Amphiphilic polysaccharides: Stabilization of dispersed two-phase systems and gelling properties of chitosans and propylene glycol alginates
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For many of the polysaccharides with reported surface active and emulsifying properties, this has been attributed to an associated protein fraction. However, certain polysaccharides, such as chitosans and propylene glycol alginates, possess inherent amphiphilic properties. These polysaccharides have the potential of acting as sole stabilizers in dispersed two-phase systems such as O/W emulsions and foams by virtue of combing their interfacial and gel forming properties, thereby omitting the use of surfactants and conveying system interconnectivity and increased long-term stability. In this thesis the potential for exploiting the total functionality of these polysaccharides were evaluated. The emulsion destabilizing and stabilizing properties of chitosan were investigated. Chitosans efficacy in inducing flocculation and creaming in an O/W emulsion was evaluated and it was found that flocculation was more pronounced at lower chitosan concentrations for chitosans of higher fractions of acetylation (FA). It was concluded that under the given conditions the mechanism of flocculation was bridging flocculation mediated by hydrophobic acetyl-rich stretches of chitosan chains. Secondly, an oil-in-water (O/W) emulsion was prepared with chitosan (FA ~ 0.5) as the sole emulsifier. On the optical tweezers (pH 5) emulsion droplets displayed a high degree of stability as well as adaptability to external influence. The observations highlight the suitability of highly acetylated chitosan as emulsifier in O/W emulsions. Chitosan gels (FA ~ 0.5) covalently crosslinked with diethyl squarate (DES) were rheologically described with respect to gelling kinetics, mechanical properties and long term stability as a function of variable DES concentration and molecular weight (Mw). More rigid gels were formed for higher concentration of DES and higher Mw due to formation of a more optimal gel lattice. The gels were highly stable for 12 months in isotonic buffer at 40°C. A series of propylene glycol alginates (PGA) were characterized as a function of their degree of esterification (D.E.). The measured surface tension decreased with increasing D.E. The apparent equilibrium shear storage modulus values for PGA gels decreased with increasing D.E. While the PGAs of low and intermediate D.E. formed functional crosslinking zones between non-substituted Gblocks with Ca2+, the gelling mechanism for the PGAs of highest D.E. (84 % and 93 %) is assumed to be hydrogen bonding in combination with electrostatic point interactions with Ca2+. The rheology of the high D.E. PGA solutions could easily be manipulated by addition of sodium chloride and urea. Self-supporting and homogeneous gelled PGA foams were prepared, as well as PGA/unmodified alginate gelled foams omitting the use of other stabilizers. The foams displayed tailorable properties with respect to density and elastic properties as a function of the D.E. of the PGAs. In a proof-of-concept study the optical tweezers were applied in a novel manner to study the interaction forces between single pairs of emulsion droplets. One of the major observations was the assumed rearrangement of the polysaccharide emulsifier layers at close contact between the emulsion droplets, resulting in a decrease in repulsive force. The characteristic effect was observed for both chitosan (FA 0.5) and sugar beet pectin, demonstrating the dynamics of these interfacial layers. Other interesting observations were the van der Waals attractive force between polystyrene beads and the depletion interaction between emulsion droplets in the presence of surfactant micelles in the continuous phase. The optical tweezers method requires optimization in order to obtain quantifiable data for deformable droplets, but shows promise of becoming a useful tool for scientists in the research on emulsion stability.