Study On Water Vapor Pressure Above Vitrified Sucrose Solution

As a fundamental physical property that reflects the vapor-liquid or vapor-solid equilibrium, vapor pressure is necessary in many scientific and practical researches. It could also be used to deduce enthalpy, entropy, water activity and other useful chemical and physical properties. In the processes of cryopreservation and freeze-drying, the vapor pressure above aqueous solution determines the direction of moisture transfer between the solution and the environment. Moreover, it is also closely related to the water activity of the solution, and reflects the state of water molecules in the solution.Vitrified solutions are often encountered in food and biomaterial preservation and freeze-drying. Numerous studies have been conducted on different physical and chemical properties including the vapor pressure above sucrose solution at room temperature. However, few research concerned vitrified sucrose solution in low temperatures. Experimental data are scarce below0℃. In this thesis, the following work have been carried out:Firstly, an experimental apparatus capable of measuring vapor pressures using the static method was established, and verified with measurements of the saturated vapor pressures of ice. Then, the vapor pressures above vitrified sucrose solution with concentration ranging from75%～79%(w/w) were measured.20g sucrose solution was vitrified by cooling in liquid nitrogen, then placed in a thermostat bath. The vitrification of sucrose solution was verified with DSC.20data points were obtained, ranging from1.7to8.2Pa. The standard uncertainty of pressure and temperature measurements were respectively0.04Pa and45mK.The obtained data were then compared with two different water activity models. It was found that the empirical Zobrist model fitted the experimental data well with an average difference of2.4%. The performance of Vrentas model could be improved by fitting the interaction parameter to experimental data. The average discrepancy could be reduced from8.0%to3.0%. It was found that the value of the glass transition temperature has a great impact on the Vrentas model predictions. The implication of the currently obtained results was demonstrated by the calculation of driving force of water removal in a secondary drying process. Experimental data and theoretical model of the vapor pressure above vitrified sucrose solution could be used to design and optimize process parameters of freeze-drying.