Size Distribution Of Water Clusters Research Articles

Molecular Dynamics Insights into the Nanoscale Structural Organization and Local Interaction of Aqueous Solutions of Ionic Liquid 1-Butyl-3-methylimidazolium Nitrate.

Considering the growing number of applications of the aqueous ionic liquids (ILs), atomistic molecular dynamics (MD) simulations were used to probe the effect of water molar fraction, xw, ranging from 0.00 to 0.90, on the nanoscale local structure of 1-butyl-3-methylimidazolium nitrate, [bmim][NO3], IL. The results prove that, with water addition, the cation-anion, cation-cation, and anion-anion structural correlations are weakened, while strong anion-water and unconventional cation-water hydrogen bonds are formed in the solutions. Water molecules were detected as bridges between nitrate anions, and the water cluster size distribution at different xw's was investigated. Simulation shows a similar pattern of probability densities for water and anion around the acidic hydrogen atoms of the reference cation ring, while both species move away from the cation butyl chain. Increasing the water concentration to xw = 0.90 causes decreasing of the local arrangement of the nearest-neighboring cations, because of the weakening of cation-cation π-π stacking. In addition, this dilution reduces the probability of the in-plane cation-anion conformation, disrupts both the polar ionic network and nonpolar domains, and diminishes the nanoaggregation of the cation butyl chains compared to those of the neat IL. These results can rationalize the origins of the fluidity enhancements and transport property trends upon adding water to the imidazolium-based ILs. The current study proposes a deep atomistic-level insight into the complex coupling between water concentration, microscopic structure, and local interactions of aqueous imidazolium-based ILs with hydrophilic anions.

Tracing Local Nanostructure of the Aqueous Solutions of the Biocompatible [Cho][Gly] Ionic Liquid: Importance of Hydrogen Bond Attraction between Like-Charged Ions.

The neat and aqueous solutions of the cholinium glycinate ionic liquid (IL), [Cho][Gly], at different water mole fractions, xws, are studied by molecular dynamics simulations. The changes in the local nanostructure of systems with composition have been determined by calculation of various structural distribution functions. Hydrogen bond (H-bond) attractions determine the major relative orientations of the oppositely and like charged nearest neighbors. The cation-anion H-bonds mainly form between the hydrogen of the hydroxyl or methyl groups of the cation and the carboxylate oxygen of the anion. A preferred (antiparallel) arrangement between adjacent [Cho]+ cations is due to the effective H-bond between the hydroxyl oxygen and the methyl hydrogen sites that promotes the like-charge cluster formation. Adding water decreases the occurrence probability of the [Cho]+···[Gly]-···[Cho]+ bridge structure in the aqueous solutions due to the formation of the [Gly]-···HOH···[Gly]- structure via H-bonding. Observed density trend versus xw is interpreted based on an interstice model and investigating the water cluster size distribution. Finally, the effect of xw on the infrared (IR) vibrational spectra were studied and blue and red shifts were observed for the stretching and bending vibrational modes of the hydroxyl group of [Cho]+, respectively. Current findings will improve the efficient engineering design and task-specific applications of aqueous solutions of bio-ILs consist of [Cho]+ and amino acid anions.

Geochemical controls on wettability alteration at pore-scale during low salinity water flooding in sandstone using X-ray micro computed tomography

We performed a pore-scale multiphase flow experiment on a sandstone core sample. We imaged the core sample at initial oil saturation, residual oil saturations after high salinity and low salinity water flooding. Moreover, we examined fluid occupancy maps and water cluster size distribution at pore-scale before and after low salinity waterflooding. Furthermore, we performed a geochemical study to relate physicochemical process at sub-pore scale to account for in-situ wettability alteration at pore-scale.Micro-CT imaging shows that low salinity waterfloodng yielded 5% of residual oil saturation reduction after high salinity water flooding. Fluid occupancy maps within pore network show water film propagation at pore surface during low salinity water flooding, suggesting the oil film detachment from pore surfaces due to in-situ wettability alteration. Micro-CT imaging analysis also shows that the large size water cluster (greater than 107 µm3) occupies 87.7% of water volume after high salinity water flooding, whereas the same size water cluster occupies 89.6% pore volume after low salinity water flooding, implying that water clusters coalesce into each other to transport in pore network during low salinity water flooding in line with fluid occupancy maps. Geochemical modelling predicts a pH increase (from 7 to 8.9) during low salinity water flooding largely due to ankerite and albite dissolution. This study sheds light on the significance of geochemical controls over wettability alteration at pore-scale through water film propagation.

Ion-Specific Effects on Hydrogen Bond Network at a Submicropore Confined Liquid-Vacuum Interface: An in Situ Liquid ToF-SIMS Study.

The hydrogen bond (HB), one of the essential properties of water, tends to link water molecules to form dynamic water clusters. Extrinsic ions could change the size distribution of water clusters by influencing HBs. But the mechanism, especially the influence range of ions on HBs, is still in dispute due to limitation of analytical methods. Herein, we use in situ liquid ToF-SIMS analysis combined with density functional theory calculation to study the influence of different halide anions on HBs at a submicropore confined liquid-vacuum interface. Our experimental results demonstrated that anions show synchronous local and long-range effects on HBs. Specifically, the larger the anion is, the greater degree the long-range HB network and the local hydration number of anions are influenced. More importantly, we found that the long-range effect on the HB network is influenced by nuclear quantum effects, whereas the local effect on water molecules in the first hydration shell is not.

Effects of Side Chain Length on the Structure, Oxygen Transport and Thermal Conductivity for Perfluorosulfonic Acid Membrane: Molecular Dynamics Simulation

Many researches have indicated that the side chain length of perfluorosulfonic acid (PFSA) polymers had nonnegligible effects on PFSA membrane properties. However, the effects have not been completely revealed yet. This paper uses molecular dynamics simulation to explore the effects of side chain length on the structure, oxygen transport and thermal conductivity of PFSA membrane. In PFSA membrane with longer side chains, the maximum size of water clusters is larger, while the connectivity of water clusters is poorer and size distribution of water clusters is more heterogenous. And the structural characteristics have a strong effect on the diffusion and solubility coefficient of oxygen molecules. The diffusion of oxygen molecules may be hindered due to the existence of small pores in PFSA membrane with longer side chains even at high water content, leading to a lower diffusion coefficient. In addition, with the increment of water content λ, the solubility coefficient of oxygen molecules in PFSA membrane with shorter side chains decreases more quickly at λ < 12, resulting in a most obvious difference of solubility coefficients for different side chain lengths when λ is about 12. Moreover, the thermal conductivity of PFSA membrane with shorter side chains is higher.

In-situ discrimination of the water cluster size distribution in aqueous solution by ToF-SIMS

The size distribution and molecular structure of water clusters play a critical role in the chemical, biological and atmospheric process. The common experimental study of water clusters in aqueous solution is challenged due to the influence of local H-bonding environments on vibration spectroscopies or vacuum requirements for most mass spectrometry technologies. Here, the time-of-flight secondary ion mass spectrometry (ToF-SIMS) combining with a microfluidic chip has been applied to achieve the in-situ discrimination of the size distribution for water clusters in liquid water at room temperature. The results demonstrated that the presented method is highly system stable, reproducible and accurate. The comparison of heavy water with pure water was made to further demonstrate the accuracy of this technique. These results showed that (H2O)3H+ and (D2O)4D+ are the most dominant clusters in pure and heavy water, respectively. This one water molecule difference in the dominant cluster size may due to the nuclear quantum effects on water’s hydrogen bonded network. It is the first time to experimentally show the size distribution of water clusters over a wide range ( n =1–30) for pure (H2O) and heavy (D2O) water from molecular level. This technique provides an achievable method for liquid water, which offers a bridge to close the gap between theoretical and experimental study of water cluster in aqueous solution.

Molecular dynamics simulation analysis of hydration effects on microstructure and transport dynamics in sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) fuel cell membranes

Molecular dynamics simulations were used to study microstructure and transport behavior of hydrated sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) membranes at different hydration levels. Simulation results have shown that SPPO membranes swell upon hydration and become phase segregated into hydrophobic and hydrophilic domains with sulfonic acid groups located at their interface. Evaluation of radial distribution function revealed that with increasing the hydration level, sulfonic acid groups were found to be solvated with more water molecules which cause the sulfonic acid–sulfonic acid and sulfonic acid–hydronium ion distances to increase. By examining the water cluster size distribution, larger well connected clusters containing almost all the water molecules and hydronium ions were formed inside the hydrated SPPO membranes at increased water contents. Furthermore, the calculated hydronium ion and water diffusion coefficients in SPPO based membranes were increased by increasing the hydration level and were smaller than those in Nafion membrane.

Progress in In Situ X-Ray Tomographic Microscopy of Liquid Water in Gas Diffusion Layers of PEFC

Water management is an important factor for optimizing polymer electrolyte fuel cells (PEFC) under high current density conditions as required for the automotive application. The characteristics of the local liquid saturation of the gas diffusion layer (GDL) is of particular interest. Here we report on the development of in-situ X-ray tomographic microscopy (XTM) with a pixel sizes in the order of 2 μm and sensitivity for carbon and liquid water for the quantitative analysis of liquid water in GDLs. In-situ XTM of PEFC is a major experimental challenge. A complete cell needs to be operated under realistic conditions in the constraint space of the small field of view on the beamline sample stage. Further phase segmentation of the images is required to successfully analyze the quantitative properties of the different phases. For this a workflow, applying differential images between dry and wet structures has been developed. Cells with Toray TGP-H-060 GDLs were analyzed in-situ. Droplets that appear on the GDL surface are connected to a significant water structure inside the GDL. Further the water cluster size distribution in the GDL shows that while small droplets (<100 pl) are numerous, most of the water is contained in few larger clusters.

A theoretical study of water equilibria: The cluster distribution versus temperature and pressure for (H2O)n, n=1–60, and ice

The size distribution of water clusters at equilibrium is studied using quantum-chemical calculations in combination with statistical thermodynamics. The necessary energetic data is obtained by quantum-chemical B3LYP computations and through extrapolations from the B3LYP results for the larger clusters. Clusters with up to 60 molecules are included in the equilibrium computations. Populations of different cluster sizes are calculated using both an ideal gas model with noninteracting clusters and a model where a correction for the interaction energy is included analogous to the van der Waals law. In standard vapor the majority of the water molecules are monomers. For the ideal gas model at 1 atm large clusters [56-mer (0-120 K) and 28-mer (100-260 K)] dominate at low temperatures and separate to smaller clusters [21-22-mer (170-280 K) and 4-6-mer (270-320 K) and to monomers (300-350 K)] when the temperature is increased. At lower pressure the transition from clusters to monomers lies at lower temperatures and fewer cluster sizes are formed. The computed size distribution exhibits enhanced peaks for the clusters consisting of 21 and 28 water molecules; these sizes are for protonated water clusters often referred to as magic numbers. If cluster-cluster interactions are included in the model the transition from clusters to monomers is sharper (i.e., occurs over a smaller temperature interval) than when the ideal-gas model is used. Clusters with 20-22 molecules dominate in the liquid region. When a large icelike cluster is included it will dominate for temperatures up to 325 K for the noninteracting clusters model. Thermodynamic properties (C(p), DeltaH) were calculated with in general good agreement with experimental values for the solid and gas phase. A formula for the number of H-bond topologies in a given cluster structure is derived. For the 20-mer it is shown that the number of topologies contributes to making the population of dodecahedron-shaped cluster larger than that of a lower-energy fused prism cluster at high temperatures.

Simulation of water clusters in vapour, alkanes and polyethylenes

The Gibbs Ensemble Monte Carlo (GEMC) technique has been used to study the clustering of water in vapour, alkanes and polyethylene, where the water clusters are in equilibrium with liquid phase water. The effect of an external electric field and ionic impurities on the clustering of water in the hydrocarbons (alkanes and polyethylene) has also been studied. The simulations of water clustering in polyethylene were made more efficient by using a connectivity altering osmotic Gibbs ensemble method. It was found that trends in the size distribution of water clusters in the hydrocarbons are similar to those found in the pure vapour, but that fewer and smaller clusters are formed as the length of the hydrocarbon chain increased. Also, large external electric fields decrease the solubility of water in hydrocarbons, whereas the presence of ionic species dramatically increases the solubility.

Water Clusters in Gas Phases Studied by Liquid Ionization Mass Spectrometry

Studies of water clusters under atmospheric pressure are very important. In previous methods, however, clusters existing in vacuum at low temperature have been measured. Therefore, the size distributions of observed cluster ions are likely different from those existing at normal conditions. Liquid ionization (LPI) mass spectrometry we have developed provides information about hydrogen-bonded clusters in a gas phase and also at the liquid surface at ambient temperature under atmospheric pressure. Thus, LPI-MS was applied to study water clusters in gas phases. The results indicated that the sizes of water clusters in vapor increased with the temperature of liquid water and those in the air increased with the relative humidity, while they decreased with increasing the temperature of the gas phase. In general, the size distribution of water clusters shows one smooth envelope, except for a few magic numbered clusters. Adiabatic condensation is negligible to occur in this method and thus, LPI mass spectra give useful information about clusters in gas phases under the normal conditions.

The polymerization of water in benzene and toluene

A model is described for the polymerization of water in benzene and toluene as solvents, which treats the water molecule like an ordinary tetrafunctional monomer. As in the quasi-chemical model of regular solutions, the law of mass action is applied to contacts between water molecules (hydrogen bonds), while the solvent is treated as an inert continuum. The size distribution of water clusters becomes identified with the classical molecular size-distribution law for tetrafunctional polymerization derived by Flory. In accordance with an idea suggested by Stockmayer, the critical saturation point in these solution systems corresponds to the gel point in polyfunctional polymerization. The model is tested in three ways. First, specific volume measurements of water solutions in benzene and toluene are successfully fitted. The results show a marked expansion resulting from the addition of monomeric water to hydrocarbons (i. e. in extremely dilute solutions). As the water concentration is raised, the partial specific volume of the water falls owing to hydrogen bond formation. The amount of contraction at various concentrations agrees well with the law of mass action. The amount of contraction at the saturation point agrees well with prediction. This is based on the critical conversion ( = 33·3 %) of hydrogen bonds to be expected at the gel point and on the known specific volume of ice (corresponding formally to 100% conversion of hydrogen bonds). Secondly, the model receives some support from relative viscosity measurements, though these effects are rather near the limit of experimental detectability. It does appear that water clusters raise the viscosity more strongly than loose clusters occurring in other binary systems in the absence of hydrogen bonding. On the other hand, the viscosity increments are much smaller than expected for a truly covalent polymer. Thirdly, the heat of solution of water in benzene and toluene can be simply interpreted in terms of the heat of hydrogen-bond formation. The failure of previous workers to find any polymerization of water in benzene by cryoscopic and spectroscopic measurements is discussed. An instrument is described which measures simultaneously the density and viscosity of 3 ml. of a liquid. The range of densities for a given instrument is about 0·001, and differences in density can be rapidly measured to about 0·000002 g/ml. The viscosity is measured to about 0·25 %. The principle employed is that of the usual falling sphere viscometer, but use of two (Pyrex glass) spheres of slightly different density permits the density of the liquid to be calculated as well as the viscosity. The instrument is useful in polymer chemistry generally, e. g. for intrinsic viscosity measurements at extremely low rates of shear.