solutes is studied with molecular dynamics simulation by taking aqueous solutions of C60 and C60H60 as examples. In the hydration shell around a single C60 or C60H60, dipoles of simulated water molecules tend to be aligned to form the vortexlike coherent pattern which lasts for 100 ps, while individual water molecules stay within the hydration shell for about 10 ps. This structural pattern organized by fluctuating and diffusively moving molecules should be called a "fluctuating cage". In the narrow region between a pair of C60 molecules or a pair of C60H60 molecules, water density strongly fluctuates and is correlated to the mean force between solutes. The fluctuating caging and drying between solutes affect the hydrophobic interaction and dynamical behaviors of solutes.

The dielectric properties of humid cellulose

The electrical characteristics of cellophane have been measured at variable moisture contents and temperatures in the frequency range 10 −3–10 5 Hz. The data show very strong dependence on moisture content, which is typical of hydrophilic solids, and exhibit constant phase angles over wide frequency ranges. Complex admittance is proportional to ( jω) −( β−1) where ω is the frequency and β ≈ 0.95. We use a model of hopping charge transport to explain the frequency and moisture dependence of the measured spectra. At low frequencies conduction occurs predominantly by phonon-assisted proton hopping between absorbed water molecules, with β dependent on the energetic disorder of the protonic states. The moisture dependence of the conductivity σ is shown to arise from its exponential dependence on the critical percolation distance R C and to have the form exp{− BN −1/3}, where N is the water concentration and B is a constant. This result explains the universally observed logarithmic dependence on N of the low frequency conductivity in hydrophilic solids. The model also explains the characteristic low frequency dispersion observed in solid ionic conductors. In hydrophilic solids in-phase charge transport due to rotating water molecule dipoles increases with frequency and eventually becomes the dominant contribution to the total conductivity.

Dispersive ionic space charge relaxation in solid polymer electrolytes. I. Experimental system polyethylene oxide

As a model system for solid polymer electrolytes thin polyethylene oxide (PEO) films (0.6–30 μm) are prepared as Al–PEO–Al and as Al–PEO–Si structures with blocking electrodes. Dielectric measurements (capacitance and loss angle) are carried out in a frequency range of 3 mHz⩽f⩽1 MHz in atmospheres of different relative humidities (0%–75%) and at different temperatures (293–313 K). The nominal permittivity varies from 4 at high frequencies to up to 150 000 at low frequencies. We find true volume polarization in the high frequency range and a thermally activated space charge relaxation process in the low frequency range. Its time constant exhibits a remarkable dependence on the sample thickness, the relative humidity, the ion concentration, and the temperature. Due to the absorbed dipolar water molecules the activation energy of this process is decreased, leading to higher mobility of the ions. An increase in ion concentration also leads to a lower activation energy. The ions are able to move through the sample to the electrode interfaces and build up ionic space charge. We assume that at the interfaces oxide layers are formed, and these are almost impermeable to these ions. The space charge relaxation process has a broad frequency spectrum. The imaginary part of the nominal permittivity εr″(ω)∝ωβ has a slope of β=−0.8 to 0.7 above the main relaxation frequency. In the time domain, after application of a voltage step a Kohlrausch behavior of the relaxation current is found. In the short time range we find a current of j∝t−0.3 and in the long time range we find j∝t−1.1. A comparison of the frequency and the time domain by Fourier transform shows that the space charge relaxation process is time invariant for small applied voltages. Nonlinear properties that appear at higher voltages are investigated in the frequency domain and in the time domain.

Interfacial water dielectric-permittivity-profile measurements using atomic force microscopy.

The arrangement of water molecules at charged aqueous interfaces is an important question in biology, electrochemistry, and geochemistry. Theoretical studies suggest that the molecules become arranged in several layers adjacent to a solid interface. Using atomic force microscopy we have measured the water dielectric-permittivity profile perpendicular to mica surfaces. The measured variable permittivity profile starting at epsilon approximately 4 at the interface and increasing to epsilon=80 about 10 nm from the surface suggests a reorientation of water molecule dipoles in the presence of the mica interfacial charge.

Dielectric exchange force: a convenient technique for measuring the interfacial water relative permittivity profile

Water relative permittivity profiles perpendicular to mica surfaces have been measured by atomic force microscopy using the force acting on uncharged tips when immersed in the mica double-layer. This force is modelled by the gradient of the electrostatic energy variation (dielectric exchange force) involved in the immersion of the tip with a relative permittivity εTip in the double layer region with εDL. The measured variable permittivity profile starting at ε≈4 at the interface and increasing to ε≈80 about 10 nm from the surface suggests a reorientation of water molecule dipoles in the presence of mica interfacial charges. Changes in water polarization are therefore responsible for the hydration or structural forces acting on the tips immersed in the inner double layer. Corroboration for the proposed model (dielectric exchange force) is given by the observation of an attractive force when metal-coated tips (εTip≈∞) are immersed in the mica double layer. Support for the change in the water relative permittivity at the interface is given by measurements of only a repulsive force component when silicon nitride and silicon tips are immersed in solvent where there is no interaction between the mica surface and the solvent and, consequently, no solvent structuring at the interface.

Structural, electronic, and bonding properties of liquid water from first principles

We study, from first principles, structural, electronic, and bonding properties of liquid water. Our system is twice as large as that used in previous ab initio simulations and our computed structural properties are in good agreement with the most recent neutron scattering experiments. Moreover, the use of a novel technique, based on the generation of maximally localized Wannier functions, allowed us to describe the molecular charge distribution and the polarization effects in liquid water with a degree of accuracy not previously possible. We find that, in the liquid phase, the water molecule dipole moment has a broad distribution around an average value of about 3.0 D. This value is 60% higher than that of the gas phase and significantly larger than most previous estimates. A considerable increase is also observed in the magnitude of the average eigenvalues of the quadrupole moment tensor. We also find that the anisotropy of the electronic charge distribution of the water molecule is reduced in the liquid. The relevance of these results for current modeling of liquid water is discussed.

Erratum: Water Molecule Dipole in the Gas and in the Liquid Phase [Phys. Rev. Lett. 82, 3308 (1999)

Erratum: Water Molecule Dipole in the Gas and in the Liquid Phase [Phys. Rev. Lett. 82, 3308 (1999)

Comparison of Dielectric Relaxations of Water Mixtures of Poly(vinylpyrrolidone) and 1-Vinyl-2-pyrrolidinone

Microwave dielectric studies on the dynamics of water in poly(vinylpyrrolidone) (PVP)−water mixtures and 1-vinyl-2-pyrrolidinone (VP)−water mixtures were performed to compare the relaxation processes of water in polymer−water and monomeric compound−water mixtures. For the PVP−water mixtures, a relaxation peak for water was observed and the shape of the absorption curve was symmetrical about the logalism of frequency, which the Cole−Cole equation describes well. In this case, the polymer behaves as a geometric constraint for rotational motion of water molecules, and a variation of the local structure of water yields the symmetrical relaxation curve. On the other hand, one relaxation peak due to the rotational motion of the dipoles of water and VP molecules was also observed for VP−water mixtures. However, the shape of the absorption curve is asymmetric and is described well by the KWW function. Cooperative motion of water and VP molecules resulted in a variation of local structure which is observed as an asymmetric relaxation curve.

Water Molecule Dipole in the Gas and in the Liquid Phase

We study with ab initio molecular dynamics the change that the electric dipole moment of water molecules undergoes in passing from the gas to the liquid phase. Our analysis is based on the recently introduced maximally localized Wannier functions and is devoid of the ambiguities that have affected previous attempts. We find that in the liquid the dipole moment has an average value of about 3 D, 60% higher than in the gas phase. This value is much larger than is currently assumed (2.6 D). Furthermore, a broad distribution around this average value is observed. The relevance of these results for current modeling of water is discussed.

Observation of chloride-ion hydration in high-temperature liquid and supercritical water by spherical harmonic expansion analysis

The structure of the Cl− hydration shell in high-temperature (77 and 197 °C) and supercritical (375 °C) water at moderate densities (roughly twice the critical density of 0.356 g/cm3) as well as in water at an ambient temperature, is investigated by means of neutron diffraction with isotope substitution on Cl− in aqueous 3 and 9 m solutions of LiCl in D2O. These relatively high densities (compared to the critical density) are used here to maintain the salt solubility at a sufficiently high value that a measurable signal is obtained in the isotope difference. The data are subjected to an empirical potential structure refinement which enables individual site-site radial distribution functions to be estimated. The same structure refinement is also used to estimate the spherical harmonic coefficients of the Cl-water orientational pair correlation function, and these are used in turn to reconstruct the local environment around Cl− in solution. A marked disordering of the Cl− hydration shell is observed as the critical point is approached. The orientational correlation functions of the water-molecule dipole vectors around Cl− in ambient water show a strong peak at a distance of about 3.2 Å, and centered at an angle of about 126°±30°, with respect to the Cl-O(D2O) axis, confirming an almost linear Cl⋯D-O hydrogen bond but with a significant spread of orientations about this value. At the higher-temperatures the peak remains at about the same position, but becomes progressively weaker in amplitude and with a larger spread of angles, suggesting a gradual decrease in orientational correlation of the water molecules around Cl−. At the supercritical temperature, the peak is even weaker, and there is evidence for a broader hydration shell developing at a distance of r≈3.9 Å in which the water molecules are apparently randomly oriented: they apparently become more loosely packed than at lower temperatures, without being dispelled from the ion hydration shell.

Ferroelectric ice

Three recent papers have demonstrated the existence of ‘ferroelectric’ ice, in which some of the dipolar water molecules are aligned. The evidence came from experiments with normal ice, either grown on a substrate or doped with impurities, and although it resolves the debate about whether such ice exists, it reopens the thermodynamic question of whether nature prefers order at low temperatures.

The polarization model in bacterial photosynthesis.

The widely accepted model of reaction center /RC/ functioning is proved to come into contradiction with some recent data. In particular, it cannot explain why only a minor part of electronic excitations (approximately 10%) escapes from excited RC special pairs back to antenna BChls. Therefore we believe that the model must be substantially modernized. In 1981 we developed a new model/1,2/. We suggested a femtosecond state to precede primary e-transfer reaction due to reorientation of water molecule dipole in the electric field of excited RC dimer. This mechanism is responsible for energy trapping before the primary e-transport occurs. During last years his mechanism got support from various experimental works. Now this polarization model claims to fit all reliable experimental data at least in bacterial photosynthesis.

Spectroscopic and dielectric properties of liquid water: A molecular dynamics simulation study

The spectroscopic and dielectric properties of liquid water under an ambient condition are studied via a molecular dynamics (MD) computer simulation method. By employing the recent TAB/10D potential model [B. D. Bursulaya, J. Jeon, D. A. Zichi, and H. J. Kim, J. Chem. Phys. 108, 3286 (1997)], the evolving solvent electronic structure is incorporated into the simulation. Thus both the induced dipole and polarizability variations of individual water molecules with the fluctuating nuclear configuration are accounted for. The MD results on far-IR absorption, depolarized Raman scattering (DRS) and optical Kerr effect (OKE) spectroscopy are in reasonable agreement with experiments. It is found that the nonlinear electronic response of water to its fluctuating environment plays an important role in the DRS and OKE; it significantly enhances the contribution of the water librational motions to the spectra, compared with that of hindered translations. This indicates that not only molecular dynamics but also accompanying electronic structure modulations are essential to quantitative understanding of various electronic spectroscopy. The effects of H/D isotope substitution are briefly discussed.

Ionic Space Charge Relaxation and High Dielectric Permittivity in Polyethylene Oxide

ABSTRACTThin films (0.3μm to 11.2 μm) of Polyethylene Oxide (PEO) with molecular weights from 6 × 103 to 4 × 105 were prepared from aqueous solutions by a spin technique as AI-PEO-AI structures, or as AI-PEO-Si structures. Dielectric measurements (capacitance and loss angle) were carried out in a frequency range 3 mHz ≤ f ≤ 1 MHz in atmospheres of different relative humidity (0% r.h. to 75% r.h.) and at different temperatures (293 K to 323 K). The nominal dielectric permittivity exhibits a remarkable dependence on the sample thickness and the relative humidity. We find a true volume polarization in the high frequency range and a thermally activated relaxation process in the low frequency range, whose time constant is shifted towards high frequencies with increasing r.h.. It is considered that due to the absorbed dipolar water molecules chemical bonds within the sample are broken and quasi-free ions are generated. These ions move through the sample to the electrode interfaces and form an ionic space charge. We assume that at the PEO-Al interface an oxide layer is formed, which is impermeable for these ions. The transit times and the drift velocities of the ions are almost independent of the electric field strength in the low-field limit. Therefore we conclude that the movement of the ions can be described by a multiwell potential model, where the transition probability between neighbored wells is thermally activated.

A MECHANISTIC VIEW OF THE NON-IDEAL OSMOTIC AND MOTIONAL BEHAVIOR OF INTRACELLULAR WATER

It is commonly assumed that essentially all of the water in cells has the same ideal motional and colligative properties as does water in bulk liquid state. This assumption is used in studies of volume regulation, transmembrane movement of solutes and electrical potentials, solute and solution motion, solute solubility and other phenomena. To get at the extent and the source of non-ideally behaved water (an operational term dependent on the measurement method), we studied the motional and colligative properties of water in cells, in solutions of amino acids and glycine peptides whose surface characteristics are known, and in solution of bovine serum albumin, hemoglobin and some synthetic polypeptides. Solutions of individual amino acids with progressively larger hydrophobic side chains showed one perturbed water molecule (structured-slowed in motion) per nine square angstroms of hydrophobic surface area. Water molecules adjacent to hydrophobic surfaces form pentagonal structural arrays, as shown by X-ray diffraction studies, that are reported to be disrupted by heat, electric field, hydrostatic pressure and phosphorylation state. Hydrophilic amino acids demonstrated water destructuring (increased motion) that was attributed to dielectric realignment of dipolar water molecules in the electric field between charge groups. In solutions of proteins, several methods indicate the equivalent of 2–8 layers of structured water molecules extending beyond the protein surface, and we have recently demonstrated that induced protein conformational change modifies the extent of non-ideally behaved water. Water self-diffusion rate as measured in three different cell types was about half that of bulk water, indicating that most of the water in these cells was slower in motion than bulk water. In different cell types the extent of osmotically perturbed water ranged from less than half to almost all of the intracellular water. The assumption that essentially all intracellular water has ideal osmotic and motional behavior is not supported by the experimental findings. The non-ideality of cell water is an operational term. Therefore, the amount of non-ideally behaving water is dependent on the characteristics of water targeted, i.e. the measurement method, and a large fraction of it is explainable in mechanistic terms at a molecular level based on solute–solvent interactions.