Alfons Geiger

Molecular dynamics study of the hydration of Lennard‐Jones solutes

In order to clarify the nature of hydrophobic interactions in water, we have used the molecular dynamics simulation method to study a system comprising two Lennard‐Jones solute particles and 214 water molecules. Although the solutes were placed initially in contact, forces in the system drive them slightly apart to permit formation of vertex‐sharing solvent ’’cages.’’ Definite orientational preferences have been observed for water molecules in the first solvation layer around the Lennard‐Jones solutes; these preferences are loosely reminiscent of structure in clathrates. Nevertheless, substantial local disorder is obviously present. The dynamical data show that translational and rotational motions of solvation–sheath water molecules are perceptibly slower (by at least 20%) than those in pure bulk water.

Aspects of the percolation process for hydrogen-bond networks in water

Sets of configurations selected from three molecular dynamics simulations for liquid water have been analyzed for the distribution of hydrogen‐bond clusters. Two simulations correspond to water at 1 g cm−3, while the third corresponds to highly compressed water at 1.346 g cm−3. An energy criterion was adopted for existence of a hydrogen‐bond between two molecules. As the cutoff value for bonding increases (becomes more permissive), a bond percolation threshold is encountered at which initially disconnected clusters suddenly produce a large space‐filling random network. At least for the model studied, any chemically reasonable definition of ’’hydrogen‐bond’’ leads to this globally connected structure though a few disconnected fragments inhabit its interior. Although some polygonal closures can exist, the critical percolation threshold is apparently well predicted by Flory’s theory of the gel point for dendritic polymerization.

Network defects and molecular mobility in liquid water

As a step toward elucidating the connection between the structure and mobility of liquid water, we analyze quenched molecular dynamics configurations at different densities. We find that the mobility is directly related to the existence of ‘‘topological defects’’ of the tetrahedral network. The defects act as catalysts, providing lower energy pathways between different tetrahedral local arrangements.

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 molecul...

Liquid-liquid phase transitions in supercooled water studied by computer simulations of various water models

Liquid-liquid and liquid-vapor coexistence regions of various water models were determined by Monte Carlo (MC) simulations of isotherms of density fluctuation-restricted systems and by Gibbs ensemble MC simulations. All studied water models show multiple liquid-liquid phase transitions in the supercooled region: we observe two transitions of the TIP4P, TIP5P, and SPCE models and three transitions of the ST2 model. The location of these phase transitions with respect to the liquid-vapor coexistence curve and the glass temperature is highly sensitive to the water model and its implementation. We suggest that the apparent thermodynamic singularity of real liquid water in the supercooled region at about 228 K is caused by an approach to the spinodal of the first (lowest density) liquid-liquid phase transition. The well-known density maximum of liquid water at 277 K is related to the second liquid-liquid phase transition, which is located at positive pressures with a critical point close to the maximum. A possible order parameter and the universality class of liquid-liquid phase transitions in one-component fluids are discussed.

Water in nanopores. I. Coexistence curves from Gibbs ensemble Monte Carlo simulations

Coexistence curves of water in cylindrical and slitlike nanopores of different size and water-substrate interaction strength were simulated in the Gibbs ensemble. The two-phase coexistence regions cover a wide range of pore filling level and temperature, including ambient temperature. Five different kinds of two-phase coexistence are observed. A single liquid-vapor coexistence is observed in hydrophobic and moderately hydrophilic pores. Surface transitions split from the main liquid-vapor coexistence region, when the water-substrate interaction becomes comparable or stronger than the water-water pair interaction. In this case prewetting, one and two layering transitions were observed. The critical temperature of the first layering transition decreases with strengthening water-substrate interaction towards the critical temperature expected for two-dimensional systems and is not sensitive to the variation of pore size and shape. Liquid-vapor phase transition in a pore with a wall which is already covered with two water layers is most typical for hydrophilic pores. The critical temperature of this transition is very sensitive to the pore size, in contrast to the liquid-vapor critical temperature in hydrophobic pores. The observed rich phase behavior of water in pores evidences that the knowledge of coexistence curves is of crucial importance for the analysis of experimental results and a prerequiste of meaningful simulations.

Multiple liquid–liquid transitions in supercooled water

Three distinct liquid–liquid coexistence regions were observed for ST2 model water by restricted ensemble Monte Carlo simulations of the isotherms of homogenized systems and by phase equilibria simulations in the Gibbs ensemble. The lowest density liquid–liquid transition meets the liquid–vapor phase transition at a triple point and ends in a metastable critical point. A percolation analysis evidences, that the phase separations at the lowest and highest densities can be attributed to the separation of differently coordinated water molecules. The densities of the obtained four phases of supercooled water correlate with experimentally observed densities of amorphous ice.

Interpretation of the unusual behavior of H2O and D2O at low temperature: Are concepts of percolation relevant to the “puzzle of liquid water”?

This talk will summarize the present status of an ongoing research program designed to answer the question posed in the title. Since a snapshot of liquid water with a subpicosecond shutter speed reveals that this system (a hydrogen-bonded liquid) is above its percolation threshold, it is tempting to imagine that connectivity concepts of the sort encompassed in percolation theory may prove useful. We find that the traditional approach of random-bond percolation theory-developed to describe the onset of gelation - is not sufficient, since water is well above its gelation threshold. Hence we develop a new correlated-site percolation model, whose predictions are found to be in quantitative agreement with molecular dynamics calculations and in qualitative agreement with a wide range of experimental data on low-temperature water.

The HCl–HCl interaction: From quantum mechanical calculations to properties of the liquid

Analytical HCl–HCl pair potentials are derived from large quantum mechanical calculations at CEPA‐(SD) level. The computed well depth for the dimer is 1.9 kcal/mol. Those pair potentials are used in molecular dynamics simulation studies at T=297 K and ρ=0.8354 g/cm3. The results for the static (e.g., pair distribution functions) and dynamic properties are compared with experimental and other molecular dynamics results. Comparison of the results from different simulation runs allows a check on their sensitivity with respect to the pair potential employed. It turns out that the potential yielding the best fit to the computed interaction energies gives a good representation of properties of liquid HCl. However, a readjustment of this potential—in order to account for well‐known deficiencies of the quantum mechanical calculations—results in a slight improvement especially in the mean potential energy and the pressure. Finally, we used the computed pair potential to determine the structure and association ener...

Distinguishing liquids from amorphous solids: Percolation analysis on the Voronoi network

The mutual arrangement of the Delaunay simplices (configurations of four nearest atoms) has been studied for molecular dynamic (MD) models of liquid and quenched rubidium obtained by M. Tanaka [J. Phys. Soc. Jpn. 55, 3108 (1986)]. The Delaunay simplices with large circumradii and low local density of atoms, the simplices with small circumradii representing dense atomic configurations, and the simplices close in shape to perfect tetrahedron representing ‘‘rigid’’ arrangements of atomic quadruplets were delineated. The Delaunay simplices form clusters; consideration of the latter constitutes a site percolation problem on the Voronoi network [N. N. Medvedev, V. P. Voloshin, and Yu. I. Naberukhin, J. Phys. A: Math. Gen. 21, L247 (1986)]. Analysis of the MD results in these terms shows that low density atomic configurations in the liquid phase form a percolative cluster. Such a cluster does not occur in a solid phase. On the contrary, there is a percolative cluster in the solid sample, but formed by high density configurations which are nearly tetrahedral.The mutual arrangement of the Delaunay simplices (configurations of four nearest atoms) has been studied for molecular dynamic (MD) models of liquid and quenched rubidium obtained by M. Tanaka [J. Phys. Soc. Jpn. 55, 3108 (1986)]. The Delaunay simplices with large circumradii and low local density of atoms, the simplices with small circumradii representing dense atomic configurations, and the simplices close in shape to perfect tetrahedron representing ‘‘rigid’’ arrangements of atomic quadruplets were delineated. The Delaunay simplices form clusters; consideration of the latter constitutes a site percolation problem on the Voronoi network [N. N. Medvedev, V. P. Voloshin, and Yu. I. Naberukhin, J. Phys. A: Math. Gen. 21, L247 (1986)]. Analysis of the MD results in these terms shows that low density atomic configurations in the liquid phase form a percolative cluster. Such a cluster does not occur in a solid phase. On the contrary, there is a percolative cluster in the solid sample, but formed by high densi...