Unbound water

Unbound moisture in a hygroscopic material is that moisture in excess of the equihbrium moisture content corresponding to saturation humidity. All water in a nonbygroscopic material is unbound water. 

Particulate matter, in accordance with U.S. legislation, includes all substances (with the exception of unbound water) that under controlled conditions are present as solids (ash, carbon) or liquids in exhaust gases. 

The bulk of the cement is extremely porous as the fractured surface of a specimen shows (Figure 6.3c). The pores are 0-5 pm in diameter and more abundant in the depth of the cement. The porosity arises from excess unbound water which separates out as globules in the cement and is trapped by the rapid setting. Subsequent diffusion of these globules leaves the cement porous. This makes the cement permeable to dyes (Wisth, 1972). 

However, these reactions do not always occur. In some instances there may be steric factors in the API molecule that restrict access to the reactive group and the reaction does not occur, or occurs at a much-reduced rate. For almost all chemical interactions, a key component is presence of free (unbound) water (23,24). In the absence of a sufficient amount of free water, the reactions do not proceed. This is the basis for using very low humidity manufacturing and packaging facilities for the manufacture of effervescent products. The free water layer serves to dissolve sufficient of the drug and the excipient, or to form bridges between particles, such that the components/reactants come into sufficiently close contact for the reaction to occur. 

Proton NMR by pulsed techniques to permit the study of relaxation effects provides information on the distribution of protons, and thus of water molecules, between different environments. An early study (S81) indicated that, at RH < 70%, the evaporable water in hep was in an environment similar to that of the interlayer water in clay minerals or in certain crystalline hydrates. Subsequent work (B111,M71,M72,S82) has given indications of the distributions of both chemically bound and unbound water among a number of environments. The method, which can be applied most effectively only to materials very low in paramagnetic atoms (including iron), has so far been used primarily to follow the course of hydration, but it would appear that it could also provide much information on pore structure. 

Bound or unbound water in a molecule can give rise to sharp or broad bands. In alkali halide disks a water band at 3350 cm" may appear Lens tissues... 

In order to obtain information from nuclear resonance, the proceedings must be a bit complicated. One adds a paramagnetic ion to a solution in which the solvation of a diamagnetic entity is to be measured. Then, two types of water around, for example, an ion, bound and unbound waters, can be distinguished by observing the resulting... 

Free moisture, unbound Water present in a solid exhibiting a vapor pressure greater... 

The moisture content of a dried substance varies from product to product. It must be kept in mind that drying is a relative term, and means simply that the moisture content has been reduced from some initial value to some acceptable final value. This final value depends on the material being dried. For example, a stable hydrate may be considered dry after all free, or chemically unbound water has been removed. An acceptable final value does not necessarily imply the lowest possible value achievable with the drying equipment. Overly zealous drying could lead to final product that is susceptible to static charge and issues with product segregation. 

Bound moisture is water (or other solvents in nonaqueous systems) held by a material in such a manner that it exerts a lower vapor pressure than that of the pure liquid at the same temperature. Water may be chemically or physically bound. Unbound moisture is therefore moisture in association with a solid that exerts the same vapor pressure as the pure liquid. In a discussion of bound versus unbound water, it should be pointed out that are not only different equilibria to be considered, but that the binding energies and kinetics are different. 

The category of unbound moisture can be described as the moisture in excess of the bound moisture. A hygroscopic material may contain bound as well as unbound moisture. A nonhygroscopic material contains unbound moisture only. The equilibrium vapor pressure of unbound water is equal to that of pure water at the same temperature. 

The weight losses around the first and the second endotherm correspond roughly with 2 moles respectively 1 mole of crystal water (theoretically 5.0 respectively 2.5%). The weight loss at the first endotherm includes some unbound water (< 1 %). The position as well as the surface of the endotherms strongly suggests that the /3-polymorph is the more stable form. 

The thermal curve for calcium oxalate is considerably more complex than the others shown in Figure 12-6. Below about 135°C, unbound water is eliminated to give the monohydrate CaC204 H2O. Thi.s compound is then converted to the... 

Many models have been proposed, but none has adequately explained all properties of liquid water. Iceberg models postulated that liquid water contains disconnected fragments of ice suspended in a sea of unbounded water molecules. 

Indeed, water activity is a more important factor than water content for fungi growth and metabolism [29, 33-37]. Water activity gives the amount of unbounded water available in the immediate surroundings of the microorganism. Water activity is closely related, but not equal to, the water content. The definition of the water activity is ... 

Examples of Excipient Stability Information Stability of Excipients—Bound and Unbound Water Conclusion... 

Various standardized analyses have been developed to determine the chemical composition of coals. Among them are the proximate analyses, which quantify the volatile and non-volatile components, and the ultimate analyses, which determine the elemental composition. These, and examples of other types of analyses, are listed in Table 4.5. Data are often recorded on a dry and ash-free (daf) basis, because of the variable amount of unbound water (particularly in brown coals) and inorganic minerals that may be present. A mineral-matter-free (mmf) rather than simple ash-free basis is often used for elemental composition in order to take account of the oxides, sulphides etc., and also the water of crystallization in inorganic minerals, when calculating the composition of the organic matter. 

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