Water-solid interactions relative humidity

Maintenance of constant relative humidity environments is essential for studying water-solid interactions. There are primarily four techniques that are frequently employed to maintain constant relative humidity ... 

Solids that form specific crystal hydrates sorb small amounts of water to their external surface below a characteristic relative humidity, when initially dried to an anhydrous state. Below this characteristic relative humidity, these materials behave similarly to nonhydrates. Once the characteristic relative humidity is attained, addition of more water to the system will not result in a further increase in relative humidity. Rather, this water will be sorbed so that the anhydrate crystal will be converted to the hydrate. The strength of the water-solid interaction depends on the level of hydrogen bonding possible within the lattice [21,38]. In some hydrates (e.g., caffeine and theophylline) where hydrogen bonding is relatively weak, water molecules can aid in hydrate stabilization primarily due to their space-filling role [21,38]. 

Trace contaminants are also significant at charged solid surfaces, affecting both the charging process and the surface conductivity. In ambient air atmospheres their effect is often determined by interaction with adsorbed water vapor, whose dominant concentration may be sufficiently large to form a monolayer. Topical antistatic agents for solids typically rely on interaction with adsorbed water and can lose effectiveness at low relative humidity (4-2.1). 

It is the objective of this chapter to discuss the various mechanisms whereby water can interact with solid substances, present methodologies that can be used to obtain the necessary data, and then discuss moisture uptake for nonhydrating and hydrating crystalline solids below and above their critical relative humidities, for amorphous solids and for pharmaceutically processed substances. Finally, transfer of moisture from one substance to another will be discussed. 

This analysis can be applied to a small dry salt particle exposed to increasing relative humidity. The particle remains solid until, if it is hygroscopic, a characteristic relative humidity less than 100% at which it absorbs water and dissolves, forming a saturated solution. The relative humidities at which this occurs for saturated solutions of various salts are shown in Table 9.3. These values will vary with crystal size because of the Kelvin effect. For sodium chloride,. solution takes place at a relative humidity of 75% at which the diameter about doubles. With increasing relative humidity, the equilibrium relationship between drop size and vapor pressure is determined by the interaction of the Kelvin effect and vapor pressure lowering. 

Many compounds and salts are sensitive to the presence of water vapour or moisture. When compounds interact with moisture, they retain the water by either bulk or surface adsorption, capillary condensation, chemical reaction and, in extreme cases, a solution (deliquescence). Deliquescence is where a solid dissolves and saturates a thin film of water on its surface. It has been shown that when moisture is absorbed to the extent that deliquescence takes place at a certain critical relative humidity, the liquid film surrounding the solid is saturated. This process is dictated by vapour diffusion and heat transport rates (Kontny et al. 1987). 

Water held by capillary tension in pores of diameters smaller than 50 nm wiU evaporate at lower values of the relative humidity as the diameter of the pores decreases. Indicatively, values of relative humidity from 95 % to 60 % are required when the diameter of capillary pores decreases from 50 nm to 5 nm [17]. In this case, evaporation can produce significant shrinkage of the cement paste. In addition, the mobihty of ions (thus the electrical conductivity of the solution in these micropores) is affected by chemical and physical interactions between the Hquid and the solid and is therefore lower than that of a solution of the same composition. 

In a recent article [5] dealing with the properties of adsorbed water layers and the effect of adsorbed layers on interparticle forces, it was clearly stated that even under common room conditions (relative humidity in the region 40-60%), two or three adsorbed monolayers of water are often present on particles, dominating the interactions, and therefore the physical characteristics of the material. For a two-phase equilibrium system containing hydrophilic silica plates (surface of a-quartz covered by silanol groups) and water molecules, a molecular dynamic simulation expected at least one adsorbed monolayer to be present. Quite different behavior would be expected for less hydrophilic surfaces. The material character and chemical properties of solid materials are of crucial importance in the hydration interaction. Therefore, some common adsorbents which are Irequently used in aqueous electrolyte solutions are discussed separately. 

Until now we did not mention the interaction between liquid drops in a gas. In principle, such drops interact hke solid surfaces. In the absence of electrostatic charging, this interaction is dominated by van der Waals attraction. We just have to take into account a possible deformation of the surfaces. Therefore, we do not discuss it here. We would, however, like to mention one effect, which is typical for the interaction of fluid interfaces very often, the systems are not in equilibrium and the interaction between fluid interfaces is influenced or even dominated by nonequilibrium effects [733, 745]. One is unique for drops of volatile liquids. If the liquid is not in a saturated atmosphere of its vapor, it will evaporate. The flow of the vapor emanating from the liquid surface can lead to an effective repulsive force. Such a repulsion was indeed noticed by Prokhorov, who measured the interaction between two water drops [746]. He observed a repulsion that increased with decreasing relative humidity. 

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