Peter G. Kusalik

The spatial structure in liquid water.

Liquid state structure has traditionally been characterized with the radial distribution functions between atoms. Although these functions are routinely available from x-ray diffraction and neutron scattering experiments or from computer simulations, they cannot be interpreted unambiguously to provide the spatial order in a molecular liquid. A direct approach to determining the spatial structure in the liquid state is demonstrated here. Three-dimensional maps representing the local atomic densities are presented for several water models. These spatial maps provide a picture of the short-range order in liquid water which reveals specific details of its local structure that are important in the understanding of its properties.

Structure in liquid water: A study of spatial distribution functions

Despite the fact that an enormous literature has now accumulated on the structure in liquid water, the focus has been primarily limited to the average radial distributions of particles; local (atomic) pair‐density maps which span both the radial and the angular coordinates of the separation vector have remained largely unexplored. In this work, we have obtained the spatial distribution functions gOO(r,Ω) and gOH(r,Ω) for liquid water and have applied them to an analysis of the equilibrium structure. Molecular dynamics simulations of SPC/E water have been carried out at temperatures of −10, 25, and 100 °C and the local liquid structure examined. It is found that the unfolded O...O distribution demonstrates, in addition to peaks consistent with a continuous tetrahedral network pattern, a distinct maximum in the local atomic pair density at ‘‘interstitial’’ separations of about 3.5 A. This local maximum is lost in the spatially folded radial distribution function gOO(r) due to averaging over the entire angul...

Polarizable point‐charge model for water: Results under normal and extreme conditions

Molecular dynamics simulations of liquid water under normal and extreme conditions are performed using the polarizable point‐charge (PPC) model. This efficient three‐site model explicitly incorporates results from ab initio studies of the water molecule in applied electric fields. The structural, thermodynamic, and dielectric properties, and the self‐diffusion coefficient are examined at a number of temperatures ranging from 263 to 573 K. These simulation results are compared with available experimental data along the liquid–vapor coexistence line; the agreement is very good for all properties studied. The temperature of maximum density for the PPC model is found to coincide with the experimentally observed value of 277 K. The spatial coordination of water molecules in the liquid and the anisotropy of the self‐diffusion tensor are analyzed at various state points. Increased directional anisotropy in the local translational diffusion, suggestive of prenucleation phenomena, can be observed at T=263 K. Above...

The total molecular dipole moment for liquid water

For the water molecule, the dipole is the first nonzero multipole moment; it represents the polarity of the molecule and has been widely used in describing solvation behavior. A rather wide range of theoretically determined values for the total molecular dipole moment of water in condensed phases has been reported in the literature. This paper describes a means by which the average total dipole moment for the water molecule in the liquid state can be linked to experimental refractive index data. Three components comprise the mean-field approach that is employed. A formal framework is developed that relates the temperature dependence of the effective molecular polarizability to the average local electric field experienced by a liquid water molecule over a chosen temperature range. A characterization of the distributions of local fields and field gradients is also necessary, and this has been determined from the computer simulations of liquid water samples at several different temperatures for two standard ...

Structure in liquid methanol from spatial distribution functions

A structural approach that employs the spatial distribution functions of atoms has been shown recently [J. Chem. Phys. 99, 3049 (1993)] to greatly improve our understanding of the local structure in liquid water. In the present study we obtain the oxygen–oxygen and oxygen–carbon spatial distribution functions, gOO(r,Ω) and gOC(r,Ω), respectively, for liquid methanol and use them to characterize its equilibrium structure. For this purpose molecular dynamics simulations with the three‐site model of Haughney, Ferrario, and McDonald are carried out at a temperature of 25 °C. Using the spatial distribution functions we demonstrate that the dominant H‐bonded structure in this liquid is an open, nonlinear (‘‘zig–zag’’) chain of monomers packed spatially in a tetrahedral manner. gOO(r,Ω) yields an average coordination number of 1.92 which agrees well with results from chain length analysis. There is no evidence in our structural data to support a local planar assembly of oxygen sites. We also observe features in ...

Quantum effects in light and heavy liquid water: A rigid-body centroid molecular dynamics study

The centroid molecular dynamics (CMD) method is applied to the study of liquid water in the context of the rigid-body approximation. This rigid-body CMD technique, which is significantly more efficient than the standard CMD method, is implemented on the TIP4P model for water and used to examine isotopic effects in the equilibrium and dynamical properties of liquid H(2)O and D(2)O. The results obtained with this approach compare remarkably well with those determined previously with path integrals simulations as well as those obtained from the standard CMD method employing flexible models. In addition, an examination of the impact of quantization on the rotational and librational motion of the water molecule is also reported.

Crystal growth simulations of methane hydrates in the presence of silica surfaces

We present a molecular dynamics simulation study of the crystal growth of methane hydrates in the presence of model silica (SiO(2)) surfaces. The crystal growth under apparent steady-state conditions shows a clear preference for bulk solution. We observe rather disordered water arrangements very close to the silica surface within about 5 Å in both liquid and crystalline regions of the system. These disordered structures have dynamic and structural properties intermediate between those exhibited by molecules in bulk liquid and crystalline phases. The presence of methane molecules appears to help stabilize these structures. We observe that under appropriate conditions, the hydroxylated silica surfaces can serve as a source of methane molecules which can help promote hydrate growth near the surfaces.

Molecular Insights into Clathrate Hydrate Nucleation at an Ice–Solution Interface

Clathrate hydrates are specific cage-like structures formed by water molecules around a guest molecule. Despite the many studies that have been performed on clathrate hydrates, the actual molecular mechanism of both their homogeneous and heterogeneous nucleation has yet to be fully clarified. Here, by means of molecular simulations, we demonstrate how the interface of hexagonal ice can facilitate the heterogeneous nucleation of methane clathrate hydrate from an aqueous methane solution. Our results indicate an initial accumulation of methane molecules, which promote induction of defective structures, particularly coupled 5-8 ring defects, at the ice surface. Structural fluctuations promoted by these defective motifs assist hydrate cage formation next to the interface. The cage-like structures formed then act as a sink for methane molecules in the solution and enhance the stability and growth of an amorphous nucleus, which can evolve into a hydrate crystal upon annealing. These results are illustrative of how a surface that is structurally incompatible can serve to facilitate heterogeneous nucleation of a new crystalline phase. They should also further our general understanding of the formation of gas hydrates and their critical roles in various industrial and environmental processes, including carbon capture and storage.

Heterogeneous crystal growth of methane hydrate on its sII [001] crystallographic face.

This paper presents a systematic molecular simulation study of the heterogeneous crystal growth of methane hydrate sII from supersaturated aqueous methane solutions. The growth of sII hydrate on the [001] crystallographic face is achieved through utilization of a recently proposed methodology, and rates of crystal growth of 1 A/ns were sustained for the molecular models and specific conditions employed in this work. Characteristics of the crystals grown as well as properties and structure of the interface are examined. Water cages with a 5(12)6(3) arrangement, which are improper to both sI and sII structures, are identified during the heterogeneous growth of sII methane hydrate. We show that the growth of a [001] face of sII hydrate can produce an sI crystalline structure, confirming that cross-nucleation of methane hydrate structures is possible. Defects consisting of two methane molecules trapped in large 5(12)6(4) cages and water molecules trapped in small and large cages are observed, where in one instance we have found a large 5(12)6(4) cage containing three water molecules.

The multipole polarizabilities and hyperpolarizabilities of the water molecule in liquid state: an ab initio study

A charge perturbation variant of the finite-field method has been used to calculate dipole and quadrupole moments, dipole polarizability, hyper- and principal components of high-order polarizabilities of the water molecule in gas and in liquid phase conditions. Calculations were performed for the ground-state water molecule at the MP2 and MP4 levels of theory. The gas phase values determined allow our methodology for extracting polarizabilities to be tested and a properly balanced, moderate-sized basis set to be selected; the results obtained are in very good agreement with experiment and the most accurate previous theoretical estimates. A local field approach is introduced to mimic the electrostatic environment experienced by a water molecule in the liquid. Within this approach, sets of fixed charges are used to generate the desired electric fields and field gradients. Three different liquid phase models and the corresponding sets of electrical properties are examined. The values obtained from these models and for gas-phase are compared. The magnitudes of the dipole and the quadrupole moments increase moving from gas to liquid phase, where the latter shows greater sensitivity to the choice of liquid model. For a liquid phase water molecule the first hyperpolarizability (β) and first higher polarizability (A) increase markedly, actually changing sign, the second hyperpolarizability (γ) also increases but much less dramatically, and components of the second high-order polarizability tensor (B) demonstrate a rearrangement of contributions. The values reported for the hyper- and high-order polarizability tensors are the first such theoretical estimates for liquid water.