Models for Water

Lattice models for liquids were very common between the 1930s and 1950s but are rarely used nowadays. The main reason for their falling out-of-fashion was the recognition that solids are fundamentally different from liquids. Therefore, lattice models for liquids were largely discarded. One would have expected that this were true for liquid water as well. Yet quite surprisingly lattice models for water were used, and still are used, in the study of water.  

The model presented in this section is of interest for three reasons. First, it is the simplest interstitial model having features in common with many models proposed for water and used successfully to explain some of the outstanding properties of water and aqueous solutions. Second, this model may be viewed either 

The interstitial model has a serious drawback, however, which is similar to the shortcomings of applying a lattice model to a fluid in general. It is therefore important to make a clear-cut distinction between results that pertain strictly to the model and results that have more general validity. In this section, we shall describe a simplified version of Pauling s model and briefly outline some of the results. The details are left as an exercise for the reader. A partial theoretical treatment of Pauling s model was carried out by Frank and Quist (1961). 

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Table A2.3.2 Halide-water, alkali metal cation-water and water-water potential parameters (SPC/E model). In the SPC/E model for water, the charges on H are at 1.000 A from the Lennard-Jones centre at O. The negative charge is at the O site and the HOH angle is 109.47°.

The simplest model for water at the electrode surface has just two possible orientations of the water molecules at the surface, and was initially described by Watts-Tobin [22]. The associated potential drop is given by... 

Z1, P Cieplak, W D Cornell and P A Kolhnan 1993. A Well-Behaved Electrostatic Potential Based 5thod for Deriving Atomic Charges - The RESP Model. Journal of Physical Chemistry 97 10269-10280. sen H C, J P M Postma, W F van Gunsteren and J Hermans 1981. Interaction Models for Water in lation to Protein Hydration. In Pullman B (Editor). Intermolecular Forces. Dordrecht, Reidel, I. 331-342. 

Sprik M and M L Klein 1988. A Polarisable Model for Water Using Distributed Charge Sites. Journal of Chemical Physics 89 7556-7560. 

A complete set of intermolecular potential functions has been developed for use in computer simulations of proteins in their native environment. Parameters have been reported for 25 peptide residues as well as the common neutral and charged terminal groups. The potential functions have the simple Coulomb plus Lennard-Jones form and are compatible with the widely used models for water, TIP4P, TIP3P and SPC. The parameters were obtained and tested primarily in conjunction with Monte Carlo statistical mechanics simulations of 36 pure organic liquids and numerous aqueous solutions of organic ions representative of subunits in the side chains and backbones of proteins... 

In view of the importance of water in chemistry and biology, there have been many attempts to construct simple yet effective intramolecular potentials for water molecules. Water monomers are traditionally left rigid. The early three-site model for water took positive charges on the hydrogens ( h) and a negative charge (qo = on the oxygen, and wrote the pair potential between two... 

For Hg, the temperature coefficient of Ea=0 was determined by Randies and Whiteley78 and found to be equal to 0.57 mV K l.On the basis of a simple up-and-down molecular model for water,79 this positive value has been taken to indicate a preferential orientation, with the negative end of the molecular dipole (oxygen) toward the metal surface. While this may well be the case, the above discussion shows that the analysis of the experimental value is far more complex. 

A controversy exists over the interpretation of such a correlation. According to the simple two-state model for water at interfaces, the higher the preferential orientation of one of the states, the higher the value of BEa=Q/BT. If the preferentially oriented state is that with the negative end of the dipole down to the surface, the temperature coefficient of Ev is positive (and vice versa). Thus, in a simple picture, the more positive BEa=0/BTt the higher the orientation of water, i.e., the higher the hydro-philicity of the surface. On this basis, Silva et al.446 have proposed the... 

Michaelides A, Ranea VA, de Andres PL, King DA. 2003b. General model for water monomer adsorption on close-packed transition and noble metal surfaces. Phys Rev Lett 90 216102. 

In all cases the calculations were performed using QM/MM methodology that includes the pseudobond model for the QM/MM boundary [13,39,41]. This methodology has been implemented in a modified version of Gaussian 98 [42], which interfaces to a modified version of TINKER [43], The AMBER94 all-atom force field parameter set [44] and the TIP3P [45] model for water were used. 

Sprik M, Klein ML (1988) A polarizable model for water using distributed charge sites. J Chem Phys 89(12) 7556-7560... 

Stillinger FH, David CW (1978) Polarization model for water and its ionic dissociation products. J Chem Phys 69(4) 1473... 

Svishchev IM, Kusalik PG, Wang J, Boyd RJ (1996) Polarizable point-charge model for water results under normal and extreme conditions. J Chem Phys 105(11) 4742 1750... 

Gouws, J.F., Majozi, T., Gadalla, M., 2008. Flexible mass transfer model for water minimization in batch plants. Chem. Eng. Process., 47 2323-2335... 

Gouws, J.F., Majozi, T., Gadalla, M., 2008. Flexible mass transfer model for water minimization in batch plants. Chem. Eng. Process., 47 2323-2335 Gouws, J.F., Majozi, T., 2009. Usage of inherent storage for minimisation of wastewater in multipurpose batch plants. Chem. Eng. Sci., 64 3545-3554 Majozi, T., 2005. Wastewater minimization using central reusable storage in batch plants. Comput. Chem. Eng., 29 1631-1646... 

Lobaugh, J. Voth, G. A., A quantum model for water equilibrium and dynamical properties, J. Chem. Phys. 1997,106, 2400-2410... 

Frank HS (1974) Restrictions for an acceptable model for water structure. In Luck WAP (ed) Structure of water and aqueous solutions. Verlag Chemie Physik Verlag, Weinheim, p 9... 

It is important to propose molecular and theoretical models to describe the forces, energy, structure and dynamics of water near mineral surfaces. Our understanding of experimental results concerning hydration forces, the hydrophobic effect, swelling, reaction kinetics and adsorption mechanisms in aqueous colloidal systems is rapidly advancing as a result of recent Monte Carlo (MC) and molecular dynamics (MO) models for water properties near model surfaces. This paper reviews the basic MC and MD simulation techniques, compares and contrasts the merits and limitations of various models for water-water interactions and surface-water interactions, and proposes an interaction potential model which would be useful in simulating water near hydrophilic surfaces. In addition, results from selected MC and MD simulations of water near hydrophobic surfaces are discussed in relation to experimental results, to theories of the double layer, and to structural forces in interfacial systems. 

In simulating interfacial water, it is important to use a model for water-water interactions which yields accurate results in simulations of bulk water. Each of the models discussed here have obvious advantages and disadvantages. The CF model is generally more... 

Several weaknesses and disadvantages of the computer simulation methods can also be mentioned. Foremost among these limitations is the fact that none of the commonly used models for water interactions... 

What is the likely future use of MC and MD techniques for studying interfacial systems Several promising approaches are possible. Continued investigation of double layer properties, "hydration forces", "hydrophobic effects", and "structured water" are clearly awaiting the development of improved models for water-water, solute-water, surface-water, and surface-solute potentials. 

Study of hydrated kaolinites shows that water molecules adsorbed on a phyllosilicate surface occupy two different structural sites. One type of water, "hole" water, is keyed into the ditrigonal holes of the silicate layer, while the other type of water, "associated" water, is situated between and is hydrogen bonded to the hole water molecules. In contrast, hole water is hydrogen bonded to the silicate layer and is less mobile than associated water. At low temperatures, all water molecules form an ordered structure reminiscent of ice as the temperature increases, the associated water disorders progressively, culminating in a rapid change in heat capacity near 270 K. To the extent that the kao-linite surfaces resemble other silicate surfaces, hydrated kaolinites are useful models for water adsorbed on silicate minerals. 

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