Department of Chemical Engineering, University of California, Santa Barbara, CA 93106.
Department of Civil, Architectural and Environmental Engineering, University of Texas at Austin, Austin, TX 78712.
Proc Natl Acad Sci U S A. 2021 Jan 5;118(1). doi: 10.1073/pnas.2020205118.
Performance of membranes for water purification is highly influenced by the interactions of solvated species with membrane surfaces, including surface adsorption of solutes upon fouling. Current efforts toward fouling-resistant membranes often pursue surface hydrophilization, frequently motivated by macroscopic measures of hydrophilicity, because hydrophobicity is thought to increase solute-surface affinity. While this heuristic has driven diverse membrane functionalization strategies, here we build on advances in the theory of hydrophobicity to critically examine the relevance of macroscopic characterizations of solute-surface affinity. Specifically, we use molecular simulations to quantify the affinities to model hydroxyl- and methyl-functionalized surfaces of small, chemically diverse, charge-neutral solutes represented in produced water. We show that surface affinities correlate poorly with two conventional measures of solute hydrophobicity, gas-phase water solubility and oil-water partitioning. Moreover, we find that all solutes show attraction to the hydrophobic surface and most to the hydrophilic one, in contrast to macroscopically based hydrophobicity heuristics. We explain these results by decomposing affinities into direct solute interaction energies (which dominate on hydroxyl surfaces) and water restructuring penalties (which dominate on methyl surfaces). Finally, we use an inverse design algorithm to show how heterogeneous surfaces, with multiple functional groups, can be patterned to manipulate solute affinity and selectivity. These findings, importantly based on a range of solute and surface chemistries, illustrate that conventional macroscopic hydrophobicity metrics can fail to predict solute-surface affinity, and that molecular-scale surface chemical patterning significantly influences affinity-suggesting design opportunities for water purification membranes and other engineered interfaces involving aqueous solute-surface interactions.
用于水净化的膜的性能受到溶剂化物种与膜表面相互作用的高度影响，包括在污染时溶质在表面的吸附。目前，抗污染膜的努力通常追求表面亲水化，这通常是由亲水性的宏观测量来驱动的，因为疏水性被认为会增加溶质-表面亲和力。虽然这种启发式方法推动了各种膜功能化策略的发展，但在这里，我们基于疏水理论的进展，批判性地研究了宏观溶质-表面亲和力的相关性。具体来说，我们使用分子模拟来量化模型羟基和甲基功能化表面对小的、化学多样化的、带电荷中性的溶质的亲和力，这些溶质在采出水中被代表。我们表明，表面亲和力与两种常规的溶质疏水性测量方法（气相水溶解度和油水分配）相关性较差。此外，我们发现所有溶质都对疏水性表面和大部分亲水性表面具有吸引力，这与基于宏观的疏水性启发式方法相反。我们通过将亲和力分解为直接的溶质相互作用能（在羟基表面上占主导地位）和水重构罚分（在甲基表面上占主导地位）来解释这些结果。最后，我们使用逆设计算法展示了具有多个功能基团的非均匀表面如何被图案化以操纵溶质亲和力和选择性。这些发现，重要的是基于一系列溶质和表面化学，说明了传统的宏观疏水性度量可能无法预测溶质-表面亲和力，并且分子尺度表面化学图案化显著影响亲和力，这为水净化膜和其他涉及水相溶质-表面相互作用的工程界面提供了设计机会。
