Network In Liquid Water Research Articles

Solvated water molecules and hydrogen-bridged networks in liquid water

We have analyzed the molecular-dynamics (MD) trajectories for the oxygen and hydrogen atoms of liquid water, at six temperatures (from hot, T=361 K, to supercooled water, T=242 K); in the MD simulations the Nieser-Corongiu-Clementi ab initio potential has been used, since it yields reliable x-ray and neutron-diffraction data as well as infrared, Raman, and neutron-scattering spectra. Our analysis leads to two complementary models where we can consider each water as a solvated molecule (placed at the center of a solvation shell) or as a component of a cyclic polymer, a substructure of the hydrogen-bonded network. In the first solvation shell all water molecules are solvated with coordination values in the range 2–8. The most probable solvation number is four, at low temperature, and five at high temperature considering oxygen–oxygen pairs; however, the coordination number is four at all the temperatures if we consider oxygen–hydrogen pairs. The lifetime of the tetra coordinated complexes is the largest one and increases as temperature decreases. The computed population of cyclic polymers is highest for the pentameters in the studied temperature range, the second most probable cyclic structure is for hexamers. The average O–O distances in the liquid are temperature dependent and shorter than those in the gas phase, approaching ice values at low temperature (except for cyclic trimers, for which the O–O distance is nearly temperature independent). As a preliminary result, the lifetime of the polygons is estimated to be around 0.01 ps.

Hydration of Cyclic Oligo(oxyethylene) Compounds. Ultrasonic Velocities and Compressibilities of Binary Systems of Water with 12-Crown-4, 15-Crown-5, and 18-Crown-6

Abstract Ultrasonic velocities of binary systems of water with 12-crown-4, 15-crown-5, and 18-crown-6 were measured and the adiabatic compressibilities were evaluated therefrom. The results indicate that the most compacted hydration structure is formed at the mole ratio, approximately 2:1, of water to oxyethylene monomer unit and that the poly(oxyethylene) chain enhances structuring of the liquid water network.

Instabilities of hydrogen bond network in liquid water

A new model of the hydrogen bond network in liquid water is developed. Using the technique of polymer science, the grand canonical partition function of water is expressed by the functional integral of the field variables. The method is applied to anomalies of liquid water. Using the mean-field lattice-gas approximation it is shown that there are two instability lines, one in the supercooled region and the other in the stretched (P<0) region. Approaching the instability line in the supercooled region, the fluctuation of ice-likeness grows anomalously large and liquid water becomes unstable to be ice. Approaching the instability line in the stretched region, the fluctuation of the concentration of non-hydrogen-bonded sites grows large and liquid water becomes unstable to be gas. Divergence of the thermodynamic response functions and the mean-field values of exponents are discussed.

ChemInform Abstract: Model for the Structure of the Liquid Water Network

ChemInform Abstract: Model for the Structure of the Liquid Water Network

Model for the structure of the liquid water network

The state of a water molecule in liquid water is defined by its time-average network environment. Two states are characterized. State A is the familiar four-coordinated state of the Bernal-Fowler model with tetrahedral hydrogen bonds. State B is five-coordinated. Reexamination of the static dielectric constant by the method of Oster and Kirkwood confirms the marked polar character of the four-coordinated state but shows that the five-coordinated state is only about half as polar. Explicit five-coordinated models are proposed which are consistent with polarity and satisfy constraints of symmetry and hydrogen-bond stoichiometry. The potential energy due to the dipole-dipole interaction of the central water molecule with its time-average solvent network is derived without additional parameters. This permits prediction of barriers to rotation, frequencies for hindered rotation and liberation in the network, and ..delta..H/sub A,B/ and ..delta..S/sub A,B/. The results are in substantial agreement with relevant experiments. In particular, the barriers to rotation permit a consistent interpretation of the dielectric relaxation spectrum. The relative importance of the two states varies predictably with the property being examined, and this can account for some of the schizophrenia of aqueous properties. Since the two-state model is based on time-average network configurations, it does not applymore » when the time scale of observation is short compared to network frequencies, i.e., at infrared frequencies where continuum models may be successful.« less

P,T,c‐Dependence of 2H Spin‐Lattice Relaxation Rates in Supercooled LiCl‐D2O Solutions

AbstractLong‐ranged structural correlations develop within the random, transient hydrogen‐bonded network of liquid water at low temperatures and cause characteristic anomalies in the physical properties of supercooled water. Hydrostatic pressure suppresses these anomalies and turns liquid water into a normal viscous liquid. The addition of salt‐is considered another mean to suppress long‐ranged density‐density correlations, hence the pressure‐, temperature and composition dependence of deuterium (2H) spin‐lattice relaxation times (T1) is reported for supercooled LiCl‐D2O solutions over a large range of concentration (c ≤ 11 molal), pressure (p ≤ 226 MPa) and temperature (180 K ≤ T ≤ 283 K). The anomalous pressure dependence of the spin‐lattice relaxation times becomes weaker with increasing salt concentration

Comparison of the percolation model with computer simulation results on different water models

The predictions of the connectivity properties of the hydrogen-bonded networks in liquid water based on the percolation model proposed by H. E. Stanley and co-workers has been compared with Monte Carlo computer simulations using the ST2 and MCY potentials at different temperatures. The comparison is based on a geometric hydrogen-bond definition. Reasons for the good agreement found for the average number of waters with exactly j hydrogen bonds are discussed. Also, the conflicting conclusions on the density of 4-bonded patches were re-examined and were found to be the result of using different water models in the conflicting studies.

Connectivity of hydrogen bonds in liquid water

We report on a molecular dynamics (MD) study of the connectivity of hydrogen bond networks in liquid water, focusing primarily on the microscopic distribution functions giving the weight fraction of molecules belonging to a ‘‘net’’ of M molecules (M=1,2,3,...). The MD data compare favorably—using no adjustable parameters—with predictions of random bond percolation theory. We also study the connectivity of those molecules with four intact hydrogen bonds, and compare the corresponding distribution functions with correlated-site percolation theory. Our analysis supports the proposal that when looking at the bond connectivity, water appears as a macroscopic space-filling network—as expected from continuum models of water. When looking at the correlated site percolation problem defined by the four-bonded molecules, water appears as a myriad of tiny ramified low-density patches, somewhat reminiscent of mixture theories and cluster models. In Appendix A, we find a strong correlation between the number of molecules within a sphere of radius rc around a given molecule and the total interaction energy of that molecule with its neighbors residing in that sphere; for most choices of rc, the energy becomes less negative when more molecules are in the sphere, in contrast to the behavior of a normal fluid. This result supports the finding of Geiger and Stanley that regions of high bond connectivity are correlated with regions of low density. In Appendix B we describe in detail how we adapt conventional percolation theory to the calculation of cluster size distribution functions for hydrogen bond networks in water.

Gelation models of hydrogen bond networks in liquid water

Cluster statistics of hydrogen bond networks in water are calculated with the use of molecular dynamics (MD) and are compared with the predictions of gelation models. For small bond networks the MD calculations agree well with Flory theory (which neglects cycles), but they disagree for the larger networks. The MD data for the connectivity of the larger networks agree much better with our ice lattice calculations, providing the first MD test of the ability of Flory theory to describe polyfunctional condensation network statistics.

Low-Density "Patches" in the Hydrogen-Bond Network of Liquid Water: Evidence from Molecular-Dynamics Computer Simulations

The connectivity properties of liquid water are studied with molecular dynamics. It is found that the hydrogen-bonded network includes tiny spatially correlated "patches" of four-bonded molecules, and that the local density near a "patch" is lower than the global density. The diameter of the finite patches increases with decreasing temperature, and is comparable to the characteristic length scale of density fluctuations recently measured by small-angle x-ray scattering.