Molecular interactions in N-[(2-hydroxyl)-propyl-3-trimethyl ammonium] chitosan chloride-sodium alginate polyelectrolyte complexes

Several N-[(2-Hydroxyl)-propyl-3-trimethyl ammonium] chitosan chlorides (HTCCs) with various degree of substitution (DS) were synthesized. The interaction of these compounds with sodium alginate(SA) in solutions and films was studied by rheology, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and dynamic-mechanical analysis (DMA). The effects of some key factors such as DS, mass ratio of HTCC to SA (m_HTCC/m_SA), and temperature on the intermolecular interactions were investigated. The rheological results revealed the polyelectrolyte complex (PEC) solutions to be non-Newtonian fluids in all cases, showing a shear-thinning behavior. The apparent viscosity and viscoelastic properties of these HTCC compounds were observed to depend on DS and m_HTCC/m_SA. Thus, a high DS and low m_HTCC/m_SA resulted in low apparent viscosities, which was ascribed to a shielding effect of N,N,N-trimethylammonium (–N(CH₃)₃⁺) on the carboxyl (–COO^-) groups. Below the critical temperature (25 °C), intermolecular interactions were dominated by hydrogen-bonds, while above this temperature electrostatic interactions prevailed. Both biomacromolecules showed maximum synergic effects and resulted in optimum HTCC/SA PEC structures for DS, m_HTCC/m_SA, and temperature of 96%, 1/10, and 25 °C, respectively. The properties of the HTCC-SA films confirmed a strong hydrogen-bonding and electrostatic interaction synergy between both biomacromolecules. The intermolecular interactions were elucidated by molecular dynamics studies. These results can provide a theoretical approach for synthetizing HTCC-SA based edible packaging films.