Vicinal water solid interface

The above picture of water/oxide interface does not obviously show the simultaneous, primary and secondary adsorption on non-dissociated water molecules. In their review, Etzler and Drost-Hausen wrote [89] Furthermore, as mentioned elsewhere in this paper (and other papers by the present author and associates), it is obvious that vicinal water is essentially unaffected by electrical double layers . Several properties of the vicinal water appear to be similar for various solid surfaces characterized by various point of zero charge (PZC) values (the paradoxical effect ). It is therefore to be expected that the contribution to the changes of the heat of immersion with changing pH, produced by the secondarily adsorbed vicinal water, is negligible. 

A remarkable feature of vicinal water is the observation that, to a first approximation, vicinal water occurs adjacent to most (or all) solid interfaces, regardless of the chemical nature of the surface of the solid and relatively independently of the nature and concentration of solutes in solution. This substrate independence has been termed the paradoxical effect for obvious reasons. 

Sun et al. (1986) have observed that various types of clay exhibit the same influence on the vicinal water. They concluded that a substrate independence exists. Packer (1977) also noted the substrate independence of water-structuring effects. He quotes Woessner s NMR studies showing that the ratio of the deuteron-to-proton splittings for water, oriented by proximity to a clay surface (3.75), appears independent of the type of clay and that the same ratio is found for water in oriented collagen, Li-DNA, and rayon. Thus, Packer suggests that it is merely the presence of a static surface and not its nature that matters in producing dynamic orientation of that water, and that the predominant effect is water-water interaction. The importance of the paradoxical effect lies in the prediction that vicinal water occurs at all solid interfaces and must, therefore, also occur in cellular systems—the cellular interior offers vast structural areas for the induction of vicinal water. 

It is difficult to estimate the decay length of vicinal water. However, on the basis of data from several laboratories, an acceptable estimate would be a decay length in the range of 20 to 50 molecular diameters (of a water molecule)—that is, roughly 50 to 150 A (Drost-Hansen 1969,1978,1982). What this means is that vicinal water lies adjacent to most solid interfaces, representing structural changes that may be readily sensed over a distance of about 100 A. In all probability, the structural characteristics of vicinal water are close to maximal at the interface they decay away from the interface, possibly exponentially. 

In the immediate vicinity of the interface between free water and vapor, the vapor pressure at equilibrium is the saturated vapor pressure. Very moist products have a vapor pressure at the interface almost equal to the saturation vapor pressure. If the concentration of solids is increased by the removal of water, then the dissolved hygroscopic solids produce a fall in the vapor pressure due to osmotic forces. Further removal of water finally results in the surface of the product dried. Water now exists only in the interior in very small capillaries, between small particles, between large molecules, and bound to the molecules themselves. This binding produces a considerable lowering of vapor pressure. Such a product can therefore be in equilibrium only with an external atmosphere in which the vapor pressure is considerably decreased. 

Structural Component of the Disjoining Pressure This component of the disjoining pressure is caused by orientation of water molecules in the vicinity of an aqueous solution-solid interface or aqueous solution-air interface. Water molecules can be modeled as an electrical dipole. 

The bulk properties of water and of solutions of electrolytes in water have been reviewed by VON Erichsen [1955]. Furthermore, the theory of ionic solution was the theme of a discussion sponsored and published by the Faraday Society [1957]. An account of the more recent literature is contained in a review on clay-water relationships by Graham [1964] and in an article by Luck [1964]. Therefore, the treatment in this chapter will be limited to the effects of the solid interface in soils and clays on the properties of the liquid phase in the vicinity of the solid surface. 

For the weathering of rock-forming minerals, the solution kinetics is determined by the solubility product and transport in the vicinity of the solid-water-interface. If the dissolution rate of a mineral is higher than the diffusive transport from the solid-water interface, saturation of the boundary layer and an exponential decrease with increasing distance from the boundary layer results. In the following text this kind of solution is referred to as solubility-product controlled. If the dissolution rate of the mineral is lower than diffusive transport, no saturation is attained. This process is called diffusion-controlled solution (Fig. 23 right). 

The beginning of flocculation in the vicinity of the iep and redispersion of the floes below but not far from the cmc confirm a bilayer character of the adsorbed structures at saturation. Comparing the overall limiting areas per one adsorbed molecule of 0.44 nm (BDDAB/quartz) and of 0.39 nm (SHBS/zirconia) with those at the air-water interface, the surface only partly covered with bilayer can be envisaged. In the case of quartz the bilayer coverage is much greater because this solid is farther from its pzc (initial pH 6) than the other (initial pH 3.8). 

Discussing interface inhibition one finds that the pure squeezing out effect (salting out effect), which may concentrate inhibiting neutral molecules at the metal electrolyte interface, will be rather rare. However, it is possible that the activity of ions or molecules taking place in the corrosion reaction is decreased simply by the accumulation of neutral molecules in the vicinity of the metal surface. Such substances could be alcohol, water soluble inert solids in general, or inert ions. 

For liquids and solids, specific orientation and conformation of msymmetrical molecules (ions) in the interfacial regions result not only in the maximization of their interaction energy, but also yield entropy effects that cannot be neglected. For example, molecular dynamics calculations of the intermolecular potential function points to a predominant orientation of the water dipoles at the Liquid-Gas interface [45]. Other examples are an icelike structuring of water molecules in the vicinity of crystalline solid surfaces [46] and a specific orientation of the alcohol molecules in the interface between a liquid n-alkanol and water [47]. 

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