Hydration temperature effects

Water Content and Hydration Temperature Effects. As mentioned earlier, the Dow membrane is more amorphous than Nafion 117. This allows the Dow membrane polymer matrix to adsoib more water than Nafion 117. The temperature at which the membrane is hydrated also influences the swelling of the ionomer matrix. The highest temperature used during the membrane preparation procedure controls the water content of the membrane. The water content of the membrane was calculated by dividing the weight of water absorbed by the total weight of the hydrated membrane. The water content of a membrane is primarily controlled by the inherent structure of the ionic polymer (Table I). 

The-effects cannot be due to hydrates because we get them in alcohol and in acetone as well as in water and with the ester just as well as with the free acid. Rising temperature has the same qualitative effect as decreasing the concentration. Since rising temperature must decompose hydrates, we should have to condude that there are fewer hydrates in dilute solutions than in more concentrated ones, which does not seem probable. Further, we can get the temperature effect with the anhydrous add, where there can be no hydrates. 

Calcium carbonate solubility is also temperature and pressure dependent. Pressure is a 6r more important fector than temperature in influencing solubility. As illustrated in Table 15.1, a 20°C drop in temperature boosts solubility 4%, whereas the pressure increase associated with a 4-km increase in water depth increases solubility 200-fold. The large pressure effect arises from the susceptibility of the fully hydrated divalent Ca and CO ions to electrostriction. Calcite and aragonite are examples of minerals whose solubility increases with decreasing temperature. This unusual behavior is referred to as retrograde solubility. Because of the pressure and temperature effects, calcium carbonate is fer more soluble in the deep sea than in the surfece waters (See the online appendix on the companion website). 

As a special case of this example, one might specify that a natural gas mixture is in very large excess relative to the water phase (as in a gas-dominated pipeline), so that the gas composition does not change upon hydrate formation. Effectively, C = 1 for the gas components, with an additional component for the water (total C = 2). With three phases (Lw-H-V), there must be one intensive variable (F = 2 - 3 + 2) for a constant gas mixture composition (in large excess) relative to the water phase, specifying that the highest pipeline pressure is sufficient to determine the temperature (and the other intensive variables) at which hydrates form with a gas of fixed composition. 

A major disadvantage of the HLB concept is that it makes no allowance for temperature effects. With increasing temperature, the hydration of lyophilic (particularly poly(ethylene oxide)) groups decreases and the emulsifying agent becomes less hydrophilic - i.e. its HLB decreases. 

On the other hand, for the liquefied acid gas reducing the water content to 3.2 g/m3[std] has a dramatic effect on the hydrate temperature. For the case where there is plenty of water, the hydrate forms at between 24° and 26°C. On the other hand, the reduced water case, the hydrate forms at less than -16°C. 

FIGURE 6 A n.lS for the hydrates of FeCl,. The IS values have been corrected for the effects of temperature, using the procedure described In the text, assuming no intrinsic temperature effects. The uncertainty limits shown correspond to 0.005 mm/s. The effect of an order of magnitude increase in the experimental uncertainty is illustrated in the bottom of the figure. 

Electrolyte conductivity depends on three factors the ion charges, mobilities, and concentrations of ionic species present. First, the number of electrons each ion carries is important, because A, for example, carries twice as much charge as A . Second, the speed with which each ion can travel is termed its mobility. The mobility of an ion is the limiting velocity of the ion in an electric field of unit strength. Factors that affect the mobility of the ion include (1) the solvent (e.g., water or organic), (2) the applied voltage, (3) the size of the ion (the larger it is, the less mobile it will be), and (4) the nature of the ion (if it becomes hydrated, its effective size is increased). The mobility is also affected by the viscosity and temperature of the solvent. Under standard conditions the mobility is a reproducible physical property of the ion. Because in electrolytes the ion concentration is an important variable, it is usual to relate the electrolytic conductivity to equivalent conductivity. This is defined by... 

K. Iwasaki and T. Fujiyama, Light-scattering study of clathrate hydrate formation in binary mixtures of tert-butyl alcohol and water. 2. Temperature effect. J. Phys. Chem., 83 (1979), 463-468. 

Odler, L, Abdul-Maula, S., andLu, Z. (1987) Effect of hydration temperature on cement paste structure. Materials Research Society Symposium Proceedings 85,139-144. 

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