INTRODUCTION
Water sorption of pharmaceutical materials is considered as an important, sometimes critical, factor that affects selection of the salt and crystal form of a drug substance, manufactur-ing and performance of solid dosage forms. In this honorary edition of Journal of Pharmaceutical Sciences, devoted to Pro-fessor George Zografi, it is noted that he along with colleagues and students began to investigate the water sorption properties in the 1970s, building a prototype moisture sorption system to evaluate pharmaceutical materials.1 In 1988, Prof. Zografi re-viewed the critical aspects of water–solid interactions from a fundamental, scientific perspective, highlighting the distinc-tion between adsorption in crystalline and absorption in amor-phous solids, capillary condensation, deliquescence, and for-mation of hydrates2—physicochemical factors that continue to challenge the development of drug products 25 years later. Over the years, it has been recognized that water sorption can affect powder bulk density, blending, flow and compaction,3–5 capsule mechanical properties,6 tablet hardness,7–9 disintegration7,10–12 and dissolution,10–14 excipient compatibility,15–17 and chemical stability.18–20 The moisture properties of pharmaceutical solids are an important part of package selection.21–23 Many commer-cial drug products contain a storage statement on the label around moisture protection, and may contain a desiccant or special packaging, underscoring water’s relevance to detrimen-tal changes that it can produce on storage.
Most drugs are crystalline solids that generally sorb only a small amount of water from the atmosphere, for example, 0.1%–0.2% water at a relative humidity (RH) as high as 90%. The reason for the low sorption level is that water molecules
mainly have access to the solid surface, not to the bulk of crys-talline particles. After only a few layers of water molecules deposit onto a surface, the sorbed water properties start to ap-proach that of bulk water, that is, the vapor pressure or RH approaches 100%. For example, if a crystalline solid has a typ-ical specific surface area of 0.5–1 m2/g, using water’s area of 0.10 nm2/molecule value,24 one molecular layer would occur at a water content of 0.03%. Considering that multilayers can form before a monolayer is established, it is not too surprising that at a few multiples of 0.03%, the RH will approach 100%, which is consistent with the high humidity water content of crystalline solids in the 0.1%–0.2% range. The important physical perspec-tive is that although the moisture level is small for crystalline solids, water molecules occupy a significant fractional coverage of a solid’s surface even at a moderate RH of 50%.
Another important aspect of water sorption is that it can vary from one batch of drug or excipient to another, and in cases where the moisture level is critical, the water sorption profile becomes an important characterization technique for raw ma-terials. There can be energetic and micromeritic reasons for the water sorption level variation. Typically, raw materials are also tested for particle size and specific surface area, and the results can be used to try to normalize the moisture sorption profile by these properties, for example, water content per unit of surface area. It is also possible to analyze a moisture sorption isotherm in terms of the Brunauer–Emmett–Teller (BET) equation to extract a CB value related to energy of adsorption of the first layer or the Guggenheim-Anderson-de Boer (GAB) equation to extract CG and K values related to adsorption energies of the first and an intermediate layer.25,26 Both models also provide the Wm parameter formally related to monolayer capacity, and although it does not provide specific surface areas in accord with values from N2 gas adsorption, it can have interpretive value.27 The BET or GAB sorption energy parameters fail to give the complete characterization of a vapor’s interaction with
a solid, in particular water’s energetic interaction with a drug or excipient. Specifically, the BET or GAB models do not in-corporate interactions between adsorbed vapor molecules, in particular water–water interactions, which presumably are an important factor in the sorption process as the RH increases, es-pecially for the water molecule that exhibits strong H-bonding with itself. Indeed, IR spectroscopic experiments of moisture sorption on NaCl have shown H-bonding of water with itself at submonolayer coverage.28 It is in fact well established from sorption calorimetry experiments that heats of sorption vary continuously as the vapor pressure (or RH in the case of water) of the probe increases, indicating that a single energy param-eter such as CB can miss important detail in characterizing water–solid interactions.25,29
Sorption calorimetry is less commonly used in routine char-acterization of drug substances and raw materials. In contrast, automated water sorption equipment is a standard technique that requires minimal sample preparation and effort, making it more appropriate for routine characterization. The main data obtained by the water sorption technique is water weight ver-sus RH. Although it is necessary to quantify the water content of materials and the isotherm provides practical value in char-acterizing their “hygroscopicity,”30 the isotherm contains far greater information regarding water–solid thermodynamic in-teractions. In this research, a more detailed analysis of water sorption isotherms is executed. The water sorption isotherms were measured for several anhydrous crystalline solids, an isolated site hydrate, a channel hydrate, and an amorphous polymer. Specifically, the compounds selected represent “non-hygroscopic” anhydrous drugs (genistein, indomethacin, and griseofulvin), the nonhygroscopic excipient lactose mono-hydrate, the hygroscopic channel hydrate erythromycin A dihydrate, and the hygroscopic amorphous excipient polyvinylpyrrolidone (PVP K29-32). The water sorption isotherm data were analyzed to provide partial molar enthalpy and partial molar entropy of sorbed water, as well as the partial molar quantities for the solid component in the case of PVP and erythromycin A dihydrate. It was the intent of this research (1) to illustrate and execute the methodology and (2) to provide the interpretation of the thermodynamics of pharmaceutical solids with sorbed water.