Temperature versus water content solute

This method involves preparation of a standard curve by equilibration of a specific amount of dry standard material in duplicate or triplicate over different saturated salt solutions. The standard curve is a plot of aw versus water content of the standard material. The standard must be stable during reuse of the material, and the chamber used should be exactly the same as will be used later. For the measurement, the size of each standard should be in a controlled narrow range, (e.g., —1.6 0.1 g). Once the standard curve is made, dry aliquots of the same mass of standard are equilibrated over a large quantity of the food material (-10 to 20 g). The moisture content of the standard material is then measured (e.g., by mass gain) and aw is estimated from the standard curve (Vos and Labuza, 1974). This method avoids preparation or storing of saturated salts for each determination moreover, use of a standard shortens the equilibration, thus requiring less time for measurement, and less abuse of temperature for sample as well as standard. 

It can precipitate as potassium hydrogen tartrate (KHT) or as calcium tartrate (CaT), the latter being practically insoluble in aqueous solutions. Their equilibrium solubility varies with temperature, pH, and alcohol content, while the presence of a few wine components, such as polysaccharides and mannoproteins, may hinder spontaneous nucleation even if the solution is supersaturated. From Figure 14 that shows the equilibrium tartaric acid-dissociated fractions versus pH and ethanol volumetric fraction (Berta, 1993 Usseglio-Tomasset and Bosia, 1978), it can be seen that in the typical pH range (3 4) of wines KHT is predominant. As temperature is reduced from 20 to 0°C, KHT solubility in water or in a 12% (v/v) hydro-alcoholic solution reduces from 5.11 to 2.45 kg/m3 or from 2.75 to 1.1 kg/m3, respectively (Berta, 1993). Each of these data also varies with pH and reaches a minimum at the pH value associated with the maximum concentration of the hydrogen tartrate anions. For the above-mentioned solutions, the solubility minimum shifts from pH 3.57 to pH 3.73 as the ethanol content increases from 0 to 12% (v/v) (Berta, 1993). 

It is well known that polymer concentration plays an essential role in membrane formation through the phase inversion process. The effect of polymer concentration on the morphology of PVDF hollow-fiber membranes has been demonstrated in many studies [21,34,37-42]. Generally, a concept of critical polymer concentration is often employed as a guideline for a proper selection of polymer content in dope solutions [4,5]. The critical polymer concentration can be determined from the correlation of viscosity versus polymer concentration at a specific shear rate and temperature. An example of the critical polymer concentration of PVDF/NMP dope solutions is illustrated in Figure 7.5. Depending on the specific application, PVDF dopes with a polymer content below the critical concentration are usually adopted to fabricate microporous PVDF hollow fibers for water-related applications such as MF [43], UF [40], and MD [8,10]. On the other hand, dopes possessing a polymer concentration above the critical value are chosen to fabricate PVDF hollow fibers with a relatively dense selective skin for gas separation and pervaporation [12,13,39]. 

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