Water, freezable

PEMs lEC (mmol g Water ) uptake (%) Total Non- freezable water Freezable water a (S cm ) Refs. 

To survive freezing, a cell must be cooled in such a way that it contains little or no freezable water by the time it reaches the temperature at which internal ice formation becomes possible. Above that temperature, the plasma membrane is a barrier to the movement of ice crystals into the cytoplasm. The critical factor is the cooling rate. Even in the presence of external ice, most cells remain unfrozen, and hence, supercooled, 10 to 30 degrees below their actual freezing point (-0.5 °C in mammalian cells). Supercooled cell water has a higher chemical potential than that of the water and ice in the external medium, and as a consequence, it tends to flow out of the cells osmotically and freeze externally (Figure 1). 

Differential scanning calorimetry measurements have shown a marked cooling/heat-ing cycle hysteresis and that water entrapped in AOT-reversed micelles is only partially freezable. Moreover, the freezable fraction displays strong supercooling behavior as an effect of the very small size of the aqueous micellar core. The nonfreezable water fraction has been recognized as the water located at the water/surfactant interface engaged in solvation of the surfactant head groups [97,98]. 

C/min) all freezable water is crystallized, or if the rewarming is interrupted before the melting starts and the product cooled down again to e. g. -150 °C, the rewarming curves resemble that in Fig. 1.33. There is no water left, which can crystallize at Td. There are only two events, which are denoted (as by Mac Kenzie) with antemelting and incipient melting. 

Curve B After cooling down to -100 °C, the product has been warmed up at 10 °C/min to -48 °C, kept for 15 min at this temperature (thermal treatment), cooled down again at 10 °C/min to -100 °C, and the DSC plot (B) measured during rewarming. During thermal treatment all freezable water is crystallized, and Tg is increased to -58 °C, respectively -57 °C. During rewarming, no crystallization can be detected (Fig. 2 from [1.33]). 

Table 1.1 shows the non-freezable water (UFW) in various foods. The reasons and the consequences are described in Sections 1.1.3 and 1.1.4. In comparing these data with other publications, e. g. [1.3], smaller values may be found. This can depend not only on the different raw materials and the history of the probe until measurement, but also the methods of measurement. 

Other research using NMR techniques to study mobility in starch systems includes Li et al. (1998), who investigated the mobility of unfreezable and freezable water in waxy cornstarch using H and 2H NMR Choi and Kerr... 

ESR spectroscopy can be used with adsorbed paramagnetic ions to study the liquid associated with mineral surfaces. Cu(II) and Mn(II) have been used in his type of investigation, although difficulties are encountered with observing a resonance from Mn(II) in distorted environments. Measurements of Cu(II) on silica at room temperature and above have shown that adsorbed water behaves in the same manner as bulk water, but at lower temperatures it experiences a decreased mobility (61). On freezing two types of water are found one which is freezable and undergoes crystallization and the other which is unfreezable, in which the ice structure cannot be formed because of the surface interaction. NMR, IR and differential thermal... 

As much as the nanophase segregated morphology of Nafion has been a controversial issue in the literature over several decades, the need for understanding the structure and distribution of wafer in PEMs has sfimulafed many efforfs in experimenf and theory. Major classifications of water in PEMs distinguish (1) surface and bulk wafer, (2) nonfreezable, freezable-bound, and free wafep and (3) wafer vapor or liquid water. Anofher fype of wafer offen discussed is that associated with hydrophobic regions. 

For Cu(ll) in water adsorbed on silica gels it was reported 5 that in gels with small pores isolated hydrated ions are detected at 77 K5 in pores larger than -4 nm a broad signal is superimposed on the spectrum of isolated ions. The appearance of the broad signal indicates aggregation of cations and the presence of bulk or freezable water. In a recent publication the Cu(ll) probe was used to test the possibility of ice formation in microemulsions, jce formation was detected in one of the microemulsions studied for very slow cooling rates from -300 K to 77 K. 

Examination of the ESR spectra measured in this study, Figures 1 and 2, shows no indication of the dipolar broadened line at 77 K in the networks studied even when the samples were cooled to 77 K from ambient temperature during more than four hours. We particularly checked the S-band spectra for this line. We expect the dipolar broadening to be the same at the two frequencies but much more conspicuous at S-band because the spectrum from isolated ions is spread over a smaller range of magnetic fields at this microwave frequency. The absence of the broad line indicates that in all the networks measured Cu(ll) hydrated by freezable, or bulk, water is not detected. These results are in agreement with those presented in ref. 25 which indicate the absence of bulk water in water absorbed on silica gels with pores smaller than 6 nm. 

The water reduction afforded (about 20-25%) reduces the freezable free water content which usually serves as a heat sink for the heat liberated by the initial hydration reactions. Hence, lower amounts of antifreeze admixtures can be used. 

When concrete is to be placed in cold weather, it is preferable that accelerators or antifreezers be used in combination with air-entraining agents and water-reducing admixtures. These combinations not only reduce the amount of freezable water in the mix but also generally reduce the quantity of antifreezers and accelerators needed to obtain desired effects compared to the amounts that have to be used when these are used separately. In addition these combinations may be useful in increasing the resistance of concrete to frost action and to corrosive agents. 

The unfreezable water may have more than a theoretical meaning. Adding more water than the total of the unfreezable to biopolymers, small amounts of freezable water are observed but the freezing point is reduced (see Figs. 16 and 20 in 181 ). So in fish (cod) muscle water freezes at —10 °C 208. Feeney has found a special anti-... 

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