Grafting Superhydrophobic Moieties on Cellulose Nanofibers via Ionic Bonding

Magnesium stearate (MS) as a derivative product of stearic acid (SA) biomass was chosen as the superhydrophobic agent for in-situ modification of citric acid (CA) crosslinked and alkaline treated cellulose nanofibers (CNFs). MS is a common food additive and possesses high thermal stability (T_m = 200 °C, T_d = 359 °C).¹Our approach is taking advantage of ionic bonding and the ion exchange interaction to induce the self-assembly between MS and CA-CNF^-K⁺ to prepare the highly hydrophobic and rough CNF substrates, thus obtaining a superhydrophobic biomass-based surface with controlled superhydrophobicity, roughness and self-cleaning function via a simple and green way.²<br/>Through vacuum filtration and air drying processes, a MS-CA-CNF disk with distinctly different roughness on each side was prepared, of which the appearance is similar to that of the biowax-mineral hybrid modified paper substrate.³ Side A is rough, with spherical particles of diameter from 200 to 500 mm and Side B is smooth. Water contact angle test reveals the high hydrophobic Side A (150 °) and hydrophilic Side B. The quite high WCA of Side A cah be explained by its low surface energy and high roughness. X-ray diffraction (XRD) has been used to analyze the crystalline structure of crystalline structures of both sides. Side A displays a similar crystalline diffraction peak to MS while Side B exhibits more similar crystalline pattern to CNFs. Based on this, surface of Side A is speculated to be mainly composed of stearic chains. SEM energy dispersive spectrum (SEM EDS) characterization has specified the distribution and concentration of Mg on Side A and Side B and strongly supported above speculation. The wt% of C, O, Mg elements on both surfaces has been calculated. Quite low concentration of Mg was detected on Side A and Side B. Side A possesses a remarkably higher concentration of C and much lower concentration of O compared with Side B, which may be due to the tight aggregation of stearic chains on Side A. The distribution of Mg on the cross-section of MS-CA-CNF disk has also been tested. The concentration of Mg near the rough surface Side A is much higher than other locations, indicating that Mg²⁺ may act as the linkers between stearic chains and CA-CNF^- so that MS is able to sit stably on CA-CNF^-.<br/>Based on this, a mechanism has been proposed to explain the self-assembly phenomenon of this disk. Due to the very low surface energy of stearic chains and the ionic binding role of Mg²⁺, MS will highly self-assemble and be stably attached to CA-CNF^-. As a result, the white layer is mainly composed of stearic chains while Mg²⁺ acts as the bridge between stearic chain and CA-CNF^-. Due to the preferential evaporation of ethanol (EtOH) and better affinity between MS and EtOH, MS will move toward the surface while EtOH is evaporating. The evaporation-induced self-assembly mechanism was once used to explain the different aggregation pattern of zein on bacterial cellulose membrane. The synergetic effect of ionic bonding and solvent effect can assist the self-assembly of MS-CA-CNF and explain the distinctly different water barrier property on each side of disk.<br/><br/><br/>References<br/>1. M. He, M. Xu and L. Zhang, Controllable Stearic Acid Crystal Induced High Hydrophobicity on Cellulose Film Surface. ACS Appl. Mater. Interfaces 2013, 5 (3), 585-591.<br/>2. T. S. G. Raja and K. Jeyasubramanian, Tuning the Superhydrophobicity of Magnesium Stearate Decorated Zno Porous Structures for Self-Cleaning Urinary Coatings. Appl. Surf. Sci. 2017, 423, 293-304.<br/>3. J. Wan, P. Wang, X. Qian, M. Zhang, S. Song, M. Wang, Q. Guo and J. Shen, Bioinspired Paper-Based Nanocomposites Enabled by Biowax–Mineral Hybrids and Proteins. ACS Sustainable Chem. Eng. 2020, 8, 9906-9919.