Water sorption and glass transition of pig gastric mucin studied by QCM-D.

Hydration of films of pig gastric mucin was studied using a quartz crystal microbalance with dissipation monitoring (QCM-D) equipped with a humidity module. As a prerequisite, the water adsorption isotherm of a clean silica surface was determined. Atomic force microscopy was used to characterize the changes occurring on the silica surface after repeated sorption/desorption and cleaning cycles. The water sorption isotherms of several hundreds of nanometers thick mucin films were obtained in QCM-D experiments using analysis of overtone behavior. The results show that the sorption isotherms are not dependent on the film thicknesses and are in good agreement with sorption calorimetric data on mucin in the bulk phase. Moreover, hydration-induced changes of rheological properties of mucin films were investigated using a model-free approach. The ratio of G'/G″ was evaluated as a function of relative humidity. The transition from solidlike behavior to liquidlike behavior was observed in the same humidity range as in sorption calorimetric experiments. Thus, ability of QCM-D to monitor glass transition in biopolymers was demonstrated.