Moisture, sorbed

To obtain a stable effect of hydrophobicity, the cloth should be thoroughly cleaned and prepared, because natural fats and size interfere with the uniform distribution of the finishing film. Silicone film on fibre reduces its sorption, penetration of moisture into the fibre and thus reduces the amount of moisture sorbed by the fibre. 

The amount of moisture sorbed by amorphous solids is typically much greater than that sorbed by non-hydrating... 

The quantity of moisture sorbed by a colloidal solid varies tremendously with atmospheric conditions, as shown in Figure 35-3. In contrast to the behavior of adsorbed water, however, the sorption process may require days or even weeks to attain equilibrium, particularly at room temperature. Also, the amounts of water retained by the two processes are often quite different from each other. Typically, adsorbed moisture amounts to a few tenths of a percent of the mass of the solid, whereas sorbed water can amount to 10% to 20%. 

Samples were stored in a desiccator, since Ellison and Zisman [4] have shown that nylon wettability is much influenced by moisture sorbed from the atmosphere. 

The amount of moisture sorbed by the lyophilized protein sample is directly related to the activity of DjO vapor phase (i.e., RH) to which the protein is exposed. To maintain constant RH throughout the experiment, solid-state amide HX is usually carried out inside a sealed desiccator. The desired RH can be easily maintained by using a suitable saturated salt solution [62]. Some commonly used salts and the resulting %RH are LiCl (11% RH), KCjH30 (23% RH), MgCl (33% RH), I C03 (43% RH), and NaCl (75% RH) [60]. The RH values correspond to the RH over aqueous solutions of these salts at room temperature the humidity over D O is assumed to be identical. To control the rate of moisture sorption and prevent powder collapse in formulations containing hygroscopic excipients... 

Water molecules combine the tendency to cluster, craze and plasticize the epoxy matrices with the characteristic of easily diffusion in the polymer1 10). The morphology of the thermoset may be adversaly influenced by the presence of the sorbed moisture. The diffusion of the water in glassy polymers able to link the penetrant molecules is, therefore, characterized by various mechanisms of sorption which may be isolated giving useful information on the polymer fine structure. 

The correct and critical estimate of the diffusion coefficients may be used to get informations on the morphological modifications occurring in presence of the sorbed moisture. 

SFE usually requires pre-extraction manipulation in the form of cryogenic grinding, except in cases where analytes are sorbed only on the surface or outer particle periphery. The optimum particle diameter is about 10-50 p,m. Diatomaceous earth is used extensively in SFE sample preparation procedures. This solid support helps to disperse the sample evenly, allowing the SCF to solvate the analytes of interest efficiently and without interference from moisture. 

Since water molecules occupy regular positions within the lattice of a hydrate with a specific stoichiometry (e.g., 1 1 monohydrate, 2 1 dihydrate, 5 1 pentahydrate) to the solid, relatively large quantities of water are sorbed. Figure 3 shows a moisture uptake isotherm for ipratropium bromide [39]. This substance undergoes an apparent hydration of the crystal between 63% and 75% relative humidity. Above 75% relative humidity, approximately 4.6% water is sorbed (theoretical monohydrate is 4.4 g/g). Interestingly, as anhydrous ipratropium bromide is equilibrated for extended time periods (e.g., 2 months and 5 months respectively, as shown in Fig. (3), hydration of the crystal appears to occur at... 

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... 

To illustrate this more quantitatively, consider the hypothetical sucrose example discussed by Ahlneck and Zografi [80]. Assuming that all the sorbed water is taken up by the amorphous portion of material, 0.1% total moisture would correspond to approximately 20%, 10%, 4%, and 2% moisture content in the amorphous material, respectively, for 0.5%, 1%, 2.5%, and 5% of amorphous solid. The glass transition temperatures for the amorphous portions... 

Combining solids that have previously been equilibrated at different relative humidities results in a system that is thermodynamically unstable, since there will be a tendency for moisture to distribute in the system so that a single relative humidity is attained in the headspace. As shown in Fig. 7, moisture will desorb into the headspace from the component initially equilibrated at a higher relative humidity and sorb to the component initially equilibrated at a lower relative humidity. This process will continue until both solids have equilibrated at the final relative humidity. The final relative humidity can be predicted a priori by the sorption-desorption moisture transfer (SDMT) model [95] if one has moisture uptake isotherms for each of the solid components, their initial moisture contents and dry weights, headspace volume, and temperature. Final moisture contents for each solid can then easily be estimated from the isotherms for the respective solids. 

Sites suitable for conventional SVE have certain typical characteristics. The contaminating chemicals are volatile or semivolatile (vapor pressure of 0.5 mm Hg or greater). Removal of metals, most pesticides, and PCBs by vacuum is not possible because their vapor pressures are too low. The chemicals must be slightly soluble in water, or the soil moisture content must be relatively low. Soluble chemicals such as acetone or alcohols are not readily strippable because their vapor pressure in moist soils is too low. Chemicals to be removed must be sorbed on the soils above the water table or floating on it (LNAPL). Volatile dense nonaqueous liquids (DNAPLs) trapped between the soil grains can also be readily removed. The soil must also have sufficiendy high effective porosity (permeability) to allow free flow of air through the impacted zone. 

As discussed earlier, the proportion of the molecules in the vapor state within the soil near a source is around 6 orders of magnitude less than the sorbed or dissolved portions. This condition was stated for 50% moisture saturation. For extremely dry soils, this ratio becomes even more pronounced. Phelan and Barnett defined a soil-air partitioning coefficient, Kd>, in a manner parallel to the soil-water partitioning coefficient, Kd [12] ... 

Note here that Kd/ is indicated to be a function of w, the gravimetric moisture content in grams/grams. That dependence on soil moisture is most pronounced in very dry soils, up to about 11% saturation, above which Kdj seems to asymptotically approach moisture content independence. In the most dry soils K may be 8 or more orders of magnitude greater than at 50% saturation. A plausible explanation for this is that, as the soil becomes wetter, water displaces some of the explosive molecules sorbed on the soil particles, causing them to be released into the vapor state. 

Soil moisture has a tremendous effect on soil-vapor sorption. Dry soils will sorb about 10,000 times more landmine signature chemicals than damp soils. This depresses the vapor levels the same amount. This process is reversible, so daily morning dew is valuable for vapor sensing, and afternoon drying is detrimental for vapor sensing. 

---