Water-surface interaction potential surfaces

We would expect that the amplitude B of the leading singular term in equation (13) should not depend on the water-surface interaction potential, at least in the first approximation. This term arises from the bulk order parameter, whose amplitude Bq is determined by the water-water interaction only. Therefore, we believe that the water-water interaction gives a major contribution to the amplitude B. In contrast, the parameters of the asymmetric terms in equation (13) should strongly depend on the water-surface interaction. In particular, Pc in the surface layer is essentially below the bulk critical density, when a weak fluid-wall interaction provides preferential adsorption of voids, whereas pc may exceed the bulk critical density in the case of a strong water-surface interaction. It is difficult to predict the values of the temperature-dependent terms in the asymmetric contribution, as the surface diameter reflects interplay between the natural asymmetry of liquid and vapor phases, described by the bulk diameter, and preferential adsorption of one of the component (molecules or voids). 

The dipole density profile p (z) indicates ordered dipoles in the adsorbate layer. The orientation is largely due to the anisotropy of the water-metal interaction potential, which favors configurations in which the oxygen atom is closer to the surface. Most quantum chemical calculations of water near metal surfaces to date predict a significant preference of oxygen-down configurations over hydrogen-down ones at zero electric field (e.g., [48,124,141-145]). The dipole orientation in the second layer is only weakly anisotropic (see also Fig. 7). 

Figure 1. Comparison of ST2 water-surface interactions computed from Equations 7 or 8 using parameters for the Lennard-Jones 9-3 potential in Table II and the delocalized charge magnitude for smectite and mica surfaces in Table III.

FIGURE 16.1 Hydro-gen bonding between diethyl ether and water. The dashed line represents the attractive force between the negatively polarized oxygen of diethyl ether and one of the positively polarized hydrogens of water. The electrostatic potential surfaces illustrate the complementary interaction between the electron-rich (red) region of diethyl ether and the electron-poor (blue) region of water. 

Most of the water-mediated interactions between surfaces are described in terms of the DLVO theory [1,2]. When a surface is immersed in water containing an electrolyte, a cloud of ions can be formed around it, and if two such surfaces approach each other, the overlap of the ionic clouds generates repulsive interactions. In the traditional Poisson-Boltzmann approach, the ions are assumed to obey Boltzmannian distributions in a mean field potential. In spite of these rather drastic approximations, the Poisson-Boltzmann theory of the double layer interaction, coupled with the van der Waals attractions (the DLVO theory), could explain in most cases, at least qualitatively, and often quantitatively, the colloidal interactions [1,2]. 

In Ref. 49 the orientational distribution of water near the Pt(lOO) surface was investigated in great detail. In spite of the preference for adsorption of isolated water molecules through the oxygen atom, which is incorporated into the water-metal interaction potential, relatively few configurations were observed in which the dipole moment of the molecule points into the solution. The analysis will not be repeated here the interested reader is referred to Ref. 49. 

Note that the extensive HB network is compromised near both the hydrophilic and the hydrophobic surfaces, but differently. In the case of the hydrophilic surface, the enthalpic gain from the water-surface interaction compensates for the twin losses of enthalpy and the entropy of water arising from the molecular rearrangement imposed by the surface. However, for a hydrophobic surface, such a compensation is not present. Therefore, the chemical potential of a water molecule near a hydrophobic surface is higher than that in a bulk. 

A systematic study of physical effects that influence the water structure at the water/metal interface has been made. Water structure, as characterized by the atom density proflles, depends most strongly on the adsorption energy and on the curvature of the water-metal interaction potential. Structural differences between liquid/liquid and liquid/solid interfaces, investigated in the water/mercury two-phase system, are small if the the surface inhomogeneity is taken into account. The properties of a polarizable water model near the interface are almost identical to those of unpolarizable models, at least for uncharged metals. The water structure also does not depend much on the surface corrugation. 

By now we should be acutely aware of the intricacies of the water-water interaction potential surface. Hence it is only fair to raise the question of whether water-solute interactions have as much reliability as the water-water interactions. Evaluating the reliability of a force field that describes these interactions is virtually impossible. The reliability of the fitted parameters can be tested only for sample cases. There are few systems that have been as thoroughly studied as water. Consequently the database of experimental information for other systems may be smaller. 

With the strengthening of the water-surface interaction, the critical temperature of the layering transition starts to decrease. When the water-surface potential Uq changes from -4.62 to -7.70 kcal/mol, T drops from 400 to 360 K, whereas the surface density of a water monolayer... 

The obtained electrostatic potential profiles and ion distributions can in principle be used to calculate surface or interfacial tensions. However, up to now only few PMFs for ion-water surface interactions are available from MD simulations and there are no reliable experimental data of interfacial tensions for SAM-solution interfaces. Therefore it is not yet possible to check if the correct Hofmeister series can be obtained with this new approach. 

Pc- (c) Dipole density p. (d) Water contribution to the surface potential x calculated from the charge density Pc by means of Eq. (1). All data are taken from a 150 ps simulation of 252 water molecules between two mercury phases with (111) surface structure using Ewald summation in two dimensions for the long-range interactions. 

The orientational structure of water near a metal surface has obvious consequences for the electrostatic potential across an interface, since any orientational anisotropy creates an electric field that interacts with the metal electrons. Hydrogen bonds are formed mainly within the adsorbate layer but also between the adsorbate and the second layer. Fig. 3 already shows quite clearly that the requirements of hydrogen bond maximization and minimization of interfacial dipoles lead to preferentially planar orientations. On the metal surface, this behavior is modified because of the anisotropy of the water/metal interactions which favors adsorption with the oxygen end towards the metal phase. 

Recently, many experiments have been performed on the structure and dynamics of liquids in porous glasses [175-190]. These studies are difficult to interpret because of the inhomogeneity of the sample. Simulations of water in a cylindrical cavity inside a block of hydrophilic Vycor glass have recently been performed [24,191,192] to facilitate the analysis of experimental results. Water molecules interact with Vycor atoms, using an empirical potential model which consists of (12-6) Lennard-Jones and Coulomb interactions. All atoms in the Vycor block are immobile. For details see Ref. 191. We have simulated samples at room temperature, which are filled with water to between 19 and 96 percent of the maximum possible amount. Because of the hydrophilicity of the glass, water molecules cover the surface already in nearly empty pores no molecules are found in the pore center in this case, although the density distribution is rather wide. When the amount of water increases, the center of the pore fills. Only in the case of 96 percent filling, a continuous aqueous phase without a cavity in the center of the pore is observed. 

In the previous chapter we considered a rather simple solvent model, treating each solvent molecule as a Langevin-type dipole. Although this model represents the key solvent effects, it is important to examine more realistic models that include explicitly all the solvent atoms. In principle, we should adopt a model where both the solvent and the solute atoms are treated quantum mechanically. Such a model, however, is entirely impractical for studying large molecules in solution. Furthermore, we are interested here in the effect of the solvent on the solute potential surface and not in quantum mechanical effects of the pure solvent. Fortunately, the contributions to the Born-Oppenheimer potential surface that describe the solvent-solvent and solute-solvent interactions can be approximated by some type of analytical potential functions (rather than by the actual solution of the Schrodinger equation for the entire solute-solvent system). For example, the simplest way to describe the potential surface of a collection of water molecules is to represent it as a sum of two-body interactions (the interac-... 

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