The influence of chitosans with defined degrees of acetylation on the thermodynamic data for copper coordination

The interaction of copper with three different chitosans having degrees of deacetylation of 77.5, 81.5, and 86.1%, named C, A, and F, respectively, was followed by the batch method at 298±1K and the values obtained were fitted to a modified Langmuir equation. These interactions were also obtained by calorimetric titration. Experimentally, 50.0 mg of each chitosan was suspended in doubly distilled water at 298.15±0.02K under mechanical turbine stirring. The titration was performed by adding increments of 10 μl of a 0.10 mol dm3 Cu(NO3)2 aqueous solution and the calorimetric isotherms obtained were adjusted to a modified Langmuir equation. From the net thermal effects K and ΔH values were calculated, also permitting the acquisition of other thermodynamic data for the chitosan-copper interaction at the solid/liquid interface. The exothermic enthalpic values of −45.65±1.97, −49.91±1.57, and −48.64±0.82kJ mol−1, for chitosans C, A, and F, respectively, reflect the degree of deacetylation. The spontaneity of the systems is shown by the negative ΔG values, −36.1±0.2, 36.8±0.1, and −38.1±0.3kJ mol−1 for the same sequence of chitosans. The negative entropic values, −34, −44, and −35 J mol−1 K−1, are in agreement with an ordering of solvent as the complexation occurred. The intensity of the thermal effects and the thermodynamic data obtained from the copper/chitosan interactions can be associated with the ability of these biopolymers to extract copper from aqueous solutions.