Chitosan/sodium dodecylsulfate interactions - Calorimetric titration and consequences on the behaviour of solutions and hydrogel beads

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.