Compressibility in pharmaceuticals is mainly governed by the relationship between which two factors?

Compressibility in pharmaceuticals primarily refers to the ability of a powder to decrease in volume under pressure, which is crucial for tablet formulation and other solid dosage forms. The relationship between solid fraction and compaction pressure is central to this process.

When compaction pressure is applied to a powder mixture, the individual particles are forced closer together, effectively reducing the volume or increasing the solid fraction. The degree to which the powder compresses, therefore, is highly dependent on the amount of pressure applied during compression as well as the arrangement and characteristics of the solid particles themselves. An increase in compaction pressure typically leads to a greater solid fraction, resulting in a denser and more stable tablet formulation.

Understanding this relationship is critical, as it informs the formulation process, ensuring the resulting product meets desired specifications for strength and release characteristics.

In contrast, the other options mentioned do not directly address the primary factors influencing compressibility in the context of pharmaceutical sciences. Temperature and humidity, for instance, may affect stability but not the compressibility under pressure. Similarly, API concentration and moisture content, as well as particle size and viscosity, may influence other properties such as solubility or flow, but they do not serve as the main determinants of compressibility during compaction.