(188e) Development of a Dynamic Method for Measuring Bio-Solute Solubilities in Subcritical Water
The knowledge of the aqueous solubilities of bioactive compounds such as sugars and flavonoids as a function of temperature is significant for a wide range of applications in food, pharmaceutical and biofuels technology. A few studies have been conducted in the past to measure the solubility of sugars and flavonoid compounds in water. However, such studies were primarily conducted in a batch vessel at temperatures below the boiling point of water. We have attempted to measure the solubility of some sugars and flavonoids in water at subcritical conditions using a dynamic flow system. The apparatus, based on a modification of the system used by Miller and Hawthorne , allows water to flow through a saturation cell comprised of the selected solute and a dispersant, at a constant flow rate and at a particular temperature. The concentration of sugars in the outlet sampling vial is analyzed using techniques such as HPLC and HPSEC.
Using the above-mentioned apparatus, the aqueous solubilities of sugars were measured between temperatures 25 and 200C, and that of flavonoids were measured between 25 and 140C. The effect of temperature, pressure and flow rates on the aqueous solubilities of these bio-solutes was studied using this technique. It was found that the solubilities showed an exponential trend with an increase in temperature. There was close to a four-fold increase in the solubilities of the sugars in water above its boiling point while the flavonoids showed a two-fold increase. Typical sugar solubilities between 20 ? 120C ranged from 470 ? 2380 g/L for glucose and from 500 ? 1337 g/L for maltose monohydrate. However, the flavonoids were comparatively less soluble in water with typical solubilities at the same temperatures ranging between 29 ? 773 g/L for protocatechuic acid and 2*10^(-3) ? 0.25 g/L for quercetin. Assuming the water activity to be unity, basic solution thermodynamic theory can be used to calculate the heat, enthropy and Gibbs free energy of solutions for these compounds in water. Such data can be used to examine the effect of water as a solvent for extraction and hydrolysis of such solutes from biomass substrates.
Pressure plays a minor role in the solubility of such solutes in subcritical water and was used as a mode to study and maintain an undisrupted flow of water through the flow solubility apparatus. Higher flow rates were maintained especially at higher temperatures to prevent the thermal degradation of the solutes in water. Due to the limited availability of solubility data in the literature and no data for some solutes, especially flavonoids, theoretical models such as UNIFAC and Hansen three-dimensional solubility parameter concept were used to examine the accuracy of such measurements. Such experimentally-measured solubility data along with its theoretical correlations can be used in optimizing the design parameters for extraction and/or reaction of such solutes from natural products.
 David J. Miller and Steven B. Hawthorne, Solubility of liquid organics of environmental interest in subcritical (hot/liquid) water from 298 K to 473 K, J.Chem.Eng.Data, 45 (2000), pp. 78-81.