Understanding the interfacial interactions of bioinspired chitosan-calcite nanocomposites by first principles molecular dynamics simulations and experimental FT-IR spectroscopy.

The interfacial structures and bonding mechanism of the adsorption of chitosan monomers (GlcN and GlcNAc) on calcite (104) surface are investigated using first principles molecular dynamics simulations. It is revealed that the strong coordination CaO bonds are formed between five-fold coordinated Ca atoms on calcite surface and O atoms of hydroxyl groups (OH) and acetyl groups of chitosan in the chitosan-calcite nanocomposites, improving the chemical compatibility between organic and inorganic phases at the interface in the natural occurring nacre. Although, the presence of interfacial hydrogen bonds is also confirmed by analyzing the valence electron density difference in terms of induced dipole, the evolution of the equilibrium adsorption geometry is found to be mainly governed interfacial CaO coordination bonds. The amide group (NH2) of chitosan binds with calcite at the interface mainly through the formation of weak hydrogen bond. The predicted interfacial electronic structure can be either conductive (GlcNAc) or insulating (GlcN). The frequencies of the characteristic peaks in the calculated total and partial vibrational densities of states of GlcN/calcite systems are found to be in agreement with the FT-IR spectrum of the experimentally prepared chitosan-calcite nanocomposites, crucially validating the modelling structures and computational methodology employed in current work. Theoretical results strongly indicate that the peculiar adsorption in FT-IR spectrum at 2500 cm-1 is related to the presence of free GlcN molecule due to the hydrolysis of chitosan in aqueous solution.