Amphiphilic polysaccharides: Stabilization of dispersed two-phase systems and gelling properties of chitosans and propylene glycol alginates
Doctoral thesis
Permanent lenke
http://hdl.handle.net/11250/2372788Utgivelsesdato
2015Metadata
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Sammendrag
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.