Crystalline solids water sorption

V. WATER SORPTION BY CRYSTALLINE SOLIDS A. General Model... 

The sorption of water vapor onto nonhydrating crystalline solids below RHq will depend on the polarity of the surface(s) and will be proportional to surface area. For example, water exhibits little tendency to sorb to nonpolar solids like carbon or polytetrafluorethylene (Teflon) [21], but it sorbs to a greater extent to more polar materials such as alkali halides [34-37] and organic salts like sodium salicylate [37]. Since water is only sorbed to the external surface of these substances, relatively small amounts (i.e., typically less than 1 mg/g) of water are sorbed compared with hydrates and amorphous materials that absorb water into their internal structures. 

There are many examples of second-order analyzers that are used in analytical chemistry including many hyphenated spectroscopic tools such as FTIR-TGA, IR-microscopy, as well as GC-MS, or even two-dimensional spectroscopic techniques. Another hyphenated technique that is being developed for the study of solid-state transitions in crystalline materials is dynamic vapor sorption coupled with NIR spectroscopy (DVS-NIR).26 DVS is a water sorption balance by which the weight of a sample is carefully monitored during exposure to defined temperature and humidity. It can be used to study the stability of materials, and in this case has been used to induce solid-state transitions in anhydrous theophylline. By interfacing an NIR spectrometer with a fiber-optic probe to the DVS, the transitions of the theophylline can be monitored spectroscopically. The DVS-NIR has proven to be a useful tool in the study of the solid-state transitions of theophylline. It has been used to identify a transition that exists in the conversion of the anhydrous form to the hydrate during the course of water sorption. 

The sorption of water vapor onto non-hydrating crystalline solids below RHq will depend on the polarity of the surface(s) and will be proportional to surface area. For example, water exhibits little tendency to sorb to non-polar solids like carbon or polytetrafluoroethylene... 

Kontny MJ, Grandolfi GP, Zografi G. Water vapor sorption of water-soluble substance Studies of crystalline solids below their critical relative humidities. Pharm Res 1987 4 104-112. 

With regard to crystallinity, it should be noted that amorphous materials are generally less stable than the corresponding crystalline forms. Often, amorphous phases crystallize on exposure to moisture. In amorphous solids, the net eftect of water sorption is to lower the glass transition temperature, Tg, and hence plasticize the material. In turn, this increases the molecular mobility and therefore, the chemical reactivity. 

The differences between polymorphs and hydrates are significant. The basis for all these differences is that polymorphs are different crystal structures of the same molecules(s) while hydrates are crystals of the drug molecule with different numbers of water molecules. As discussed above, the hydration state (and therefore the structure) of a crystalline hydrate is a function of the water vapor pressure (water activity) above the solid. Polymorphs, however, are typically only affected by changes in water vapor pressure if water sorption allows molecular motion, which in turn allows a reorganization into a different polymorph (i.e., a solution mediated transformation). This distinction is particularly important in defining the relative free energy of hydrates. A simple (only one molecule) anhydrous crystalline form is a one component system, and the free energy is, practically, specified by temperature and pressure. A crystalline hydrate is a two-component system and is specified... 

The RH dependency of the first-order rate constant of aspirin decomposition in the CPG 170 mixture at 50°C is illustrated in Fig. 10. An anomalous linear relationship was obtained between the RH and the rate constants as the RH was raised, the rate constant decreased. Leeson and Mattocks [21] proposed the mechanism of aspirin decomposition in the solid state as follows. After the formation of a water layer on aspirin particles, aspirin was dissolved into the water layer. The aspirin then decomposed in the solution. In the crystalline state, the amount of adsorbed water increased at a high RH therefore, faster aspirin decomposition was observed. On the other hand, the stability of aspirin in the solid dispersed system was affected by many factors such as hygroscopicity of additives, pH on the surface, specific surface area, and dispersed state. El-Banna et al. [22] reported that the aspirin in the coprecipitated samples with povidone or urea had slightly higher degradation rates due to its increased water sorption ability. In the case of a urea solid dispersion. 

An interesting use of GVS, for amorphous dispersions, is detection and quantification of crystallinity. Due to higher hygroscopicity of hydrophilic polymers and amorphous API of ASD, the exposure of a sample to elevated humidity environment triggers the sorption of water molecules by polar/hydrophilic functional groups at the air-solid interface. The a-relaxation time of amorphous material often correlates with its water sorption potential (Bhardwaj and Suryanarayanan 2013). The gravimetric vapor sorption (GVS)-desorption led to the dissimilar structurally reversal of annealed amorphous trehalose when compared to that obtained by heating beyond Tg (Saxena et al. 2013). The sorbed water molecules Tg = -137°C) increases... 

Figure 9. A RH perfusion study for crystalline sodium chloride. From 0 to 72% RH, typical water sorption exotherms are produced. Above 72% RH the water sorption no longer reaches an equilibrium, hence, the power does not return to the baseline. The sodium chloride crystal lattice begins to dissolve at 75% RH. In time all the solid will be solvated and the calorimetric signal will return to zero.

An absorbent material is one which changes either chemically, physically, or both during the sorption process. Certain chemicals, in absorbing moisture during this process, will dissolve into the water from the initial crystalline structure. Further added water results in a phase change from solid to liquid. An adsorbent is another material in which there are no chemical, phase, or physical changes during the sorption process. 

Vadas, E.B., Toma, P, and Zogra, G. (1991). Solid-state phase transitions initiated by water vapor sorption of crystalline L-660,711, a leukotriene PeceptorantagonislJharm. Res., 8 148-155. 

The three natural CDs form crystalline structures in the solid state that decompose above 200°C with no definite melting points. They are not considered hygroscopic, but they do form various stable hydrates. The water vapor sorption isotherms (Fig. 9) show two phases for the p- and y-CDs, and one phase for a-CD. At 11% RH, ot-CD absorbs 4 water molecules and upon long-term storage, forms a stable hydrate with 6 water molecules. The water content gradually increases with increasing humidity to a constant value of 6.6 water molecules per CD molecrJe at and above 79 /o Four different... 

Idealized moisture isotherms are presented in this article for substances that sorb moisture in discrete stages (e.g., crystalline materials capable of forming a hydrates) and for substances that do not interact with water in discrete stages. These idealized isotherms form a basis for the discussion of deviations and unexpected effects of moisture sorption that can influence the physical or chemical properties of the solid. 

The sorption isotherm is a vertical straight line. As soon as a pressure = / is reached, it remains constant during the entire process of sorption and cannot be further increased until all available places ate occupied. This case is obviously reali d in the formation of crystalline compounds of the solid with the vapour phase such as e. g., the hydrate CaClg. HoO from anhydrous calcium chloride and water-vapour. Crystals of the hydrate grow at the cost of the anhydrous substance. It is well known that step-ladder isotherms are then found. 

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