The hygroscopicity of a drug and pharmaceutical substances is a potential parameter to be considered in tablet formulation. The moisture uptake rate is quite variable depending on the type of drug and excipients as well as the environmental conditions. So, a concise definition of hygroscopicity is not possible.
Powders can absorb moisture by both capillary imbibition and swelling. The instantaneous water absorption properties of pharmaceutical excipients correlate with total surface area while the total absorption capacity correlates with powder porosity.
If drug and excipients are so hygroscopic, they can readily adsorb water until they deliquesce, or begin to dissolve. Moisture adsorption is important because adsorbed water can cause incorrect weighing and degradation of drug and/or excipients. The drug, excipients, and water reaction will continue as the amount of water increases.
The moisture uptake rates (MUR) can simply be obtained by weighing the sample after a given time (six days), but in such a case it is assumed that the moisture uptake is still in the linear phase.
If, for instance, the weight gain is 5 mg per 10 – g sample in six days, then the MUR is 5/(10 × 6) = 0.083 mg/g/day.
If the MUR values are plotted versus relative humidity (RH) and the curve that intercepts the x axis at 20% RH is obtained from a straight line, the compound can be stored without moisture pickup in atmospheres of less than 20% RH.
In addition, the hygroscopicity of materials is indicated as follows:
Depending on the hygroscopicity based on the EMC, various drug and excipients are classified in four groups:
- Type I: Non-hygroscopic
- Type II: Slightly Hygroscopic
- Type II: Moderately Hygroscopic
- Type IV: Very Hygroscopic
Read also: Determination of Loss on Drying
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