Physisorption of water on salt surfaces
Introduction
The study of water adsorption on salt is of fundamental importance in various areas of science and technology. Our work is motivated by the technological procedure of separating NaCl and KCl using contact electricity [1], [2]. A mixed powder of the raw salts can be separated in an electrical field. However, the control of relative humidity and of chemical additives is important. The process of grinding the mineral salts produces a considerable amount of surface defects at which water adsorption and adsorption of chemical additives play an important role in charging. Dissociative phenomena might also be important, since it is known that, for example, color centers on alkali halides can lead to water dissociation [3]. Starting with questions concerning water at surface defects of salts, we found that even the water interaction with a clean surface is not completely understood on the atomic scale. The aim of this contribution is to discuss the structure and dynamics of water adsorption on a clean NaCl(1 0 0) surface.
Recent investigations have shed new light on the H2O/NaCl(1 0 0) problem. Refined experimental techniques permitted to resolve the orientation and structure of adsorbed layers of water. From the theoretical side, computational ab initio methods developed during the last decade allowed to determine adsorption energies on the basis of quantum mechanics [4]. In particular we refer to recent experimental [5], [6], [7], [8], [9] as well as theoretical work [10], [11], [12], [13] and combination of both [7], [14], [15]. In one of the first calculations of the adsorption energy of water on NaCl a continuum ansatz was used which already gave reasonable results for the adsorption energy of a single molecule [16]. However, greater accuracy is needed, especially if one wants to understand the microscopic structure of hydrogen bonds or results of atomic force microscopy (AFM) showing surface step motion after nanoindentation induced by adsorbed water during the AFM contact [17], [18], [19], [20].
Recent self-consistent Hartree–Fock calculations concentrate on optimum adsorption sites of water molecules on NaCl(1 0 0) [11], [12], [13]. In this work, we discuss results of static calculations and molecular-dynamics (MD) simulations, both based on density functional theory. In particular, we focus on the (1×1) monolayer of H2O for which we obtain new and more systematic results on adsorption energies and structural arrangements of the H2O molecules than those published in the references given above. The (1×1) overstructure might seem to be an artificial structure, since (as MD simulations show) reorientation transitions to larger overstructures can further optimize the binding energy of the water layer. Nevertheless, calculations for the (1×1) structure allow predictions of favorable adsorption sites and have yielded accurate information about the adsorption energy of a single water molecule. Another motivation for these calculations was the postulation of ordered water layers at temperatures T<130 K [5], [6], [11]. We have investigated this by ab initio MD simulations which give new information about the dynamic stability of the different adsorption geometries. This is also of interest for structural and dynamic aspects of the c(2×4) ice structure which was predicted on experimental grounds [6], [8]. Static aspects of this ice structure were discussed in Refs. [10], [12] on the basis of model potentials and ab initio calculations.
Section snippets
VASP
All calculations are done by using the Vienna ab initio simulation package (VASP) which is based on density functional theory, plane wave basis set and a version of Vanderbilt's ultrasoft pseudopotentials [21], [22]. Furthermore, we use local density approximation (LDA) and the exchange correlation functional of Ceperley and Alder parameterized by Perdew and Zunger [23]. The Kohn–Sham Hamiltonian is diagonalized with an iterative scheme in order to optimize the electronic wave function (by
Free water
The relaxation of an isolated H2O molecule in a 10×10×10 Å3 supercell yields a bond angle of α=105° and an O–H bond length of 0.975 Å. These are close to the experimental values of α=104.5° and a=0.96–0.98 Å. We have also checked that the vibrational frequencies of the water molecule agree with experimental values: We obtained for the O–H stretch mode (109.6 THz) and for the bend-vibration mode (47.85 THz).
In the following of this section, we discuss the interaction
Adsorption of water on NaCl(1 0 0)
In this section we discuss results of static calculations for adsorption of H2O on NaCl(1 0 0). Section 4.1 deals with the ideal monolayer adsorption, in which case all H2O molecules are adsorbed and oriented in the same direction. Since Esw does not include Eww, these monolayer calculations yield an approximate value for Esw of an isolated H2O molecule in different positions. In Section 4.2 we discuss the case of half-monolayer coverage and in C the static optimization of the c(2×4) bilayer
Molecular-dynamics of water adsorption on NaCl
We have carried out MD simulations for the 2×4 surface cell with diagonal boundary conditions and coverages from 0.25 to 1.5 water molecules per NaCl formula unit. In order to achieve a monolayer coverage in this case, four water molecules have to be taken into account corresponding to one H2O molecule per NaCl unit cell. For each configuration, simulations are done up to the ps range (1–4 ps), and temperatures are varied from 100 to 300 K. In contrast to the static calculations, we allow the
Adsorption on a KCl substrate
Comparative calculations have been performed for water adsorption on KCl. The main difference with respect to NaCl is the by 10% larger lattice constant of KCl. For a satisfying description of KCl it is necessary to use pseudopotentials which take the inner p electrons into account. Optimization of bulk KCl yields a LDA lattice constant of 6.15 Å which is smaller than the experimental value of 6.28 Å. For the cleavage energy of the KCl(1 0 0) face, we obtain 0.43 eV. No tendency for surface
Summary
We have performed ab initio calculations for the adsorption of water on NaCl and KCl and have determined Esw and Eww for different adsorption geometries. The H2O molecule is physisorbed via H–Cl hydrogen bonds on both substrates. On NaCl, oxygen is also bound to the cation leading to a flat adsorption geometry with O shifted as proposed by Bruch et al. [5] (Fig. 4b), whereas on KCl, the oxygen–cation interaction is weaker so that the up-pointing position in Fig. 4d is favored. The MD
Acknowledgements
This work has been supported by the German Science Council DFG through the Graduate School Structure and Dynamics of Heterogeneous Systems at the Gerhard-Mercator-University of Duisburg. We acknowledge discussions with D. Wolf (Duisburg), M. Henzler and H. Pfnür (Hannover).
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