Chitosan/sodium dodecylsulfate interactions |
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Authors: | R Barreiro-Iglesias C Alvarez-Lorenzo A Concheiro |
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Institution: | (1) Departamento de Farmacia y Tecnología Farmacéutica, Facultad de Farmacia, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain;(2) Departamento de Farmacia y Tecnología Farmacéutica, Facultad de Farmacia, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain;(3) Departamento de Farmacia y Tecnología Farmacéutica, Facultad de Farmacia, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain |
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Abstract: | Summary The thermodynamics of the interaction of chitosan and sodium dodecylsulfate, SDS, was characterised by titration microcalorimetry
to gain an insight into the binding process of amphiphilic molecules to this biocompatible polymer and its consequences on
the behaviour of the solutions and chemically cross-linked hydrogels of chitosan. 0.2 M acetic acid was used as solvent medium,
without or with 0.9% NaCl, in order to evaluate the influence of the ionic and hydrophobic interactions with two chitosans
of different molecular mass and degree of deacetylation, DD. The critical micellar concentration, CMC, of SDS was ten times
lower in the presence of the salt (0.35 vs. 3.5 mM, as estimated by surface tension measurements). Binding to chitosan (at 0.25%) began at concentrations significantly
lower than CMC (critical aggregation concentration, CAC=0.035-0.17 mM) and saturation was reached at around 10 mM SDS, which
corresponds to a positive/negative charges ratio of about 1. The process was in all cases enthalpy-driven (strongly exothermic)
and, in the absence of the salt, also entropically favourable. The Gibbs free energy of interaction values were slightly greater
for the chitosan with lower DD but greater molecular mass. The addition of increasing amounts of SDS resulted in a continuous
decrease in the viscosity of chitosan solutions above the CAC, which ended in a macroscopic coacervation when around 1/3 of
the positive charges were neutralised. In the same range of SDS concentrations, the hydrogel beads showed a continuous decrease
in the swelling degree and a final collapsed state. The scarce tendency to redissolution or hydrogel reswelling in the presence
of greater SDS concentrations can be attributed to that the binding process is mainly caused by the ionic interaction and
did not go beyond the neutralisation point. |
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Keywords: | chitosan isoperibol microcalorimetry polymer/surfactant interactions SDS surface tension |
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