Hygroscopicity in Pharmaceutics
During formulation of new and existent drugs in pharmaceutics, hygroscopicity is one of the important physical parameters that are studied so as to enhance the efficacy, stability and handling of the drug. Hygroscopicity has always been an important aspect in pharmaceutics as the bulk of the drugs either in tablet or solution form is normally hygroscopic. Up to now, there remains no universally recognized definition of hygroscopicity because it is driven by both thermodynamic and kinetic forces. Hygroscopicity can mean the amount of moisture absorbed or adsorbed by a substance from the atmosphere as well as the rate of moisture uptake of a substance that is in a known relative humidity (Hilfiker, 2006, p. 236). A solid that can readily absorb moisture when exposed to the atmosphere of a known relative humidity is said to be hygroscopic while the ability to absorb or adsorb moisture is known as hygroscopy.
Hygroscopy is usually known to occur either through absorption, adsorption or deliquescence where the adsorbing or absorbing material changes physically or chemically in many of its physicochemical characteristics. Due to the adverse effects that it has on the end products, highly hygroscopic drugs will usually give weight fluctuations which lead to tablets or film-coat cracking (Wermuth, 2008, p. 758).This has led to development of formulations under controlled, humid conditions and expensive packaging are required. This has further driven many companies to come up with rules and regulations covering hygroscopicity.
The drug ingredients used in drug manufacture are usually aqueous salts that are prepared using strongly acidic or basic hydroxy-acids salts and counter ions, which possess a high degree of water vapor uptake (they are highly hygroscopic). Therefore, if the salts have a high solubility, hygroscopicity is bound to take place in the drug substance. Pharmaceutical ingredients are made of hygroscopic salts or ingredients and when they absorb moisture the chemical stability, rheological properties and the compactibility of the drugs are highly affected (Gaud, 2008, p. 72).
Hygroscopicity is believed to happen through three main modes namely adsorption, absorption and deliquescence. Water molecules get adsorbed onto the solid surface of the drug substance when they interact with the molecules on the surface. In this case, particle size plays a big role since it affects the surface area available for absorption or adsorption. Adsorption is the solid-water interaction mode that is strongest on the upper surface of the drug substance. It mostly contributes to very insignificant moisture content in pharmaceutical solids. The water-solid interaction modes involve absorption, which is the penetration of the water molecules into the solid forming a liquid or solution. Deliquescence on the other hand is the formation of a saturated solution around the solid when the solid exceeds its critical relative...