DSC study on hyaluronan drying and hydration

Abstract

The processes of hyaluronan (HYA) drying and hydration were studied using differential scanning calorimetry. In the first approach the isoconversional Kissinger–Akahita–Sunose (KAS) method was applied in order to determine actual activation energies of evaporation of pure water and water from concentrated HYA solutions. Since the evaporation is a single-step process, the activation energies for pure water provided results consistent with tabulated values of evaporation enthalpies. In the course of water evaporation from hyaluronan solution a break in increasing enthalpy followed by a decrease below 0.34 g of water per 1 g of HYA was observed. This result confirmed earlier observation that at this particular water content evaporation from hyaluronan is compensated by heat evolution associated with the formation of new bonds in hyaluronan supramolecular structure. Subtraction of water evaporation enthalpy from enthalpies obtained for HYA concentrated solution provided a possibility to extrapolate the evaporation enthalpies to the concentration (approximately 2 g of water per 1 g of HYA) at which free water is not present any longer and only bound water starts being evaporated from the HYA solution. Similar results were obtained in the second approach in which using slightly modified “traditional” freezing/ thawing experiment, melting enthalpy of ice was plotted against water fraction in HYA. It was found out that the melting enthalpy of ice exponentially increases from 0.8 up to 2 g of water per g of hyaluronan where it reaches and keeps the melting enthalpy of hexagonal ice. It was shown that both approaches can serve as alternatives providing an additional insight into the state of water and biopolymers in highly concentrated solutions.