Interfacial Behavior Of Cyclodextrins At Oil/Water Interface And Properties Of Their Stabilized Emulsions

Cyclodextrins(CDs) have a hydrophobic cavity and a hydrophilic shell allowing them to form inclusion complexes with various guest molecules. Due to their advantage of nontoxicty and biocompatibility, CDs have been widely used in many industrial fields such as food, pharmaceutical and catalysis. Previous studies have shown that hydrophobic molecules of oil phase can be included partially in the hydrophobic cavities of CDs with the formation of the inclusion complexes(ICs). These inclusion complexes can be self-assembled further into microcrystals by non-covalent interactions at o/w interface, which can stabilize emulsions. The emulsions stabilized by the ICs are of the Pickering emulsion. In this thesis, the key factors which may affect interfacial behavior of CDs at the o/w interface have been analyzed. The influence of the key factors on adsorption kinetics of CDs at the o/w interface and interfacial rheological properties of ICs films has been studied. The relationship between the interfacial behavior of CDs and the stability of their emulsions is gotten from the results of bulk stability and rheological properties of the emulsions. The main contents are as follows.(1) The adsorption of two CDs(α-CD and β-CD) at the o/w interface and their self-assembly behavior have been studied. We chose two types of oil with different polarities(soybean oil and n-dodecane) as the emulsion oil phase, measured the dynamic interfacial tension and interfacial shear rheological properties at different CDs concentrations(5 mM, 10 mM and 15 mM), and obtained the characteristic parameters of the microscopic interfacial relaxation processes. The relationship among the cavity structure of CDs, properties of oil phase and the interfacial tension have been evaluated. The results show that the formation and self-assembly of the ICs can be directly influenced by the matching between the structure of CD cavity and that of the oil molecules, and consequently results in different interfacial morphology and characteristic of ICs films. From the results of cryo-scanning electron microscope(croy-SEM), the ICs of n-dodecane/CDs can aggregate into spheres and their interfacial films exhibit rough appearance, wile the ICs films of soybean oil-CDs present smooth sheet structure. The morphology of the interfacial films gives significant impact on their mechanical strength. It is obvious that the dynamic adsorption of CDs at the o/w interface, self-assembly and interactions of oil-CD ICs play a very critical role in the formation and stability of interfacial films.(2) The CDs Pickering emulsions were prepared based on the optimal conditions about the formation and control on the interfacial films. The effects of CDs types, p H, NaCl concentration and the ratio of o/w on the microstructure, droplet size, creaming index, shear viscosity and viscoelasticity modulus of emulsions have been investigated. It shows that the emulsion droplet size decreases with increasing NaCl concentration, increases with increasing o/w volume ratio, but keeps almost constant with changing pH. It is obvious that the change of environmental conditions can affect the droplet size, elastic modulus and thus the stability of emulsions.(3) Competitive adsorption between β-CD and sodium caseinate(SC) at the o/w interface and its effect on the emulsion stability have been explored by developing two different routes(a simultaneous way and a sequential way). The dynamic interfacial tension of CD-SC and interfacial rheological properties of adsorbent layer were measured. The effective diffusion coefficients of SC are obtained by fitting the experimental data using a mathematical model, the dependence of interfacial elastic modulus upon the frequency is studied, and the self-assembly mechanism of CD and SC and their competitive adsorption models at the o/w interface are proposed. It shows that SC, due to its higher affinity for the interface, adsorbed preferentially at the o/w interface with blocking the formation of CD-oil inclusion complexes in a simultaneous way, and hence the emulsions are protein-stabilized. In the sequential way, due to the pre-adsorbed solid layer of ICs formed at the o/w interface, the emulsions are Pickering emulsion, but the interfacial films impaired gradually with the sequential addition of SC.