Physisorption of water

Tsurumi et al. recently reported that the rectifying I-V characteristics of the Pd-ZnO diode exhibited humidity sensitivity. With increasing humidity, the current under the forward bias increases remarkably by physisorption of water at the junction interface. The forward current change shows a linear relation with relative humidity from 0 to 90Vo r.h. [32]. 

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. 

Table 2. Characterization of the Physisorption of Water onto a Hg Surface"...

As shown in Fig. 11, surface dehydroxylation progressively removes physisorbed water, hydrogen-bonded vicinal (or gemlnal) hydroxyls, and isolated hydroxyls. Because surface hydroxyls have been shown to be the principal sites for physisorption of water (see, e.g., Fig. 7), dehydroxylation makes the surface progressively more hydrophobic. The extent of physical adsorption of other molecular species capable of hydrogen bonding, such as ammonia and alcohol, is reduced as well. 

M. Nagao, Physisorption of water on zinc oxide surface, J. Phys. Chem. 75 (1971) 3822-3828. 

The hydrophobic character exhibited by dehydroxylated silica is not shared by the metal oxides on which detailed adsorption studies have been made, in particular the oxides of Al, Cr, Fe, Mg, Ti and Zn. With these oxides, the progressive removal of chemisorbed water leads to an increase, rather than a decrease, in the affinity for water. In recent years much attention has been devoted, notably by use of spectroscopic and adsorption techniques, to the elucidation of the mechanism of the physisorption and chemisorption of water by those oxides the following brief account brings out some of the salient features. 

Physisorption and chemisorption of water on alumina, titania and ferric oxide selection of results (Morimoto ef a/. )... 

The earlier interpretation of point X in terms of a close-packed monolayer of water would thus seem untenable. As has been clearly demonstrated, the total uptake at X, 327pmolg" , contains a contribution of ISOpmolg" from chemisorption thus physisorption accounts for only 177pmolg, which corresponds to 21 h per molecule of water. The fact that the total uptake at X corresponds to 11-2A, and is therefore close to the figure 10-5 for a close-packed monolayer, must be regarded as fortuitous. 

John C. Shelley and Daniel R. Berard, Computer Simulation of Water Physisorption at Metal-Water Interfaces. 

In 1911 Zsigmondy pointed out that the condensation of a vapour can occur in very narrow pores at pressures well below the normal vapour pressure of the bulk liquid. This explanation was given for the large uptake of water vapour by silica gel and was based on an extension of a concept originally put forward by Thomson (Lord Kelvin) in 1871. It is now generally accepted that capillary condensation does play an important role in the physisorption by porous solids, but that the original theory of Zsigmondy cannot be applied to pores of molecular dimensions. 

R. Kutteh and T. P. Straatsma, in Reviews in Computational Chemistry, K. B. Lipkowitz and D. B. Boyd, Eds., Wiley-VCH, New York, 1998, Vol. 13, pp. 75-136. Molecular Dynamics with General Holonomic Constraints and Applications to Internal Coordinate Constraints. J. C. Shelley and D. R. Berard, in Reviews in Computational Chemistry, K. B. Lipkowitz and D. B. Boyd, Eds., Wiley-VCH, New York, 1998, Vol. 12, pp. 137-205. Computer Simulation of Water Physisorption at Metal-Water Interfaces. 

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