Moisture sorption profiles

Fig. 1.69.2. Moisture sorption profiles of anhydrous lactose (1), mannitol (2), trehalose (3) and sucrose (4). Top before and bottom after lyophilization (% weight change from the data in Table 1.10.4) at different relative humidity (RH) changes over 50-60 h. Before lyophilization 1, lactose — 1.1 RH 10%, 1.2 RH 60% 2, mannitol 3, trehalose - 3.1 RH 10%,...

Fig. 1.69.3. Moisture sorption profiles of dextran and povidone % weight change from the data in Table 1.10.4 (Figure 2 from [1.152])...

Malamataris, S. Dimitriou, A. Moisture sorption profiles and tensile strength of tableted phenobarbitone formulations. J. Pharm. Pharmacol. 1990, 42, 158-163. 

Nonspecific hydration, or hydration of the lattice without first-order phase transitions, also must be considered. Cox et al. [40] reported the moisture uptake profile of cromolyn sodium, and the related effects on the physical properties of this substance. Although up to nine molecules of water per molecule of cromolyn sodium are sorbed into the crystalline lattice at 90% relative humidity, the sorption profile does not show any sharp plateaus corresponding to fixed hydrates. Rather, the uptake profile exhibits a gradual increase in moisture content as relative humidity increases, which results in... 

Normally, the moisture sorption-desorption profile of the compound is investigated. This can reveal a range of phenomena associated with the solid. For example, on reducing the RH from a high level, hysteresis (separation of the sorption-desorption curves) may be observed. There are two types of hysteresis loops an open hyteresis loop, where the final moisture content is higher than the starting moisture content due to so-called ink-bottle pores, where condensed moisture is trapped in pores with a narrow neck, and the closed hysteresis loop may be closed due to compounds having capillary pore sizes. 

Of particular importance is the timescale over which diffusion occurs under various conditions of relative humidity (RH) and temperature. The RH determines the equilibrium moisture concentration, whereas higher temperatures will accelerate the moisture sorption process. In order to predict the moisture profile in a particular structure, it is assumed that Fickian diffusion kinetics operate. It will be seen later that many matrix resins exhibit non-Fickian effects, and other diffusion models have been examined. However, most resin systems in current use in the aerospace industry appear to exhibit Fickian behaviour over much of their service temperatures and times. Since the rate of moisture diffusion is low, it is usually necessary to use elevated temperatures to accelerate test programmes and studies intended to characterize the phenomenon. Elevated temperatures must be used with care though, because many resins only exhibit Fickian diffusion within certain temperature limits. If these temperatures are exceeded, the steady state equilibrium position may not be achieved and the Fickian predictions can then be inaccurate. This can lead to an overestimate of the moisture absorbed under real service conditions. 

In all the previous examples, the process was absorption, and water ultimately had access to the entire solid variations in particle size, and consequently, surface area, would not effect the profile. In many of the subsequent examples, not only is surface area a determinant of moisture content, but perturbation of the solid can occur during sorption to produce unexpected and dramatic effects. 

Fig. 10.1.3 [Kop3] Moisture transport in cylindrical catalyst support pellets from alumina with a diameter of 3.5 mm. (a) Drying profiles of water along the diameter of an initially wet piellet. Dotted lines Experimentally detected profiles. Solid lines simulated profiles. The first profile was acquired when the dry gas flow was turned on. The delay between the detection of the successive profiles is 60s. Profiles 1-7,9, 11,14, and 18 are shown, (b) Experimental profiles for water vapour sorption by an initially dry pellet containing CaC with uniform salt distribution.

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