Effect of Cooling Rate on Freezing of Wool-Water Mixtures

Abstract

The effect of the cooling rate on freezing of a wool-water system is studied using Dsc at cooling rates (CR) that vary from 1.25 to 40 K/min. By means of centrifugation, an equilibrium quantity of interstitial water in the fibers ( Ws) is determined to be 0.48 g/g of wool. Two samples with different water contents are used: sample A is more than Ws, i.e., Wc = 0.54, and sample B is 0.43 g/g of wool. Characteristic shape changes in a broad exotherm ranging from temperatures of about 230 to 260 K are observed in DSC curves. A freezing mechanism of interstitial water is proposed on the basis of supercooling. The swollen wool is assumed to be a dispersion system consisting of domains defined as structurally favored sites, permitting water migration for ice crystal growth. The shape change with CR can be explained by differences in the freezing temperature of water of different volumes in the domains. Kissinger's equation is applicable to this crystallization process. Values of activation energy for both A and B samples are nearly the same, about -115 kJ/mol. Activation energy is considered to be the sum of the two terms for crystallization and migration of water, and the latter value is presumed to be about 49 kJ/mol. The important finding is that ice crystals in sample A cooled at 1.25 K/min contain an amount of water corresponding to about 38.2% of the so-called "nonfreezing water" Wu, which remains unfrozen at a CR above 5 K/min.

Enthalpies of Water in Freezing and Thawing of Wool-Water Mixtures

“…As shown in Table I, the values were 181 J/g for Aw and 85 J/g of water for MW. A lower interaction might be expected for the former than the latter, as we proposed in the previous section and in our upcoming paper [23]. The difference between those enthalpy values is probably due to the different wool structural components related directly or indirectly to the internal unbound lipids.…”

“…In upcoming work, we report that the water corresponding to 3c, is interstitial water existing in a continuous phase, and that of 3c~ is external water between the fibers [23]. It seems unlikely that the enthalpies of freezing for 3c, and 3C2 water differ considerably.…”

“…axis. This suggests that the enthalpies for freezing and thawing of the water corresponding to I c and 2c are different from those for external water.° ' / ENTHALPY OF WATER IN FREEZING Assuming that the quantity of unfrozen water (W,,) does not change with varying water content at a scanning rate of 5 K/min [23], we can determine the enthalpy from the relationship between the heat of freezing of water corresponding to the 1 c exotherm, AQ j and W,., that is, W&dquo; + W,,, where W, is the weight of 1 c water in grams per gram of wool. The results for both Aw and Mew samples are shown in Figure 7.…”

“…This su ests that the unbound i ~ ] lipids are not directly concerned with the freezing of 1 c water. The water molecules are probably located in a highly ordered binding state to the polar groups of wool chains enclosed by a hydrophobic barrier [23]. Figures 10 and 11 show the relationships of water content We to the amounts of water corresponding to the exotherms of 1 c, 2c, and 3c and to the endotherms of 1 h and 2h for the Aw and MW samples.…”

“…This su ests that the unbound i ~ ] lipids are not directly concerned with the freezing of 1 c water. The water molecules are probably located in a highly ordered binding state to the polar groups of wool chains enclosed by a hydrophobic barrier [23].…”