Water structure, modeling

Reviews on water structure models include Mishima and Stanley (1998), Wallqvist and Mountain (1999), and Ludwig (2001). Mishima and Stanley (1998) concentrated their review on three relatively recent water structure hypotheses (1) the stability limit hypothesis (Speedy, 1982), (2) the singularity-free hypothesis (Sastry et al., 1996), and (3) the liquid-liquid phase transition hypothesis (Poole et al., 1992). 

An understanding of equilibrium phenomena in naturally occurring aqueous systems must, in the final analysis, involve understanding the interaction between solutes and water, both in bulk and in interfacial systems. To achieve this goal, it is reasonable to attempt to describe the structure of water, and when and if this can be achieved, to proceed to the problems of water structure in aqueous solutions and solvent-solute interactions for both electrolytes and nonelectrolytes. This paper is particularly concerned with two aspects of these problems—current views of the structure of water and solute-solvent interactions (primarily ion hydration). It is not possible here to give an exhaustive account of all the current structural models of water instead, we shall describe only those which may concern the nature of some reported thermal anomalies in the properties of water and aqueous solutions. Hence, the discussion begins with a brief presentation of these anomalies, followed by a review of current water structure models, and a discussion of some properties of aqueous electrolyte solutions. Finally, solute-solvent interactions in such solutions are discussed in terms of our present understanding of the structural properties of water. 

Uniformist, Average Models. We divide the current water structure models into two major categories. The first treats water essentially as an unstructured liquid while the second admits the simultaneous existence of at least two states of water—i.e., the structural models which Frank has termed the mixture models. ... 

Implications of Thermal Anomalies for Water Structure Models... 

We now return to some aspects of the different possible water structure models mentioned earlier. Whether water will be shown to possess... 

Poly(oxyethylene) (POE) (-OCH2CH2—) is an unusual polyether with practically uiilimited solubility in water, unlike other structurally related polymers. At elevated temperatures, however, the isotropic aqueous solution of POE separates into two phases. The mechanism of the water solubility of POE and the phase behavior has attracted much attention of many investigators. Various mechanistic models have in fact been proposed to account for these phenomena a water structure model, a hydrogen bond model, and a conformational model. ... 

The main types of the water structure models according to the classification given by Angell and Rodgers 18) include ... 

Krindel, P., and I. Eliezer, Coord. Chem. Rev., 1971, 6, 217 (water structure models). 

K.T. Wikfeldt, M. Leetmaa, M.P. Ljungberg, A. Nilsson, L.G.M. Pettersson, On the range of water structure models compatible with X-ray and neutron diffraction data. J. Phys. Chem. B 113(18), 6246-6255 (2009)... 

Fig. 5 shows data from a simulation of TIP4P water that is confined on both sides by a rhombohedral mercury crystal with (111) surface structure. Bosio et al. [135] deduce from their X-ray studies that a solid o-mercury lattice with a larger lattice constant in the z direction may be used as a good structural model for liquid mercury. Thus, the mercury phase was modeled as a rigid crystal in order to simplify the simulations. The surface of such a crystal shows rather low corrugation. 

Fig. 5(a) contains the oxygen and hydrogen density profiles it demonstrates clearly the major differences between the water structure next to a metal surface and near a free or nonpolar surface (compare to Fig. 3). Due to the significant adsorption energy of water on transition metal surfaces (typically of the order of 20-50kJmoP see, e.g., [136]), strong density oscillations are observed next to the metal. Between three and four water layers have also been identified in most simulations near uncharged metal surfaces, depending on the model and on statistical accuracy. Beyond about...

Frank, H. S. Structural Models, in Water — a Comprehensive Treatise, (ed. Franks, F.), Vol. 1, chapter 14, New York, Plenum Press 1972... 

The advantages of the simple approach outlined above are the limited number of water molecules needed in the simulation and the well-defined water structure. The major drawback is that, owing to the periodicity, this water structure fits best on a (111) or (lll)-like surface, e.g., (211). There are at least two other approximations to model the water interaction. One is to include a large number of water molecules and apply molecular dynamics to determine a structure for the water and include this water arrangement in the simulations [Filhol and Neurock, 2006]. The drawbacks of this approach are the computational time required and the results sensitivity to the water structure. 

Figure4.5 c(2 x 2)B structure. Left panel structural model. Right top panel corresponding simulated STM image (VB= + 1.30 V, / = 0.002nA). The protrusion maxima correspond to water molecules, whereas the depressions correspond to Rh atoms. Right bottom panel simulated current profiles along [00 1] at decreasing (blue to red) tip-surface distances. (Reprinted with permission from Ref. [18].)...

---