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 +/- 1 K 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 +/- 0.02 K under mechanical turbine stirring. The titration was performed by adding increments of 10 mul of a 0.10 mol dm(3) Cu(NO3)(2) aqueous solution and the calorimetric isotherms obtained were adjusted to a modified Langmuir equation. From the net thermal effects K and DeltaH 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 +/- 0.82 kJ mol(-1), for chitosans C, A, and F, respectively, reflect the degree of deacetylation. The spontaneity of the systems is shown by the negative DeltaG values, -36.1 +/- 0.2, 36.8 +/- 0.1, and -38.1 +/- 0.3 kJ 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. (C) 2004 Elsevier Inc. All rights reserved.