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Mechanism of Chitosan Adsorption on Silica from Aqueous Solutions
We present a study of the adsorption
of chitosan on silica. The
adsorption behavior and the resulting layer properties are investigated
by combining optical reflectometry and the quartz crystal microbalance.
Exactly the same surfaces are used to measure the amount of adsorbed
chitosan with both techniques, allowing the systematic combination
of the respective experimental results. This experimental protocol
makes it possible to accurately determine the thickness of the layers
and their water content for chitosan adsorbed on silica from aqueous
solutions of varying composition. In particular, we study the effect
of pH in 10 mM NaCl, and we focus on the influence of electrolyte
type and concentration for two representative pH conditions. Adsorbed
layers are stable, and their properties are directly dependent on
the behavior of chitosan in solution. In mildly acidic solutions,
chitosan behaves like a weakly charged polyelectrolyte, whereby electrostatic
attraction is the main driving force for adsorption. Under these conditions,
chitosan forms rigid and thin adsorption monolayers with an average
thickness of approximately 0.5 nm and a water content of roughly 60%.
In neutral solutions, on the other hand, chitosan forms large aggregates,
and thus adsorption layers are significantly thicker (∼10 nm)
as well as dissipative, resulting in a large maximum of adsorbed mass
around the p<i>K</i> of chitosan. These films are also characterized
by a substantial amount of water, up to 95% of their total mass. Our
results imply the possibility to produce adsorption layers with tailored
properties simply by adjusting the solution chemistry during adsorption