Moisture sorption/desorption

In any study of the moisture sorption-desorption properties of modified wood, the moisture content of the samples is determined using gravimetric methods. Invariably, the EMC of the sample is based upon the oven-dry weight of the modified wood ... 

Figure 1 Moisture sorption-desorption isotherm of powdered sucrose.

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. 

At this stage in the polymorph screen, enough data should be available to sort out the number and nature of solid forms obtained from crystallization experiments. As part of the characterization process, and to continue attempts to generate new forms, hot-stage microscopic and moisture sorption/desorption analyses should be carried out. 

It should be stressed, that a successful analysis of solvated crystal forms requires additional methods besides those mentioned with true polymorphs. Such methods are, for example, thermogravimetry or Karl-Fischer titration and moisture sorption/desorption analysis for hydrates. In order to perform studies... 

Determine the solvation/desolvation behaviour depending on the vapour pressure of the solvent (hydrates moisture sorption/desorption isotherm). 

King, C.J., Rates of moisture sorption-desorption in porous dried food. Food TechnoL, 22 50, 1968. 

Heat and mass transfers in porous media are coupled in a complicated way. On the one hand, heat is transported by conduction, convection, and radiation. On the other hand, water moves under the action of gravity and pressure gradient whilst the vapor phase moves by diffusion caused by a gradient of vapor density. Thus, the heat transfer process can be coupled with mass transfer processes with phase changes such as moisture sorption/desorption and evaporation/condensation. 

The moisture-solid interaction is an inevitable aspect of pharmaceutical development. Elucidation of moisture-induced physical alterations in amorphous pharmaceuticals is crucial, especially for ASD. Gravimetric measurement on the rate and extent of moisture gain (sorption) by or loss (desorption) from amorphous samples as a function of RH or as a function of time at a constant RH (isohumic condition) can provide a wealth of information of ASD. The key structural properties of ASD measureable by moisture sorption/desorption are drug-polymer interactions, moisture-induced glass transition, crystallization, hydrate formation/dehydration, etc. while that associated with particulate or bulk properties are hygroscopicity, diffusivity, pore size, surface area, etc. (Burnett et al. 2009). 

The main choices one has to decide on in performing a TGA experiment are the sample pans, the sample size, the temperature program, including possible isothermal steps and the gas environment—either inert or oxidative. Occasionally, it may be helpful to add moisture to the TGA s gas stream to measure moisture sorption/desorption properties (see Section 3.4.4 and Fig. 3.15) or to test a polymer s susceptibility to hydrolysis. Several manufacturers sell accessories that allow the operator to control the room temperature humidity of the purge gas (see Section 3.7 on instrumentation). A fritted glass bubbler can also be employed to saturate the purge gas with water. 

The coupled heat and liquid moisture transport of porous material has wide industrial applications. Heat transfer mechanisms in porous textiles include conduction by the solid material of fibers, conduction by intervening air, radiation, and convection. Meanwhile, liquid and moisture transfer mechanisms include vapor diffusion in the void space and moisture sorption by the fiber, evaporation, and capillary effects. Water vapor moves through porous textiles as a result of water vapor concentration differences. Fibers absorb water vapor due to their internal chemical compositions and structures. The flow of liquid moisture through the textiles is caused by fiber-liquid moleeular attraction at the surface of fiber materials, whieh is determined mainly by surface tension and effective capillary pore distribution and pathways. Evaporation and/or condensation take plaee, depending on the temperature and moisture distributions. The heat transfer proeess is coupled with the moisture transfer processes with phase ehanges sueh as moisture sorption/desorption and evaporation/condensation. 

However, under some transient situations where some phase ehange processes happen, such as moisture sorption/desorption and evaporation/ condensation, these two processes are coupled and interact significantly. 

Fan and Luo [12] incorporated the new two-stage moisture sorption/ desorption model of fibers into the dynamic heat and moisture transfer model for porous clothing assemblies. They considered the radiation heat transfer and the efFect of water content of fibers on the thermal conductivity of fiber material. Further, Fan and his co-woricers improved the model by introducing moisture bulk flow, which was caused by the vapor-pressure gradients and supersaturation state [12]. This improvement made up for the ignorance of liquid water diffusion in the porous textile material in previous models. The equations of the model are listed as follows ... 

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