Water sorption, amorphous solids

Two alternative explanations have been suggested which are both quite speculative. First, portions of mineral surfaces of intermediate polarity (e.g., siloxane regions, -Si-O-Si-) may permit some exchange of polar water and nonpolar organic sorbates (Hundal et al., 2001). Such surfaces occur in minerals like the faces of aluminosilicates. However, amorphous solids like silica (-Si-OH) and alumina (-A1-OH) have very hydrophilic exteriors when these inorganic materials are suspended in water. Yet these amorphous materials still clearly show sorption of apolar substances (e.g., Mills and Biggar, 1969a Schwarzenbach and Westall, 1981 Estes et al., 1988 Szecsody and Bales, 1989 Farrell et al., 1999). 

Although water vapor is absorbed into amorphous solids and not simply adsorbed on the surface, it still has been found that such sorption isotherms can be fit to the BET equation up to a. P/F of about 0.40 as with vapor adsorption, and over the entire range... 

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. 

Typically, the effect of water sorption on the T for an amorphous solid is estimated using the Gordon-Taylor equation [28a] ... 

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... 

The influence of moisture or water on ASD stability is governed by the interaction of water with either API or polymer. The amorphous solids can interact with water via two mechanisms the adsorption of the water molecules at the surface and the absorption of water into the bulk structure. Absorption is possible due to the lower density structure of amorphous solids whereby the free volume facilitates the sorption... 

The mechanism of moisture-induced relaxation remains an open question. One hypothesis is that an amorphous solid contains a finite amount of hydrophilic sites that become saturated at some critical moismre content. Above this moisture content value, adsorbed/absorbed water molecules have a greater molecular mobility. Thus, water exhibits different type of impact on the solid dispersion below and above this critical moisture content. This water-binding-site saturation hypothesis is supported by calorimetric measurements which clearly show that the heat of water sorption approaches the enthalpy of condensation of water above RH threshold (Miller and Lechuga-Ballesteros 2006). 

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. 

In summary, it is clear that water absorbs into amorphous polymers to a significant extent. Interaction of water molecules with available sorption sites likely occurs via hydrogen bonding such that the mobility of the sorbed water is reduced and the thermodynamic state of this water is significantly altered relative to bulk water. Yet accessibility of the water to all potential sorption sites appears to be dependent on the previous history and physical-chemical properties of the solid. In this regard, the water-solid interaction in amorphous polymer systems is a dynamic relationship depending quite strongly on water activity and temperature. 

The most obvious effect of changes in a solid API s water activity can be formation or loss of waters of hydration from the API crystal lattice. In some cases, loss of waters of hydration ean lead to the complete conversion of the API to an amorphous form, which in turn can lead to greater chemical instability. Eortunately, such effects are generally well-known with a given API based on moisture sorption isotherms (i.e., water uptake and loss measured using sensitive balances as a function of relative humidity). 

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