Freezable water molecules

After a PEM is cooled at very low temperatures (e.g., less than -50°C) differential scanning calorimetry (DSC) shows an endothermic peak at around 0°C as the temperature scans up. This seems to indicate that the PEM contains water that freezes af the water freezing temperature. From the peak area the amount of such "freezable" water can be determined, and its difference from the total amount of water within the membrane that is pre-determined by weighing is used to represent the amount of water that is not freezable (called non-freezable water). For example, Hou et al. found that fully hydrated Nation 212 membrane in liquid water at 25°C contains 5.3 and 15.2 freezable and non-freezable water molecules per -SOjH group, respectively while the same membrane hydrated in water vapor at 75% RH and 25°C contains 0 and 6.2 freezable and non-freezable water per -SO3H group, respectively (Hou 2008). 

In general, intracellular freezing induced with extracellular ice crystal initiates around -5°C and most freezable water freezes by the time the cells reach -20°C. Thus, freezing injury of the cells should be concentrated in this temperature region. On the other hand, water molecules cannot endure in a supercooled state under —40°C even if there is no seeding of ice crystals. This suggests that reduction of cell viability is restricted to temperatures above -40°C. The results shown in Figure 9, also support this conjecture. 

Often the solvates (hydrates) are not detected since, according the corresponding phase diagram, at ambient temperature, they can be partly or completely dissociated. Suspensions of hydrates in water should shift the equilibrium toward the formation of the stable hydrated form. The ability of DSC measurements at subambient temperatures allow to determine phase transitions. Giron et al. proposed to use the melting peak of freezable water for the analysis of suspensions of drug substances in water in combination with TG for the determination of the number of molecules of water bounded as hydrates. 

According to analysis by differential scanning calorimeter (DSC), water molecules in the membranes can be classified into three types, non-freezing, freezing bound and free water.40 Also, the weight ratios of freezable water and... 

In the absence of H pulse NMR, DSC can only distinguish freezable water from nonfreezable water. Comparative data of X (freezable and nonfreezable) determined by DSC are Usted in Table 5 for various membranes. hi complementary work [203], DTA measurements showed that below about six molecules of water per equivalent, ion-containing polymers... 

It is more plausible that the seven HjO molecules per lipid constitute real nonfreezable water (which is not detected by DSC) and freezable interlamellar water (to which the above-mentioned melting peak below 0°C may be ascribed). This number is to be compared with the five nonfreezable water molecules per molecule of lipid plus five freezable interlamellar water molecules per molecule of lipid that were evaluated by Kodama and Aoki [134] for the gel phase of the DPPC-water system, using a deconvolution analysis. The 7 A (= 65 - 58) decrease in the interbilayer distance is then due to the loss of five molecules of... 

In Aerosol OT (AOT)-water systems, there are about six molecules of non-freezable water per molecule of AOT [63]. The hydration number of the Na ... 

A characteristic feature of DSC is its ability to distinguish clearly between freezable water, for which ice-melting behavior is observed, and nonfreezable interlamellar water, for which it is not observed even at temperatures low enough to form ice. As is well known, the structure of ice is characterized by networks of hydrogen bonds formed among neighboring water molecules. Therefore, the point to note is that the water molecules present as nonfreezable water cannot participate in the formation of such hydrogen bonds even when cooled to extremely low temperatures. 

As discussed above, the water molecules in lipid-water systems are classified into three types nonfreezable interlamellar, freezable interlamellar, and bulk water. A correlation between the numbers of water molecules of these three types at a desired total water content is given by the equation... 

Table 1 Ice-Melting Enthalpies of Freezable InterlameUar and Bulk Water, and AH, Respectively, per Mole of Lipid and the Numbers of Nonfreezable and Freezable InterlameUar and Bulk Water Molecules, IVk, and N, respectively, per Lipid Molecule at Varying Water Contents (Wh o) in the Gel Phase of the DPPC-Water System...

Again, we remark on the linear relationship between and observed at water contents below the pre-region. This indicates that the molar melting enthalpy for the freezable interlamellar water is nearly the same for 5 < A < 8. However, this result does not agree with our analysis because the multiple-component deconvolutions of the ice-melting peak for the freezable interlamellar water suggest the existence of water molecules in different hydrogen bond modes. 

FIG. 14 Water distribution diagram for the gel phase of the DPPC-water system. The cumulative numbers of water molecules (per molecule of hpid) present as nonfreezable and freezable interlamellar water and as bulk water are plotted against N . 

The limiting, maximum numbers of water molecules are approximately 5 HjO and 5 (= 10-5) H2O per molecule of lipid for the nonfreezable and freezable interlamellar water, respectively. 

On the other hand, as discussed above, the L-subgel phase of the DPPC-water system involves the extra nonfreezable interlamellar water up to one molecule of HjO per molecule of lipid, compared with the gel phase. This nonfreezable interlamellar water comes from the freezable interlamellar water present in the gel phase, indicating the critical role of this freezable water in the conversion of the gel to the L-subgel phase. In fact, as shown in Fig. 25B, the conversion to the L-subgel phase by annealing is not realized for a gel sample at < 5 (see curve a), i.e., when there is no freezable interlamellar water (see Fig. 14). Furthermore, as shown in Fig. 25B, the fixed peak of the L-subgel-to-gel phase transition is observed above 12-13 where the subgel phase is fully hydrated (see... 

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