Pharmaceutical solids water sorption

VII. WATER SORPTION BY PHARMACEUTICAL SOLIDS SUBJECTED TO PROCESSING... 

Two parameters of excipients that are important to the compatibility formulator are (i) the ability of the excipients to absorb water at variable humidity and (ii) the pH that the excipient will impart in the solid-state environment. Knowledge of water sorption of excipients is well documented (27) and is used to estimate the amount of water the excipients will introduce into a solid-formulation mixture (30). The percentage of water and the dispensation of water, whether bound or variably adsorbed, was shown to have a dramatic effect on the hydrolytic stability of aspirin (31). It is well known that water will redistribute within solid pharmaceutical systems and this redistribution has been modeled and studied extensively (18). The pH influence that an excipient imparts to the formulation matrix can be directly related to... 

M. Hageman, Water sorption and solid state stability of proteins, in Stability of Protein Pharmaceuticals. Part A. Chemical and Physical Pathways of Protein Degradation (T. Ahem and M. C. Manning, eds), Plenum Press, New York, 1992, pp. 273-309. 

Kontny, M.J. Zografi, G. Sorption of water by solids. In Physical Characterization of Pharmaceutical Solids Brittain,... 

The tendency of a solid to take up water from the atmosphere, as it is subjected to a controlled RH program under isothermal conditions, is referred to as hygroscopicity (35). The hygroscopicity of a solid can be classified based on the amount or rate of water uptake when a solid is exposed to controlled RH values at a specified temperature (Table 1 from Ref. 35). The RH of a saturated solution of the API, RHo, can also be of importance. From a preformulation perspective, it is important to know the effect of water sorption on the pharmaceutically relevant properties of the API. 

Dalton, C. R. and Hancock, B. C. Processing and storage effects on water vapor sorption by some model pharmaceutical solid dosage formulations. Int. ]. Pharm. 156 143—151, 1997. 

In the preformulation study, the comprehension of physicochemical properties regarding water-solid surface interaction is beneficial to the handling, formulation, and manufacture of the finished products. Data on sorption/de-sorption isotherm, hydration of salts of drug product, water sorption of pharmaceutical excipients, and kinetics of water adsorption or desorption of a substance can be obtained effectively by the dynamic vapor sorption method. The knowledge may be utilized for dosage form design and supports the understanding of the mechanism of action. 

Biomaterial films can be used to prevent water sorption by low moisture foods or pharmaceutical solids and, thus, to maintain product quality, and to improve shelf life and storage stability. For many biomaterials water acts as a plasticizer and therefore affects stability and quality (Roos and Karel, 1990, 1991). Hence, it is important to control water sorption properties of low moisture foods or pharmaceutical products. Edible films can be used as barriers or retarders of water sorption of low moisture products. 

Water vapor sorption (WVS) is used to assess the hygroscopicity of the hot melt extruded formulations. WVS is used in quantification of amorphous content in pharmaceutical solids. Prodduturi et al. investigated the use of rapid dynamic vapor sorption techniques for analysis of films containing hydroxypropyl cellulose and clotrimazole. ... 

The sorption of water by a solid is of extreme interest to the pharmaceutical scientist since the stability of a drug entity in either its bulk or formulated state often critically depends on the nature of water contained within the solid [78],... 

Water is introduced into closed pharmaceutical systems either accompanying the input materials or in the headspace as relative humidity [79]. Whatever water is contained within the dosage form and its container will ultimately equilibrate among the components according to its affinity for the solid ingredients and the number of association sites. The Sorption-Desorption Moisture Transfer model has been used to evaluate the thermodynamically favored state that will result after the equilibration process is complete [79]. 

The isotherm for aspirin would be classified as Type III, indicating a low affinity for water followed by multilayer sorption. These four isotherms cover a broad range of moisture interaction with solids of pharmaceutical interest. 

Sorptive properties of the pharmaceutical products as represented by their sorption isotherms reflect their ability to absorb water from the air and are the source of valuable information for the selection of dryers as well as the packaging conditions. The sorption isotherm is the relationship between the relative humidity of the air and the moisture content of solid in equilibrium with this air. 

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