Investigations Of The Viscoelasticity And Stability Of The Natural Silk Fibroin At Different Oil/Water Interfaces

In this thesis,novel emulsions with strong mterfacial stability as well as good biocompatibility were prepared by using silk fibroin extracted from natural Bombyx mori cocoon shells.Through investigating the mterfacial rheological behaviors of silk fibroin at different oil/water interfaces and the microstructure and stability of the emulsions stabilized by silk fibroin,the influence of pH and ionic strength on the mterfacial viscoelasticity at the oil/water interface and emulsion stability was discussed,and the relationship between mterfacial rheological characteristics and emulsion stability for silk fibroin was obtained,which helps to reveal the mterfacial stabilization mechanism of silk fibroin molecules at the interface and the control and analysis methods for mterfacial stability.We hope these results will provide new idea for the preparation and investigations of the emulsions stabilized by silk fibroin and solid theoretical foundation for the applications of these emulsions in the fields of drug delivery,food industry as well as cosmetics.At the beginning,the silk fibroin aqueous solution was prepared through degumming in alkali solution,dissolution in LiBr solution and dialysis against deionized water.Afterwards,silk fibroin solutions with different pH and ionic strength were prepared using different pH buffer solutions and salt solutions with different ion types,and the effects of pH and ionic strength on the interfacial dilatational rheology and interfacial shear rheology of silk fibroin at the dodecane/water and butyl butyrate/water interfaces were investigated by using analysis tensiometer and biconical rheometer in order to discuss the interfacial structure,interfacial viscoelastic properties and interfacial adsorption kinetics of silk fibroin layers at different oil/water interface.The interfacial rheological results of silk fibroin with the effects of pH indicate that at pH=4 with respect to the isoelectric point of silk fibroin,silk fibroin at both nonpolar dodecane/water and polar butyl butyrate/water interfaces tends to form more compact configuration with aggregates and the interfacial layers exhibit lower storage modulus and higher brittleness.While the interfacial rheological results of silk fibroin with the effects of ionic strength indicate that with the addition of ions,the elasticity of the interfacial silk fibroin films is changed with the formation of higher content of ?-sheet configurations.As the ionic strength increases,the electrostatic repulsion between neighboring silk fibroin molecules is reduced and interactions between silk fibroin molecules are enhanced.Therefore,the shape of silk fibroin molecules is changed due to counterion screening,which results in a more compact coiled conformation that can pack more easily at the interface.The novel emulsions stabilized by silk fibroin with different pH and ionic strength were also prepared with different oils of dodecane and butyl butyrate,and the emulsion type,droplet size and distribution and stability of the resultant emulsions were investigated,in order to analyze the effect of pH and ionic strength on the emulsion stability and discuss the microstructure and stability mechanism of the emulsions stabilized by silk fibroin.The emulsions stabilized by silk fibroin is O/W type,and the emulsion droplet size is the biggest and the remained stable emulsion fraction 1s the smallest at pH=4.At pH=4 with respect to isoelectric point of silk fibroin,the net charge on the surface of silk fibroin molecules is close to zero,and the electrostatic repulsion between protein molecules is not sufficient enough to overcome the Van der Waals attractions between emulsion droplets,which makes the emulsion droplet aggregate more easily and droplet size become bigger.On the other hand,at pH=4 the structure of mterfacial films are more compact,and the resultant brittleness of mterfacial films leads to easy breakage of adsorption layers and the decrease of emulsion stability.In the case of the emulsions with different ionic strength,a drastic increase in mean particle size is observed after incorporation of ions,and these emulsions are relatively more stable with NaCI than those with CaCl2 and NdCl3Finally,we brought forward some further investigation expectations of silk fibroin.The achievements of the interfacial rheology of silk fibroin at the oil/water interface not only promote better understanding of the interfacial rheological characteristics of protein films,but also provide important theoretical basis for further applications of protein and its complex systems with other surfactants as colloid stabilizers.