Equilibrium moisture sorption

Fig. 5. First (open circles) and second (full circles) equilibrium moisture sorptions in the DGEBA-TETA neat resin. The dotted line is eq. (1)...

By equilibrium moisture sorption behavior, we refer to the moisture content of paper sheets attained when the sheets reach a final thermodynamic equilibrium state with the environment around them. This represents the limiting moisture content under a particular relative humidity but also is a function of the past history to which the paper sheet was exposed. 

Figure 1. Typical equilibrium moisture sorption isotherms for paper. I - adsorption, 2 - desorption.

Alternatively, aw may be measured by isopiestic equilibration. In this method, a dehydrated sorbent (e.g. microcrystalline cellulose) with a known moisture sorption isotherm (section 7.4) is exposed to the atmosphere in contact with the sample in an enclosed vessel. After the sample and sorbent have reached equilibrium, the moisture content of the sorbent can be measured gravimetrically and related to the nw of the sample. 

Salwin and Slawson (1959) found that stability in dehydrated foods was impaired if several products were packaged together. A transfer of water could take place from items of higher moisture-vapor pressure to those of lower moisture-vapor pressure. These authors determined packaging compatibility by examining the respective sorption isotherms. They suggested a formula for calculation of the final equilibrium moisture content of each component from the iso-... 

Numerous efforts have focused upon the nature of moisture transport of epoxy systems. Previous-sorption desorption work demonstrated that equilibrium moisture levels In an epoxy system can be related to thermodynamic states (1,2,3). Transient and equilibrium dynamic mechanical experiments are performed In this work with two epoxy systems TGEBA-TETA and N-5208. These experiments provide Insight Into the nature and extent that network changes have on the dynamic mechanical properties as a result of hygrothermal cycling. 

Moisture sorption of microcrystalline cellulose has been studied extensively. Fig. 6A includes the sorption and desorption studies for microcrystalline cellulose. The inserts are plots of moisture content versus time, which approximately represent the kinetics of sorption and desorption at each humidity. The equilibrium adsorption isotherm (Type II) has been fit to the BET equation (C = 16.48, = 0.033 g/g solid) and this curve is... 

The influence of sorbed moisture on chemical stability and the flow and compaction of powders and granulations is well established. The moisture content and hygroscopicity of excipients is particularly important as total product processing as well as finished product stability can be affected. Hygroscopicity, moisture-sorption isotherms, and equilibrium moisture content can be determined by thermogravimetric analysis and Karl Fisher titration methods. 

Moisture Sorption Isotherms. Green wood loses moisture to the atmosphere and approaches a moisture content designated as the equilibrium moisture content (EMC) for the particular atmospheric conditions. The EMC is a function of relative humidity, temperature, previous exposure history (hysteresis), species, and other miscellaneous factors. 

Effect of Relative Humidity and Sorption History. An indirect method for estimating wood moisture content is to measure its equilibrium relative vapor pressure h. This is related to wood moisture content by a sorption isotherm. The percent relative humidity (H) or relative vapor pressure (h) (H = 100 h) is the most important factor in determining the EMC for wood. A curve showing EMC as a function of percent relative humidity or relative vapor pressure at constant temperature is called a moisture sorption isotherm. 

Hailwood-Horrobin Solution Sorption Theory. The Hailwood-Horrobin (57) model treats moisture sorption as hydration of the polymer, taken here to be dry wood, by some of the sorbed water called water of hydration, m. The hydrate forms a partial solution with the remaining sorbed water, called water of solution, m,. An equilibrium is assumed to exist between the dry wood and water and the hydrated wood with an equilibrium constant K. Equilibrium is also assumed to exist between the hydrated wood and water vapor at relative vapor pressure h, with equilibrium constant K2. A third constant is defined as the moisture content corresponding to com-... 

Figure 2 Sorption-desorption isotherm for mannitol. Sorption equilibrium moisture Desorption equilibrium moisture...

The relationship between water activity and moisture content for most foods at a particular temperature is a sigmoidal-shaped curve called the sorption isotherm (Figure 3.10). The term equilibrium moisture content curve is also used. Sorption... 

X is the equilibrium moisture content and Xln, in units of g/100 g dry basis, is the B.E.T. monomolecular moisture content for the food listed in Table 3.2. However, there are nearly as many equations to sorption isotherms as there are researchers in this field. 

Water activity, glass-transition temperature, and equilibrium moisture content for freeze-dried camu-camu pulp ( ) sorption isotherm, adjusted by GAB model, (A) glass-transition temperature adjusted by Equation 62.3. 

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