Nanosized Cellulose Fibrils as Stabilizer of Emulsions
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Pickering emulsions have been a subject of research for many years due to their practical applications not only in everyday life products but also in industry. The stability of these emulsions is due to the irreversible adsorption of colloid particles at the oil/water interface which prevents droplet coalescence. Cellulose materials are among the different types of particles used as stabilizers. Most of the studies report the use of native cellulose as stabilizer of oil-in-water emulsions due to its high hydrophilic nature. The potential of cellulose for chemical modifications which alter its hydrophobicity opens up the possibility to also use cellulose particles (fibrils) for stabilization of water-in-oil emulsions. Therefore, the research focus of this thesis was on the use of surface modified nanofibrillated cellulose as emulsion stabilizers as well as understanding the mechanism responsible for the stability of these emulsions. It is shown that, in agreement with earlier studies, cellulose nanofibrils of intermediate degree of surface substitution (DSS) can be used as stabilizer of water-in-toluene emulsions. The sample with the lowest DSS which gave stable emulsions for the toluene/water system was identified. The emulsifying capacity of the cellulose nanofibrils was proportional to their concentration and wettability and inverse proportional to the oil/water ratio. The structure of nanofibrillated cellulose layers at the toluene/water interface was characterized by image analysis, atomic force microscopy (AFM) and field-emission electron microscopy (FE-SEM). The results showed that only the cellulose nanofibrils that were able to stabilize emulsions formed networks at the oil/water interface. It is likely that the development of nanofibril networks prevents the droplets coalescence and is responsible for the stability of the emulsions. Cellulose nanofibrils can be selectively oxidized by the NaClO/NaBr mixture using 2,2,6,6- tetramethyl-1-piperinidyloxy radical (TEMPO) as a catalyst. Adsorption of cationic surfactants from aqueous solutions on these TEMPO-mediated oxidized cellulose nanofibrils can lower their wettability by 25%. Adsorption depends on the charge density of the fibrils which are rendered more water-repellent increasing the amount of surfactant adsorbed. However, a definite minimum in the adhesion of water to the nanofibrils is achieved. Further adsorption of surfactant above that will render the cellulose nanofibrils more hydrophilic again, independently of their charge. This may be due to bilayer formation. The nanofibrils are never rendered truly hydrophobic, but the adsorption of surfactants may be sufficient to increase the fibrils tendency to adsorb at interfaces. A cationic surfactant (cetyltrimethyl ammonium bromide, CTAB) was also adsorbed directly on the surface of the films prepared from TEMPO-mediated oxidized cellulose nanofibrils. The hydrophobicity of the films increased with surfactant adsorption, without significantly affecting their tensile index. The average nanofibril diameter and surface porosity were quantified using computer-assisted image analysis on FE-SEM.