Potential models of water

Jedlovszky P, Vallauri R (1999) Temperature dependence of thermodynamic properties of a polarizable potential model of water. Mol Phys 97(11) 1157-1163... 

Interaction potential models of water developed for computer simulations are typically fitted to the properties of the liquid phase. The most frequently used experimental data to be matched are the heat of vaporization (or the configurational internal energy), the structure at the level of pair correlations and the density. In the case of the most popular models tests have been carried out for further properties to check their performance Thanks to their classical, nonpolarizable character rigid planar models of water like the SPC/E and TIP4P are inexpensive to implement in computer simulations. In the following table we present some alternative parametrizations suggested in the literature recently. 

G. T. Gao, X. C. Zeng, H. Tanaka, The melting temperature of proton-disordered hexagonal ice A computer simulation of 4-site transferable intermolecular potential model of water, J. Chem. Phys. 112 (2000) 8534-8538. 

L. X. Dang, T. M. Chang, and A. Z. Panagiotojmulos,/. Chem. Phys., 117, 3522 (2002). Gibbs Ensemble Monte Carlo Simulations of Coexistence Properties of a Polarizable Potential Model of Water. 

More realistic treatment of the electrostatic interactions of the solvent can be made. The dipolar hard-sphere model is a simple representation of the polar nature of the solvent and has been adopted in studies of bulk electrolyte and electrolyte interfaces [35-39], Recently, it was found that this model gives rise to phase behavior that does not exist in experiments [40,41] and that the Stockmeyer potential [41,42] with soft cores should be better to avoid artifacts. Representation of higher-order multipoles are given in several popular models of water, namely, the simple point charge (SPC) model [43] and its extension (SPC/E) [44], the transferable interaction potential (T1PS)[45], and other central force models [46-48], Models have also been proposed to treat the polarizability of water [49],... 

PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC, the oxidation of water is necessary to produce hydroxyl or oxygen species that participate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers [Taylor et ah, 2007b] have recently reported on a systematic study that examined the potential dependence of water redox reactions over a series of different metal electrode surfaces. For comparison purposes, we will start with a brief discussion of electronic structure studies of water activity with consideration of UHV model systems. 

Two cell models of water have been reported. Weissman and Blum 63> considered the motion of a water molecule in a cell generated by an expanded but perfect ice lattice. Weres and Rice 64> developed a much more detailed model, based on a more sophisticated description of the cell and a good, nonparametric water-water interaction namely the Ben-Naim-Stillinger potential 60>. The major features of the WR model are the following ... 

The most valuable of all the models of water, by far, is the computer simulated liquid with well defined water-water interaction. To date, molecular dynamics simulations for two pair potentials 3>, and Monte Carlo simulations for three pair potentials 7i>72>, have been published. The details of the methods of simulation can be found in the literature, to which the reader is referred. 

Spohr found a significant reduction in the dipole reorientation time for a different model of water (but using the same water/Pt potential). In that paper, the reorientation dynamics are characterized by the spectral densities for rotation around the three principal axes of the water molecule. These calculations demonstrated the hindered rotation of water molecules in the plane parallel to the surface. In addition, a reduction in the frequency of rotation about the molecular dipole for water molecules in the adsorbed... 

The potential model for water used in our work is taken from the work of Stillinger and David (8) (SD) as modified by Halley and co-workers (9) (HRR). Models for water that are capable of hetero-lytic dissociation into ions OH and H+ could take two different approaches. Probably the best approach, promising but not yet... 

Habib, and surface potential determination, 893 Habib llockris model of water, 899 I Iabib Bockris isotherm, 943,949 Half-crystal position, electrodeposition,... 

These results show that including quantum mechanical electronic rearrangement in dynamics calculations of the configurations of water on a metal surface can reveal effects that are not present in classical models of the water metal interface which treat the interaction of water with the surface as a static, classical potential energy function. For example, in classical calculations of the behavior of models of water at a paladium surface the interaction with one water molecule with the surface had a similar on-top binding site, a clas-... 

This section presents the continuation of Section V. In the latter a new model [10] termed the hat-curved model was described, where a rigid dipole reorients in a hat-like intermolecular potential well having a rounded bottom. This well differs considerably from the rectangular one, which is extensively applied to polar fluids. Now the theory of the hat-curved model will be generalized, taking into account the non-rigidity of a dipole that is, a simplified polarization model of water is described here. 

A. Previous models of water (see 1-6 in Section V.A.l) and also the hat-curved model itself cannot describe properly the R-band arising in water and therefore cannot explain a small isotope shift of the center frequency vR. Indeed, in these models the R-band arises due to free rotors. Since the moment of inertia I of D20 molecule is about twice that of H20, the estimated center of the R-band for D20 would be placed at y/2 lower frequency than for H20. This result would contradict the recorded experimental data, since vR(D20) vR(H20) 200 cm-1. The first attempt to overcome this difficulty was made in GT, p. 549, where the cosine-squared (CS) potential model was formally (i.e., irrespective of a physical origin of such potential) applied for description of dielectric response of rotators moving above the CS well (in this work the librators were assumed to move in the rectangular well). The nonuniform CS potential yields a rather narrow absorption band this property agrees with the experimental data [17, 42, 54]. The absorption-peak position Vcs depends on the field parameter p of the model given by... 

Potential functions based on the sp3-hybrid tetrahedral structure model of water molecules in which two positive and two negative charges are placed at the tetrahedral positions in the molecule have been proposed by Bjerrum 1), Ben-Naim and Stillinger (BNS model) (2), and Stillinger-Rahman (ST2) (3). 

Clearly it would be preferable to write down a complete Hamiltonian, containing all the interactions, and then solve the OZ equation with some closure and obtain a full potential of mean force between the colloidal particles. This has not been done and would be difficult. An even more difficult calculation would involve the OZ2 equation. This would have the advantage of greater accuracy. The OZ2 equation appears to be less sensitive to the question of the choice of the closure thus, an element of arbitrariness is removed. Simulations, using more realistic models of water, are needed. 

Figure 1.9 is the Pourbaix diagram for iron and some of its compounds in an aqueous system at 25°C. The equilibrium potential of the reaction Fe° = Fe2+ + 2e falls outside the stability region of water represented by dashed lines. Hence, measurement of the equilibrium electrode potential is complicated by the solvent undergoing a reduction reaction, while the iron is undergoing electrochemical oxidation. This is the basis of the mixed potential model of corrosion. 

For example, suppose one can choose a rigid three-point-charge model of water with an internal geometry of 109.47° and 100 pm for the HOH angle and OH distance, respectively. The interaction energy involves a Lennard-Jones 6-12 potential for electrostatic interactions between water-water and ion-water pairs, (/pair a nonadditive polarization energy, C/pg, and a term that includes exchange repulsion for ion-water and water-water pairs,... 

Figure 5.2 Assessment of electrostatic contributions to the excess chemical potential of water, following Eq. (5.15) redrawn from Hummer et at. (1995). The temperature is T = 298 K and the density is p = 0.03333 A The SPC model of water was used and the reference system interactions are those interactions with all partial charges given the value zero.

The simulation details such as force and energy calculation including the potential models of H2O and HsO are similar to the teehniques and methods described in the earher discussion. Unlike other simulations" " of an exeess proton in bulk water we... 

This means that potential models for water and solute-water interactions will be discussed. This choice, however, is much less restrictive than it might appear. In fact, due to its nature, water and aqueous solutions perfectly serve to illustrate far more general issues in the development of realistic potentials also beyond that sufficient to simple systems, e.g. the treatment of many-body effects and phase, or thermodynamic state, transferability. Moreover, water being water, the model proposed can be readily tested against a wealth of accurate experimental data, probably the largest collection for a single molecular liquid. 

Verbragge and Hill [77] have compared protonic conductivities of several different Nafion and Dow samples immersed in sulfuric acid solutions at various temperatures. Conductivities on the order of 0.06-0.085 S/cm were reported for the acid-immersed Nafion samples at 22 °C. The immersed Dow membrane samples exhibited somewhat higher conductivity (0.13-0.14 S/cm) at this temperature. These data were reported for membranes in contact with a minimum concentration of 0.3 m H2SO4 and are dependent on the sulfuric acid concentration. This study [77] presents a model of water and ion distribution based on properties of pores in the ionomer. The model, which uses Poisson s equation to describe electric potential variation in the pore, is successful in describing experimental acid partitioning results. Other earlier reports of protonic conductivity in ionomeric membranes have been given by Slade et al. [72] and by Eisman [60]. 

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