Water Cluster Size Research Articles

Valence Ionization of Water Clusters: From Isolated Molecules to Bulk

The valence photoelectron spectra of water clusters are studied experimentally and by ab initio calculations. The size dependence of the vertical ionization energy of the outermost orbitals is explicitly shown. A shift toward lower values is observed. For small cluster sizes, it can be rationalized as an effect of charge delocalization as the system is becoming more extended. Ionization energies of larger clusters decrease linearly with inverse cluster radius and asymptotically approach the value of liquid water. In the calculations, we apply a reflection principle approach based on sampling a quantum mechanical distribution of different initial-state geometries to clusters. An excellent agreement of peak shapes calculated thus with measured ones is shown. Using additional polarization fields, the extension of this approach to the photoionization of liquid water is demonstrated. Upon deuteration of the water clusters, we experimentally and theoretically find slightly larger absolute values of the vertical ionization energies. We suggest that the measurement of electron ionization energies can be used as an alternative means to characterize water cluster sizes, which can complement the use of scaling laws.

The effects of magnetic fields on water molecular hydrogen bonds

In this paper, the experimental results on the effects of a magnetic field on water are reported. Purified water was circulated at a constant flow rate in a magnetic field. After this treatment, the physicochemical properties of water were changed, shown as the decrease of surface tension and the increase of viscosity over the treatment time. The water is more stable by magnetic treatment with less molecular energy and more activation energy, as shown from the calculation based on the results. The correlation time τ c was calculated in terms of spin–lattice relaxation time of proton NMR, which verified that the rotational motions got slow down after magnetic treatments. A two-phase model was set up to prove that the proportion of free water molecules was reduced. The results suggested that the average size of water clusters became larger by magnetic treatments.

Effect of Surface Oxygen and Temperature on External and Micropore Adsorption of Water in Single-Walled Carbon Nanotubes by Gravimetric and Spectroscopic Experiments

Gravimetric water adsorption experiments (T = 5, 20, and 35 °C and 0 < P/P o < 0.95) were performed on several chemically and structurally distinct samples of single-walled carbon nanotubes including two activated carbon samples. The isotherms followed the type V curve and were fitted to a semiempirical model which allowed the adsorptive contributions of primary sites and micropores (referred to here as pseudoexperimental isotherms) to be distinguished with statistical confidence. The isosteric heats of water adsorption calculated from experimental isotherms ranged between 46 and 58 kJ/mol. The same calculations were performed on the separated adsorptive components (functional groups and micropore isotherms) and were found to be 0.5-16 kJ/mol and 1-8.6 kJ/mol, respectively. These values are similar to those available in the current literature reportedly estimated by calorimetric and molecular simulation techniques. From semiempirical modeling, we were also able to qualitatively estimate temperature sensitive water specific sample properties such as the concentration of primary sites (found directly related to % O) and the size of water clusters aggregating on primary sites (found inversely related to % O) and those filling micropores (found directly related to the dominant pore size) and adsorption equilibrium constants. We believe that our approach is useful in interpreting experimental water adsorption thus aiding purely simulation based methods of studying the behavior of water in nanocarbons.

Development of Homogeneous Water Condensation Models Using Molecular Dynamics

DOI: 10.2514/1.40360 A condensation phenomenon in free expansion plumes has been observed and studied in the past several decades. To model homogeneous condensation of small, polar molecules produced in the combustion products of chemical propellants, such as water, accurate microscopic cluster models need to be developed. In this work, the molecular dynamics method is used to develop a model of water cluster sizes, cluster–monomer collisions, and sticking probabilities necessary for the study of water homogeneous condensation in a plume expanding to low pressure, space conditions. The molecular dynamics results are integrated into a direct-simulation Monte Carlo simulation andthe simulated Rayleigh scatteringintensities arecompared with experimental datatotest the modelsanddirectsimulation Monte Carlo simulation results.

Thermodynamics of homogeneous nucleation of ice particles in the polar summer mesosphere

Abstract. We present the hypothesis of homogeneous nucleation of ice nano-particles in the polar summer mesosphere. The nucleation of condensed phase is traced back to the first step on the formation pathway, which is assumed to be the transition of water vapor to amorphous cluster. Amorphous clusters then freeze into water ice, likely metastable cubic ice, when they reach the critical size. The estimates based on the equilibrium thermodynamics give the critical size (radius) of amorphous water clusters as about 1.0 nm. The same estimates for the final transition step, that is the transformation of cubic to hexagonal ice, give the critical size of about 15 nm at typical upper mesospheric conditions during the polar summer (temperature T=150 K, water vapor density ρvapor=109 cm−3).

A molecular dynamic study of water/methane/propane

We present a molecular dynamic simulation for water/methane/propane. The simulation is performed in a constant NPT ensemble at a temperature of 240 K and a pressure of 300 bar. The simulation is analysed to calculate the propensity for hydrogen bond formation, potential energy, number density profile and radial distribution function. Compared with the binary system (water/methane), relatively slight changes in hydrogen bond number and potential energy for the ternary system (water/methane/propane) indicate that the presence of propane retards the rate of hydrate growth. It is interesting to observe that propane, which is also of hydrophobic nature as methane, does not promote the formation of hydrate, but is rather driven out of the middle aqueous region. No hydrate formation was observed even after all of the propane had been driven out of the water film. We suggest that the size of water clusters and hydrogen bond network is likely to be disrupted by propane molecules, hindering the growth of the nuclei to the critical size required for sustained growth. The presence of the propane might also affect the net flux of methane into the aqueous phase, which is critical for hydrate formation.

NMR and Viscosity Coefficients of CuCl&lt;sub&gt;2&lt;/sub&gt; and CuSO&lt;sub&gt;4&lt;/sub&gt; and Their Relationship to Water Structure

The viscosity coefficient (B) and nuclear magnetic resonance (NMR) coefficient (B') of CuCl2 and CuSO4 and their effects on the 17O NMR chemical shift (δ) of water were determined at low concentrations. From these B and B' values, individual ion's B and B' values were further calculated. These coefficients and the relationship between aqueous solution concentrations and δ(17OH2) represented the effect of ions on water structure and the median water cluster size in aqueous solutions. CuCl2 and CuSO4 were both ”structure-makers”. The effect of CuSO4 on water structure and median water cluster size was more powerful than that of CuCl2. Cl-broke more water clusters than SO(superscript 2- subscript 4). Cu(superscript 2+) had an obvious effect on shortening the proton spin-lattice relaxation time in water and broadening the line width of the NMR spectrum during the NMR relaxation process.

Nanophase segregation and water dynamics in hydrated Nafion: Molecular modeling and experimental validation

Reported results of coarse-grained molecular dynamics simulations rationalize the effect of water on the phase-segregated morphology of Nafion ionomers. We analyzed density maps and radial distribution functions and correlated them with domain structures, distributions of protogenic side chains, and water transport properties. The mesoscopic structures exhibit spongelike morphologies. Hydrophilic domains of water, protons, and anionic side chains form a random three-dimensional network, which is embedded in a matrix of hydrophobic backbone aggregates. Sizes of hydrophilic domains increase from 1 to 3 nm upon water uptake. At low water content, hydrophilic domains are roughly spherical and poorly connected. At higher water content, they convert into elongated cylindrical shapes with high connectivity. Further structural analysis provides a reasonable estimate of the percolation threshold. Radial distribution functions from coarse-grained and atomistic molecular dynamics models exhibit a good agreement. Water cluster size distributions from coarse-grained molecular dynamics and dissipative particle dynamics are consistent with small angle x-ray scattering data. Moreover, we calculated the water diffusivity by molecular dynamics methods and corroborated the results by comparison with pulsed field gradient NMR.

Percolation and clustering in supercritical aqueous fluids

Neutron diffraction data for pure supercritical water and supercritical mixtures of water andCO2 are analyzed by using the empirical potential structure refinement Monte Carlosimulation, and molecular configurations compatible with the experimental dataare recorded. The analysis of the distribution functions of water cluster size inthese fluids allows the identification of a percolation line, which separatesgas-like states from liquid-like ones, in pure supercritical water. Solvation ofCO2 in supercritical water inhibits cluster percolation, although the radial distribution functions showliquid-like behavior: this is likely to be due to the excess volume being localized around theCO2 molecules.

Application of water–activated carbon isotherm models to water adsorption isotherms of single-walled carbon nanotubes

The objective of this study is to understand the interactions of water with novel nanocarbons by implementing semiempirical models that were developed to interpret adsorption isotherms of water in common carbonaceous adsorbents. Water adsorption isotherms were gravimetrically determined on several single-walled carbon nanotube (SWNT) and activated carbon samples. Each isotherm was fitted to the Dubinin–Serpinsky (DS) equation, the Dubinin–Astakov equation, the cooperative multimolecular sorption theory, and the Do and Do equations. The applicability of these models was evaluated by high correlation coefficients and the significance of fitting parameters, especially those that delineate the concentration of hydrophilic functional groups, micropore volume, and the size of water clusters. Samples were also characterized by spectroscopic and adsorption techniques, and properties complementary to those quantified by the fitting parameters were extracted from the data collected. The comparison of fitting parameters with sample characterization results was used as the methodology for selecting the most informative and the best-fitting model. We conclude that the Do equation, as modified by Marban et al., is the most suitable semiempirical equation for predicting from experimental isotherms alone the size of molecular clusters that facilitate adsorption in SWNTs, deconvoluting the experimental isotherms into two subisotherms: adsorption onto hydrophilic groups and filling of micropores, and quantifying the concentration of hydrophilic functional groups, as well as determining the micropore volume explored by water. With the exception of the DS equation, the application of other water isotherm models to SWNTs is not computationally tractable. The findings from this research should aid studies of water adsorption in SWNTs by molecular simulation, which remains the most popular tool for understanding the microscopic behavior of water in nanocarbons.

Small-angle X-ray scattering study on structure of adsorbed molecular assemblies in carbon micropores

The in situ small-angle X-ray scattering (SAXS) of water and CCl4 adsorbed on pitch-based activated carbon fibers (ACF) were measured at 303 K. We extended the Debye-Bueche theory to the 3-phase mixed system (solid phase: carbon, adsorbed phase, and vacant space) to obtain the average dimension of each phase (ls, la, and lv). The la of water in wider pore system (pore width: ω=1.13nm) increases with the fractional filling φ. On the other hand, la in smaller pore system (ω=0.75nm) does not change with φ. These results show the difference in the cluster growing process with the pore width. It seems that the difference in the size and shape of water clusters in the adsorption and desorption processes should lead to the adsorption hysteresis. The developed analysis method may give the direct information on molecular assemblies in micropores.

Surface modification of perfluorosulfonic acid membranes with perfluoroheptane (C 7F 16)/argon plasma

This research focuses on the surface modification of a perfluorosulfonic acid (PFSA) membrane using plasma treatment. The plasma gas was a mixture of perfluoroheptane (C 7F 16) and argon (Ar). The results indicated that the plasma mixture led to the deposition of a thin fluorocarbon polymerized top layer onto the PFSA membrane. The effects of plasma operating conditions (plasma power, reactor pressure, and treatment time) on the product properties were analyzed using a factorial design. The radio-frequency (RF) power was the most influencing factor. The film deposition rate, surface roughness, and cross-linking of the top fluorocarbon film increased with RF power, associating with a higher contact angle and lower methanol permeability in the resulting membrane. Higher RF power also resulted in a decreased ion exchange capacity and water uptake in the membranes, mainly due to the lack of sulfonate functional groups and the cross-linked fluorocarbon top layer. However, the number of water molecules associated with each sulfonate group did not deviate from that of the pristine membrane, indicating that the water cluster size was not significantly altered by the plasma modification process. The effective electrical conductivity contribution from the deposited top layer was at least five orders of magnitudes higher than other fluorocarbon film. Meanwhile, the methanol permeability of the treated membrane rendered a two-order magnitude decrease, which was ascribed to the combined effect of the more hydrophobic surface and the higher level of cross-linking in the deposited fluorocarbon top layer. The suppressed methanol crossover in the modified membranes offers advantages for direct methanol fuel cell operations at high methanol concentrations.

Assessment of Two Surface Monte Carlo (TSMC) method to find stationary points of (H2O)15 and (H2O)20 clusters

The Two Surface Monte Carlo (TSMC) technique reduces computational cost by using a computationally cheap biasing potential, which guides the molecular system to explore the potential energy surface of interest. It was shown earlier that the Effective Fragment Potential (EFP) can be a good choice for this biasing potential (Bandyopadhyay, J Chem Phys 122:091102, 2005) when the potential energy surface of interest is quantum mechanical. This may help in expanding the applicability of TSMC, since finding a good biasing potential is a major challenge. In the present work, the viability of TSMC method in finding stationary points of large molecular system is investigated using EFP as the biasing potential and RHF theory as the potential of interest. TSMC is applied to find the stationary points of water clusters of size 15 and 20. A semi-automated method starting from random geometries, without using any chemical intuition, found several stationary points. The simulated annealing method was used to refine the structures obtained from TSMC. Among the several low-energy structures obtained for 15 water cluster, one minimum, about 1 kcal/mol higher than the global minimum, was found. However, for 20 water cluster, no structure very close to the global minimum was obtained. Several strategies, learned from the experience of the present work, are discussed for improving the TSMC method, including the acceptance between the two energy surfaces.

In-situ Small/Wide-Angle Neutron Scattering Studies of the Cluster Structure in Polyelectrolyte Membrane for Fuel Cells

Proton conductivity of Nafion® membrane is varied by humidity and it has been thought to be affected by the cluster structure of the membrane. We applied Small-Angle Scattering technique under humidity-controlled atmosphere with X-ray (SAXS) and neutron (SANS) to clarify the relationship between the cluster structure and molecular structure in two types of Nafion® membrane, N115 and NE151F, which have different equivalent weight (EW). The proton conductivity of N115 is higher than that of NE151F. By these two measurements, three different sized periodic structures were observed in the Nafion® membrane. Contrast variation method (D/H=60/40, 75/25, 80/20, 90/10) was also applied in SANS experiments and it was suggested that two of three peaks are originated from two different sizes of water clusters. A distinguishing peak at q=0.2 [A-1], which shifts to lower q region by humidity increase, was reproduced by a simulation of Dissipative Particle Dynamics (DPD): the shifts of the peak was interpreted as the swelling of cluster structure. The size of the cluster calculated from the peak position is positively correlated with the proton conductivity. Finally, The effect of EW on the proton conductivity of Nafion® membrane was briefly discussed from the point of its cluster structure.

Do Hydroxyl Radical−Water Clusters, OH(H2O)n, n = 1−5, Exist in the Atmosphere?

It has been speculated that the presence of OH(H2O)n clusters in the troposphere could have significant effects on the solar absorption balance and the reactivity of the hydroxyl radical. We have used the G3 and G3B3 model chemistries to model the structures and predict the frequencies of hydroxyl radical/water clusters containing one to five water molecules. The reaction between hydroxyl radical clusters and methane was examined as a function of water cluster size to gain an understanding of how cluster size affects the hydroxyl radical reactivity.

Effects of ions in natural water on the 17O NMR chemical shift of water and their relationship to water cluster

Effects of temperature and some aqueous solutions of electrolytes consisting of ions in natural water on the chemical shifts of NMR of water ( 17O NMR) have been measured. Combining the cluster model of liquid water structure and the dependence of δ( 17OH 2) on temperature a relationship deduced of δ( 17OH 2) to median water cluster size effects of ions on median water cluster and the size are analyzed. The relationships between 17O NMR and concentration of ions are linear. Cations increase the median water cluster size and anions decrease it. The bigger the radius of ions with equal ionic charge, the stronger the effect of the ion. The extent of the ionic charge effect is much bigger than that of the ionic radius. The order of the effect of cations and anions is Na +<K +<Mg 2+<Ca 2+<Al 3+<Fe 3+, and OH −<HCO 3 −<Cl −, and CO 3 2−<SO 4 2−, respectively.

Direct Simulation Monte Carlo Modeling of Homogenous Condensation in Supersonic Plumes

A particle simulation method to model water condensation process in a supersonic rocket plume is proposed and developed. Classic nucleation theory is used to predict nucleation, condensation, and the evaporation rates for water clusters. Microscopic kinetic models are developed to simulate collision processes between water clusters and monomers, between water clusters and foreign molecules, and evaporation of monomers from water clusters. These models are integrated into the direct simulation Monte Carlo method to simulate an axisymmetric multispecies gas expansion coupled with condensation. The developed computational scheme, first verified by empirical scaling laws of condensation in supersonic microjets, is then applied to predict the spatial distributions of water cluster number density, size, and temperature in a rocket exhaust plume. An empirical equation is used to correct classical nucleation rate, condensation results are compared between the original and corrected nucleation rate, and the impact of the nucleation rate on a flow with condensation is discussed in detail.

Nanospace Molecular Science and Adsorption

The relationships between the enhanced interaction potential and intensive confinement effect of slit-shaped and cylindrical nanospaces are shown. The structures of water molecules and aqueous ions confined in nanospaces of activated carbon fiber (ACF) and single wall carbon nanohorn (SWNH)s were studied by adsorption, in situ small angle X-ray scattering(SAXS), GCMC simulation, and EXAFS spectroscopy. Water molecules are associated with each other to form the clusters, being stabilized in the carbon nanospaces. This stabilization mechanism of water in carbon nanospaces were evidenced by the interaction potential calculation, GCMC simulation, and the density fluctuation analysis of in situ SAXS. The Ornstein-Zernike analysis of in situ SAXS profiles lead to the conclusion that the critical size of water clusters for predominant water adsorption in hydrophobic carbon nanospaces is about 0.5 nm corresponding to the octomer to decamer. The adsorption hysteresis of water adsorption isotherm of nanoporous carbon was interpreted by the cluster growth, which is confirmed by the density fluctuation analysis. The Rb and Br ions confined in the carbon nanospaces were examined by EXAFS spectroscopy. The remarkable decreases in the hydration number and the water-Rb ion distance of the solution confined in the nanospaces were observed. In particular, the hydration number of the Rb ion in the nanospaces of SWNH is less than 3, being much smaller than the hydration number (6) of the bulk solution. The electrical double layer structure in the nanospaces should be quite different from that in the bulk solution.

Simulation of Water Cluster Assembly on a Graphite Surface

The assembly of small water clusters (H2O)n, n = 1-6, on a graphite surface is studied using a density functional tight-binding method complemented with an empirical van der Waals force correction, with confirmation using second-order Møller-Plesset perturbation theory. It is shown that the optimized geometry of the water hexamer may change its original structure to an isoenergy one when interacting with a graphite surface in some specific orientation, while the smaller water cluster will maintain its cyclic or linear configurations (for the water dimer). The binding energy of water clusters interacting with graphite is dependent on the number of water molecules that form hydrogen bonds, but is independent of the water cluster size. These physically adsorbed water clusters show little change in their IR peak position and leave an almost perfect graphite surface.

Accurate Energies and Structures for Large Water Clusters Using the X3LYP Hybrid Density Functional

AbstractFor Abstract see ChemInform Abstract in Full Text.