Treatment of Metal-Water Interactions

The dynamics of any metal-liquid interface involves interactions both between and among particles in the metal and fluid. For the physisorption of water on metals, where the interaction between water molecules is comparable to the metal-water interaction, it is normally assumed that the metal-water interactions can be treated with model potentials and that a detailed quantum mechanical treatment of the interaction between the two phases is not necessary, provided an adequate model of the interaction is used. Howevei a simple quantum mechanical treatment for the metal, the jellium model, exists, and its role in the simulation of metal-water interactions also is considered below. 

For bulk water-metal interfaces, there have been a few studies in which the quantum nature of the metal is treated explicitly. Such studies require 

Siepmann and Sprik - developed an approach based on an angle-dependent metal-oxygen atom potential in combination with a repulsive, three-body, metal-metal-water oxygen potential to ensure that the water molecules lie on top of the metal atoms, as found in both experiments and ab initio quantum mechanical calculations. s This model has the interesting feature that 

Zhu and Philpott employed a combination of relatively simple anisotropic and isotropic pairwise additive potential energy functions for a number of metal surfaces. They also provide a collective version of their potentials. 

Most of the collective potentials mentioned above are relatively complex because they are directly related to atom-atom potentials. Rather than constructing collective potentials in this manner, it is possible to fit them directly to the overall interactions between a water molecule and the metal, providing a simpler model that is easy to implement and interpret. For example, Spohr and Shelley et al. have employed collective potentials in which the interaction is represented as a Fourier expansion in the metal surface, but the potentials are kept simple and terms due to different physical effects are separated to facilitate systematic optimization and variation of the metal-water interactions. 

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