Modeling surface chemistry and adsorption behavior of biomimetic chitosan/silica hybrid materials

Chitosan/silica hybrid powders, obtained by the sol-gel method (pH 6, 7, 8) are studied for proposing surface chemical models and rationalizing their behaviour as adsorbents. The chitosan and silica exposed groups in dried powders and their protonation state are inferred. Solid-state Si-29-NMR, XPS, zeta potential experiments and DFT computational calculations reveal the synthesis pH history of each sample. These details, summarized in surface models, are validated by copper adsorption: The pH 8 sample exhibits the highest hydrophilic and negative character, due to the deprotonated free silanol and neutral amine groups. These favor higher copper adsorption efficiency (30.4%) and loading capacity (0.077 mg Cu(II)/m(2)), through SiO--Cu(II) electrostatic interactions and [Cu-(NH2)(x)](2+) complexation. For lower synthesis-pH samples, the neutral silanol and protonated amine populations increase, and the net surface charge becomes positive. This reduces the copper adsorption through electrostatic repulsions. The protonated amine and deprotonated silanol groups are the main responsible of electrokinetic surface response of materials prepared at lower and higher pH, respectively. The complexation is also involved in copper interactions, especially in surfaces populated by neutral amines. In conclusion, this work helps to understand the pH-dependent surface activity of chitosan/hybrid materials, being helpful for their further functionalization and applications. (C) 2020 The Authors. Published by Elsevier B.V.