Adsorption\nof <i>Acacia</i> Gum on Self-Assembled\nMonolayer Surfaces: A Comprehensive Study Using QCM‑D and MP-SPR

The interfacial structuring of <i>Acacia</i> gum at various\npH values on self-assembled monolayer (SAM) surfaces was investigated\nin order to evaluate the respective importance of surface versus biopolymer\nhydration in the adsorption process of the gum. To this end, SAMs\nwith four different ending chemical functionalities (−CH<sub>3</sub>, −OH, −COOH, and −NH<sub>2</sub>) were\nused on gold surfaces, and the gum adsorption was monitored using\nmultiparametric surface plasmon resonance (MP-SPR) and quartz crystal\nmicrobalance with dissipation. Surface modification with alkanethiol\nand the subsequent adsorption of <i>Acacia</i> gum were\nalso characterized by contact angle measurements using both sessile\ndrop and captive bubble methods. According to MP-SPR results, this\nstudy demonstrated that gum adsorbed on all surfaces and that adsorption\nis the most favorable at both acid pH and hydrophobic environments,\ni.e., when both the surface and the biopolymer are weakly hydrated\nand more prone to interfacial dehydration. These results reinforce\nour recent proposal of interfacial dehydration-induced structuring\nof biopolymers. Increasing the pH logically decreased the adsorption\ncapacity, especially on a hydrophilic surface, enhancing the hydration\nrate of the layer. A hydrophilic surface is unfavorable to <i>Acacia</i> gum adsorption except if the surface presents a negative\nsurface charge. In this case, interfacial charge dehydration was promoted\nby attractive electrostatic interactions between the surface and biopolymers.\nIn the aggregate, the water percentage and the viscoelastic properties\nwere closely related to the properties of the surface function: the\nnegative charge and hydrophobicity significantly increased the hydration\nrate and viscoelastic properties with the pH, while the positive charge\ninduced a rigid and more dehydrated layer.