Thermodynamics of interaction between water and the composite films based on chitosan and chitin nanofibrils

Interaction between water and composite films based on chitosan and chitin nanofibrils was studied. Isotherms of water vapor sorption by composite films were used to calculate partial values of entropy and enthalpy of sorbate; dependences of entropy and enthalpy on water sorption value were obtained. It was demonstrated that introducing chitin nanofibrils into chitosan matrix leads to decrease in sorption capacity of composite films. Apparently, this phenomenon is caused by formation of ordered structures consisting of chitosan macromolecules on the surface of chitin nanofibrils. The hypothesis was confirmed by calculations of thermodynamic parameters of the chitosan/chitin/water system. The calculations led to the conclusion that thermodynamically stable chitosan/chitin system is formed in composite films; in addition, it was revealed that the strongest chitosan-chitin interaction arises in the composite containing 1-5 wt.% of chitin nanofibrils. In this concentration range, Gibbs energy, entropy and enthalpy of mixing pass through a minimum; this result indicates that the highest affinity between chitosan and chitin exists when concentration of chitin nanofibrils varies from 1 to 5 wt.%.