hygroscopicity

Leave a cracker out and it goes soft; leave a bag of sugar open in a humid kitchen and it clumps. Some materials pull water vapor straight out of the air, and that thirst for atmospheric moisture is hygroscopicity. A drug powder can be hygroscopic to the point of caking, picking up weight, or even slowly dissolving in the water it absorbs.

Hygroscopicity is measured by exposing a solid to controlled humidity and tracking how much water it gains, often with a moisture-sorption balance. The behavior depends strongly on the solid form: different salts, polymorphs, and the amorphous state can absorb very different amounts of water, and an amorphous solid is usually more hygroscopic than its crystalline counterpart. A highly hygroscopic material may need protective packaging, low-humidity processing, or a switch to a less thirsty form.

The consequences of unchecked moisture uptake go beyond a damp powder. Absorbed water can accelerate chemical degradation, trigger a hygroscopic form to convert into a hydrate, lower a glassy material below the moisture level at which it crystallizes, and ruin the flow and compaction needed to make tablets. Managing hygroscopicity is therefore a recurring theme in choosing a salt and a solid form.