Molecular interaction mechanism for humic acids fouling resistance on charged, zwitterion-like and zwitterionic surfaces
Qiuyi Lu, Zhoujie Wang, Shishuang Zhang, Jingyi Wang, Xiaohui Mao, Lei Xie, Qi Liu, Hongbo Zeng
Abstract
Humic acids (HA) are ubiquitous in surface waters, leading to significant fouling challenges. While zwitterion-like and zwitterionic surfaces have emerged as promising candidates for antifouling, a quantitative understanding of molecular interaction mechanism, particularly at the nanoscale, still remains elusive. In this work, the intermolecular forces between HA and charged, zwitterion-like or zwitterionic monolayers in aqueous environments were quantified using atomic force microscope. Compared to cationic MTAC ([2-(methacryloyloxy)ethyl]trimethylammonium chloride), which exhibited an adhesion energy of ∼ 1.342 mJ/m2 with HA due to the synergistic effect of electrostatic attraction and possible cation-π interaction, anionic SPMA (3-sulfopropyl methacrylate) showed a weaker adhesion energy (∼0.258 mJ/m2) attributed to the electrostatic repulsion. Zwitterion-like MTAC/SPMA mixture, driven by electrostatic attraction between opposite charges, formed a hydration layer that prevented the interaction with HA, thereby considerably reducing adhesion energy to ∼ 0.123 mJ/m2. In contrast, zwitterionic MPC (2-methacryloyloxyethyl phosphorylcholine) and DMAPS ([2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide) displayed ultralow adhesion energy (0.06–0.07 mJ/m2) with HA, arising from their strong dipole moments which could induce a tight hydration layer that effectively inhibited HA fouling. The pH-mediated electrostatic interaction resulted in the increased adhesion energy for MTAC but decreased adhesion energy for SPMA with elevated pH, while the adhesion energy for zwitterion-like and zwitterionic surfaces was independent of environmental pH. Density functional theory (DFT) simulation confirmed the strong binding capability of MPC and DMAPS with water molecules (∼-12 kcal mol−1). This work provides valuable insights into the molecular interaction mechanisms underlying humic-substance-fouling resistance of charged, zwitterion-like and zwitterionic materials at the nanoscale, shedding light on developing more effective strategy for HA antifouling in water treatment.