Water bound loosely

The nature of the water present within a PEM can also have an effect upon its performance during PEMFC operahon. At /I > 6, water exists in the three forms previously mentioned in Section 3.2.1 free water, loosely bound water, and nonfreezable water. This has been established by Eourier transform infrared (FTIR) studies and more recently by calorimetry and gravimetric analysis. Pulse NMR has also been used in conjunchon with differential scanning calorimetry (DSC) to analyze the contributions of these three different types of water in Nation and BPSH membranes. These values can be seen in Table 3.1. 

Another type of inclusion compd is the layer or sandwich compound. This includes certain hydrated clays (such as halloysite and montmoril-lonite) which form layer-or sandwich-inclusion compds with polar organic molecules (such as alcohols, glycols, some hydrocarbons, etc) which replace the water, loosely bound in clays (Ref 10, pp445-7)... 

PEMs lEC (mmolg" Water )uptake (%) Total Non- freezable water Loosely bound water Free water a (Scm )... 

Apart from the application of XPS in catalysis, the study of corrosion mechanisms and corrosion products is a major area of application. Special attention must be devoted to artifacts arising from X-ray irradiation. For example, reduction of metal oxides (e. g. CuO -> CU2O) can occur, loosely bound water or hydrates can be desorbed in the spectrometer vacuum, and hydroxides can decompose. Thorough investigations are supported by other surface-analytical and/or microscopic techniques, e.g. AFM, which is becoming increasingly important. 

Zeolites have much more open aluminosilicate frameworks than feldspars and this enables them to take up loosely bound water or other small molecules in their structure. Indeed, the name zeolite was coined by the mineralogist... 

Water occurs in glass-ionomer and related cements in at least two different states (Wilson McLean, 1988 Prosser Wilson, 1979). These states have been classified as evaporable and non-evaporable, depending on whether the water can be removed by vacuum desiccation over silica gel or whether it remains firmly bound in the cement when subjected to such treatment (Wilson Crisp, 1975). The alternative descriptions loosely bound and tightly bound have also been applied to these different states of water combination. In the glass-poly(acrylic acid) system the evaporable water is up to 5 % by weight of the total cement, while the bound water is 18-28 % (Prosser Wilson, 1979). This amount of tightly bound water is equivalent to five or six molecules of water for each acid group and associated metal cation. Hence at least ten molecules of water are involved in the hydration of each coordinated metal ion at a carboxylate site. 

Scanning electron microscopy shows the cement to consist of zinc oxide particles embedded in an amorphous matrix (Smith, 1982a). As with the zinc phosphate cement, a separate globular water phase exists since the cement becomes uniformly porous on dehydration. Porosity diminishes as the water content is decreased. Wilson, Paddon Crisp (1979) distinguish between two types of water in dental cements non-evaporable (tightly bound) and evaporable (loosely bound). They found, in the example they examined, that the ratio of tightly bound to loosely bound water was 0-22 1-0, the lowest for all dental cements. They considered that loosely bound water acted as a plasticizer and weakened the cement. 

As mentioned previously, the cement contains both tightly bound and loosely bound water. The set cement can both lose and gain water depending on its environment. Under drying conditions (say 50 % relative humidity) it loses water and shrinks. When placed in water there is an... 

A hydrate is a compound that contains water, rather loosely bound. Usually mild heating of the compound can drive off this water of hydration. Addition of water to the anhydrous salt reforms the hydrate (reversible loss/gain of water)... 

According to Evans and Haisman (1982), there are two phases of water in starch granules (1) the "tightly" bound phase, approximately 20% of the amoimt, absorbed by granules and (2) a "loosely" bound phase that is available for gelatinization. They foimd that the DSC measured onset temperature (Tq) increased rapidly below 0.6 g water per 1 g starch ( 29% water volume fraction) and stayed essentially constant between 0.6 and 2.0 g water per 1 g starch (29-57% water volume fraction) (Eig. 5.10). Changes observed in and Tp, and the peak separation pattern... 

Contributions to Total A from Free, Loosely Bound, and Nonfreezable Water for Nafion and BPSH Using a Combination of Pulse NMR and DSC... 

The effect that the amount of free water has upon proton conductivity can be seen from these data. For BPSH membranes where lEC <1.3 meq/g, there is no free water and conductivity is low, even though at least two samples have lEC values equivalent to or higher than Nation. In the case of BPSH where lEC = 2.0 meq/g, it has a higher X value and more loosely bound water than Nation. However, the amount of free water possessed by Nation is higher than for BPSH (lEC = 2.0 meq/g) and conductivity is equivalent for the two membranes, despite the considerably higher acid content for the latter. 

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