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

Abstract

Enthalpies of freezing and thawing of water in wool-water mixtures are evaluated by DSC. The two different samples are clean wool extracted with acetone and delipided wool prepared by extraction with a mixed solvent composed of chloroform, ethanol, and water in volume ratios of 19:8:1. On cooling curves at 5 K/min for both samples, there are three main peaks for crystallization at about 228, 240, and 255K. The enthalpy of water corresponding to each peak is determined from the slope of the linear relationship between heat and water content. The enthalpies estimated from the three exotherms are 17, 181, and 287 J/g of water, respectively, for the acetone extracted wool, and the values for the delipided wool are 17, 85, and 281 J/g. On the heating curves, there are two peaks—a broad endotherm beginning at a temperature of T d , which varies with the water content, and a sharp peak due to melting of ice for bulk water at 273K. The respective enthalpies are 222 and 330 J/g for the acetone-extracted wool and 110 and 326 J/g for the delipided wool. A possible explanation is proposed for the marked difference in enthalpy between the samples upon freezing and thawing. Removing the unbound internal lipids increases diffusion resistance to water transport within the fibers. The relationship of T d to water content is also discussed.

Effect of Cooling Rate on Freezing of Wool-Water Mixtures

“…There are well defined exothermic peaks on the DSC curves of both samples. When the peaks of 2c and 3c overlap, peak separation and the evaluation of peak areas occurs in the same way as described in the earlier paper [46]. Although in some cases there is very complex overlapping between 2c and 3c, and between 3c, and 3c2 peaks, differences between 2c peak areas estimated from the lines drawn for peak separation are within 5%.…”

“…On the exotherm at CR = 5 K/min for both samples, a sharp peak 3c~ appears at about 255 K, although the 3c, peak appears at about 250 K for sample B with a water content of 0.3975. For the same conditions, the freezing temperature of pure water is lowered from about 17 to 20 K from the melting point; this is the effect of supercooling2 [46]. The degree of supercooling for 3C2 water is substantially the same for pure water.…”

“…Merino wool fibers were purified by Soxhlet extraction with acetone for 24 hours according to the method described in our earlier paper [46]. The purified wool (AW) sample thus obtained was measured for water content and water retention.…”

“…The equilibrium water content of Aw samples at 65% RH and 25°C was determined,by a previous method, the value being 0.1241 g water/g dry sample [46]. Water retention was measured by a modified method of Holt [ 15] and Joko [19].…”

“…Water retention was measured by a modified method of Holt [ 15] and Joko [19]. The saturated interstitial water content defined by W, was 0.480 g water/g dry sample [46]. D sc samples were prepared according to the method described previously [46].…”