hygroscopicity

Leave a bowl of table salt out on a humid day and it clumps; leave silica gel packets in a shoebox and they swell with absorbed moisture. Hygroscopicity is that same hunger for water — a material's tendency to take up moisture from the surrounding air, the higher the humidity the more it pulls in.

Hygroscopicity ranges from mild surface adsorption to deliquescence, where a very hungry solid pulls in so much water that it dissolves into a liquid puddle. Some drugs and excipients also form defined hydrates, taking water into the crystal lattice at characteristic humidity thresholds. It is measured by exposing the material to controlled humidity and weighing the gain, often as a dynamic vapor sorption isotherm.

Absorbed water is rarely harmless in a solid product. It can trigger hydrolysis and chemical degradation, promote caking and poor powder flow, soften tablets, drive polymorph or hydrate conversion, and throw off content uniformity by adding hidden mass. Highly hygroscopic drugs therefore demand low-humidity manufacturing, protective packaging with desiccant, and sometimes a less hygroscopic salt or form chosen during salt selection.