Water determining freezable

Corti et al. [101] studied the freezing of water and water-methanol mixtures in Nafion 117 and they found that the amount of freezable water, determined from the area of the ice fusion peak of the DSC scan, increased from 1 % to 23 % as the relative humidity increases from 84 % to 100 %. However, the presence of methanol in the membrane increased the amount of freezable water up to 61 % for methanol 25 v/lw% aqueous solutions, reaching an apparent limit of 65 % for more concentrated methanol solutions. This result would indicate that the degradation of the MEA in DMFC under freezing cycles could be worse than that observed for hydrogen feed PEM fuel cells. 

In agro-polymers, especially proteins, it is often necessary to distinguish between bound water (non-freezable) and free (freezable) water. Bound and freezable water content can be determined from water melting peaks detected in DSC. The area of the endothermic peak represents the heat of fusion of the freezable fraction division by the heat of fusion of pure water yields the mass of free water. Subtracting that value from the total water content of the sample gives the mass of bound water. ... 

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. 

Fig. 25 Determination of freezable water, using the melting peak of ice. (A) Methocel K15 M, (B) cetyl palmitate, and (C) pharmaceutical gel.

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... 

Samouillan et al. (2011) studied the dielectric properties of elastin at different degrees of hydration and specifically at the limit of freezable water apparition. The quantification of freezable water was performed by DSC. Two dielectric techniques were used to explore the dipolar relaxations of hydrated elastin dynamic dielectric spectroscopy (DDS), performed isothermally with the frequency varying from 10 to 3 x 10 Hz, and the TSDC technique, an isochronal spectrometry running at variable temperature, analogous to a low-frequency spectroscopy (10 to 10 Hz). A complex relaxation map was evidenced by the two techniques. Assignments for the different processes can be proposed by the combination of DDS and TSDC experiments and the determination of the activation parameters of the relaxation times. As already observed for globular proteins, the concept of solvent-slaved protein motions was checked for the fibrillar hydrated elastin (Samouillan et al. 2011). 

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). 

As discussed above, and Ni are estimated from the enthalpy AH, which is experimentally determined from the ice-melting DSC curve. So it is understandable that AHb is a chief determinant in the accuracy of this method. From this viewpoint, to determine AH as accurately as possible, a deconvolution analysis was used to separate the ice-melting DSC curve into two components (or peaks), broad and sharp, for the freezable interlamellar and bulk water, respectively, because the two components overlap at their basis. Furthermore, the broad component was deconvoluted into multiple components. The purpose is not only to improve the accuracy of the deconvolution analysis but also to estimate iVi( f) and Ni(f) given in Eq. (3). 

FIG. 4 Comparison of ice-melting enthalpy curves of AH and for bulk and freezable interlamellar water per mole of hpid. The AHb and AH if curves are determined from the deconvoluted ice-melting curves for the bulk and freezable interlamellar water, respectively. iVw(a) and iV (b) are water/lipid molar ratios at the maximum amounts of freezable and nonfreezable interlamellar water, respectively. The designations 1,11, and 111 represent the three regions < N (b) (i.e., in the presence of only nonfreezable interlamellar water), Al (b) interlamellar water), and > N a.) (i.e., in the presence of nonfreezable and freezable interlamellar and bulk water), respectively. 

Differential scanning calorimetry (DSC) is used to study the heat effects of changing the temperature. This is a very common technique in many fields in the cement field typical applications are, e.g. quantifying gypsum and hemihydrate in anhydrous cement (Dunn et al. 1987) or determining the free (freezable) water via low-temperature DSC, assessing freezing processes (Kaufmann 2004) or hydration kinetics (Ridi et al. 2003). 

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