What makes a material hygroscopic

In general, the rate of transfer of moisture will fall off progressively as equilibrium is approached, not only because the concentration driving force becomes smaller, but also because the overall diffusional resistance to mass transfer increases as the more easily accessible elements of surface reach their equilibrium state so that, in a porous solid, the vapor must then diffuse into the more remote pores.

A particular example of hygroscopic behavior is deliquescence which is exhibited by many water-soluble solids, including inorganic salts e. At a given temperature, the vapor pressure of a saturated salt solution will be lower than that of pure water and, if it is less than the partial pressure in the atmosphere, moisture will be tranferred to the surface of the solids, part of which will dissolve to form a saturated solution.

When all the solids have dissolved, the process will continue until the partial pressure of the now unsaturated solution equals that in the atmosphere. In some cases, the formation of a hydrate or higher hydrate of the salt may precede the formation of a liquid phase.

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