Virtually all materials are susceptible to the influence of humidity in some way or other.
Knowing how material properties may change in a humid environment is the key to decisions regarding the processing, packaging, storage and stability of a product.
A sorption isotherm is the graphical representation of the changes in the water content of a sample as a function of relative humidity at constant temperature.
Sorption isotherms are the most relevant thermodynamic parameters and at the same time the most effective way to demonstrate the interaction of materials with water vapour.
Their shape and course provide information about the changes in the material and hence about important material properties and how to handle them.
Because sorption and desorption are very complex physicochemical processes, the isotherms cannot be determined by calculation but must be recorded experimentally for each product.
Determination of the sorption isotherm
The easiest and most accurate method for the determination of the sorption isotherm is to measure the change in mass of a sample at varying humidity levels.
Limitations of the procedure
The determination of sorption and desorption processes requires a great number of individual measurements.
Due to slow sorption and desorption kinetics, it may take a very long time for samples to reach equilibrium.
To enable high sample throughput (routine analysis, quality control) it is therefore necessary to examine several samples at the same time.
Especially in scientific applications, it may be important that samples can be withdrawn during a measuring cycle to perform additional analyses such as spectroscopy, X-ray powder diffractometry or microscopy.
Our new concept of moisture stability tests meets these requirements with a completely new type of water sorption systems: the SPS instruments.
The property of solid materials to absorb and retain or release water.
The amount of water present in a material (percent by weight).
Hygroscopic material always tries to be in balance with the ambient atmosphere. The presence of water in the material results in water vapour pressure on the material surface. If this pressure is the same as that of ambient air, the material is in balance with the atmosphere. Any difference in water vapour pressure between the material and the ambient air causes the exchange of water and hence a change in the water content of the material until equilibrium is regained.
Hence, the equilibrium humidity of a material is the relative humidity that has to prevail in the ambient atmosphere in order to prevent the exchange of water.
Water activity is the relative humidity that has to prevail in the ambient atmosphere in order to prevent the exchange of water between material and air. It basically corresponds to the equilibrium humidity of a material, but is stated as 0 … 1 Aw instead of 0 … 100% relative humidity.
In steady-state, the relationship between the water content and the equilibrium humidity of a material can be represented graphically by a curve, the so-called sorption isotherm.
The isotherm shows the water content of a particular material at each humidity level at a given constant temperature. Any change in the composition or quality of the material will result in a change in sorption behaviour. Due to the complexity of the sorption processes, the isotherms cannot be determined by calculation, but must be recorded experimentally for each product.
Examples of water vapour sorption isotherms of some excipients
The first figure below gives examples of sorption isotherms of some excipients analysed with the SPS.
Microcrystalline cellulose shows the typical sorption behaviour of polysaccharides with marked hysteresis between the sorption and desorption curve.
Sucrose starts to liquefy above the critical 80% relative humidity level.
Lactose monohydrate only takes up approx. 0.13% of water over the entire humidity range. However, within this range, an isotherm with good resolution can be determined.
Examples of drugs that may form a hydrate or different types of hydrates are given in the second figure below.
Drug A is a typical stochiometric monohydrate which starts to form at 40% RH and releases water below 20% RH.
Drug B does not change into a hydrate until 90% RH is reached. Desorption takes place over several well detectable hydrate stages. Hysteresis between the sorption and desorption curve is pronounced for these samples, which is a characteristic feature of stochiometric hydrates.
Ulrich J. Griesser, Institute of Pharmacy, University of Innsbruck, Austria and
Juergen Dillenz, ProUmid GmbH & Co KG, Ulm, Germany