Water-surface interaction potential

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.

An interaction potential between the surface and ions may also be needed in simulating counterion diffusion for the smectite and mica surface models. The form of such an interaction potential remains to be determined. This may not pose a significant problem, since recent evidence (40) suggests that over 98% of the cations near smectite surfaces lie within the shear plane. For specifically adsorbed cations such as potassium or calcium, the surface-ion interactions can also be neglected if it is assumed that cation diffusion contributes little to the water structure. In simulating the interaction potential between counterions and interfacial water, a water-ion interaction potential similar to those already developed for MD simulations (41-43) could be specified. 

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. 

In the case of the Pt(lOO) surface the interaction potential is derived from semiempirical quantum chemical calculations of the interactions of a water molecule with a 5-atom platinum cluster [35]. The lattice of metal atoms is flexible and the atoms can perform oscillatory motions described by a single force constant taken from lattice dynamics studies of the pure platinum metal. The water-platinum interaction potential does not only depend on the distance between two particles but also on the projection of this distance onto the surface plane, thus leading to the desired property of water adsorption with the oxygen atoms on top of a surface atom. For more details see the original references [1,2]. This model has later been simplifled and adapted to the Pt(lll) surface by Berkowitz and coworkers [3,4] who used a simple corrugation function instead of atom-atom pair potentials. 

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. 

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... 

When the profiles of the local diameters are normalized by the bulk diameter at the same temperature, they do not collapse on a single master curve, as it happens with the profiles of the local order parameter (Fig. 49, right panel). This nonuniversality may be caused by the long-range water-surface potential. As behavior of water near a surface with short-range water-surface interaction is not yet studied, this idea remains speculative. The local diameter pd calculated in the surface layer vanishes upon increasing temperature much faster the bulk diameter (Fig. 50). It is... 

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. 

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 fact, the orientation of water at the potential of zero charge is expected to depend approximately linearly on the electronegativity of the metal.9 This orientation (see below) may be deduced from analysis of the variation of the potential drop across the interface with surface charge for different metals and electrolytes. Such analysis leads to the establishment of a hydrophilicity scale of the metals ( solvophilicity for nonaqueous solvents) which expresses the relative strengths of metal-solvent interaction, as well as the relative reactivities of the different metals to oxygen.23... 

The experimental data bearing on the question of the effect of different metals and different crystal orientations on the properties of the metal-electrolyte interface have been discussed by Hamelin et al.27 The results of capacitance measurements for seven sp metals (Ag, Au, Cu, Zn, Pb, Sn, and Bi) in aqueous electrolytes are reviewed. The potential of zero charge is derived from the maximum of the capacitance. Subtracting the diffuse-layer capacitance, one derives the inner-layer capacitance, which, when plotted against surface charge, shows a maximum close to qM = 0. This maximum, which is almost independent of crystal orientation, is explained in terms of the reorientation of water molecules adjacent to the metal surface. Interaction of different faces of metal with water, ions, and organic molecules inside the outer Helmholtz plane are discussed, as well as adsorption. 

Equation (87) and analogous equations for AG , AHm, and for surface tensions apply to molten salt mixtures in which the interaction potential can be classed as conformal. These relations may also be used to test whether the ionic interaction potential in aqueous solutions may be considered as conformal. Thus, as will be shown in one simple example, the limits of usefulness of some interionic interaction potentials may be tested in ranges of concentration of salts in water too high to obtain absolute values for the partition functions. A similar test may be made for associations in salt vapors such as... 

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. 

The theoretical study (2,3) of this interface is made inherently difficult by virtue of the complex, many-body nature of the interaction potentials and forces involving surfaces, counterions, and water. Hence, many models of the interfacial region explicitly specify the forces between colloidal particles or between solutes, but few account for the many-body interaction forces of the solvent. 

Equations 3-4 show that the form of the interaction potentials used in simulating interfacial water is critical. Of interest for interfacial systems are both the interaction potential between water molecules and that between the surface and a water molecule. 

Surface Potentials. Consider the form of the surface-water Interaction potential for an interfacial system with a hydrophobic surface. The oxygen atom of any water molecule is acted upon by an explicitly uncharged surface directly below it via the Lennard-Jones potential (U j) ... 

The above forms for the Lennard-Jones surface-water interaction potential have been used as models of hydrophobic surfaces such as pyrophyl1ite, graphite, or paraffin. If the intention of the study, however, is to understand interfacial processes at mineral surfaces representative of smectites or mica, explicit electrostatic interactions betweeen water molecules and localized charges at the surface become important. 

Two methods for including explicit electrostatic interactions are proposed. In the first, and more difficult approach, one would need to conduct extensive quantum mechanical calculations of the potential energy variation between a model surface and one adjacent water molecule using thousands of different geometrical orientations. This approach has been used in a limited fashion to study the interaction potential between water and surface Si-OH groups on aluminosilicates, silicates and zeolites (37-39). 

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