Definitions
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Definitions:
Hygroskopic:
The characteristic of solids to absorb, retain or desorb humidity is identified as hygroscopic.
Water content:
The water content of a material is the amount of water contained in the material expressed in percentage of weight.
Equilibrium humidity:
A hygroscopic material tries to establish equilibrium of its humidity with that of its environment. The presence of water in the material produces a water vapor pressure on the material surface. If this pressure is equal to the water vopour pressure of the ambient atmosphere, then the material achieved equilibrium humidity with its environment. Any difference in water vapor pressure between material and ambient air produces a water exchange and therefore a change of the water content of the material concerned, until equilibrium is achieved.
Equilibrium humidity of a material is therefore defined as the relative humidity of the ambient atmosphere that has to prevail in order to cause no exchange of water.
Water activity:
The relative humidity, which must prevail in the surrounding atmosphere to avoid water exchange between material and air, is defined as water activity. It is practically the same as the equilibrium humidity of a material in the definition, but is not stated in 0...10% r.H., but in 0...1Aw.
Sorption isotherms:
At equilibrium, the relation between water content and equilibrium humidity of a material can be displayed graphically by a curve, the so called sorption isotherm.
For each humidity value, a sorption isotherm indicates the corresponding water content value at a given, constant temperature. If the composition or quality of the material changes, then its sorption behaviour also changes. Because of the complexity of sorption processes, the isotherms cannot be determined by calculation, but must be recorded experimentally for each product.
Examples for water vapor sorption isotherms of some auxiliary substances
Figure 1 shows examples for the water vapor sorption isotherm of some auxiliary substances, determined with the SPS11 analyzer.
The microcrystalline cellulose shows the classic sorptional behavior of polysaccharides with a broad hysteresis between the sorptional- and desorptional course.
Above a critical relative humidity of 80% r.H. the saccarose sample starts to liquefy.
Lactose monohydrate adsorbs only 0.13% of water over the entire humidity range. Nevertheless within this range the sorption isotherm can be determined with a good resolution.
Figure 2 shows two examples of medical products forming a hydrate or several hydrate forms. Medicament A shows a classic stoichiometric monohydrate, starting to accumulate at 40% r.H. and again ceding the water at a relative humidity below 20% r.H.
Medicament B starts metabolizing into a hydrated form not until 90% r.H. The course of the desorption courve shows several well identifiable hydrate steps. As a characteristic of stoichiometric hydrates, there is a distinctive hysteresis between sorption- und desorption courve of these samples.
Ulrich J. Griesser, Institute of Pharmacy, University of Innsbruck, Austria and
Juergen Dillenz, PMT Analytical GmbH & Co KG, Ulm, Germany
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