Choudhary Aditya, Tsunduru Aashish, Tsianou Marina, Alexandridis Paschalis, Bedrov Dmitry
Department of Materials Science & Engineering, University of Utah, 122 S. Central Campus Drive, Room 304, Salt Lake City, UT 84112, USA.
Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York (SUNY), Buffalo, NY 14260-4200, USA.
J Colloid Interface Sci. 2025 Feb;679(Pt A):1207-1218. doi: 10.1016/j.jcis.2024.10.041. Epub 2024 Oct 10.
Understanding the intricate molecular-level details of toxic per- and polyfluoroalkyl substances (PFAS) partitioning to the air-water interface holds paramount importance in evaluating their fate and transport, as well as for finding safer alternatives for various applications, including aqueous film forming foams. The behavior of these substances at interfaces strongly depends on molecular architecture, chemistry, and concentration, which define molecular packing, self-assembly, interfacial diffusion, and the surface tension.
Modeling of three PFAS surfactants, namely, longer-tail (perfluorooctanoate (PFOA)) and shorter-tail (perfluorobutanoate (PFBA) and 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy) propanoate (GenX)) has been conducted using atomistic molecular dynamics simulations. A systematic comparison between these representative PFAS of different sizes and structure reveals factors influencing their association behavior, mechanism of surface tension reduction, and interfacial mobility as a function of surface coverage.
Shorter-chain PFAS surfactants (GenX or PFBA) require lower surface coverage compared to longer chain (PFOA) PFAS to achieve the same decrease in surface tension. However, a higher concentration of GenX and PFBA is necessary in the bulk aqueous solution to achieve the same surface coverage as PFOA, due to their higher solubility in water. The PFAS molecular orientation and mobility at the interface are found to be vastly influenced by the length and architecture of the hydrophobic fluorocarbon tail. A significant ordering of the water dipole moment near the anionic headgroup is apparent at high surface concentration. A direct correlation is established between the PFAS interfacial properties and PFAS-PFAS, PFAS-counterion, and PFAS-water interactions.
了解有毒的全氟和多氟烷基物质（PFAS）在气-水界面分配的复杂分子水平细节，对于评估它们的归宿和迁移至关重要，同时对于寻找包括水成膜泡沫在内的各种应用的更安全替代品也至关重要。这些物质在界面处的行为强烈依赖于分子结构、化学性质和浓度，这些因素决定了分子堆积、自组装、界面扩散和表面张力。
使用原子分子动力学模拟对三种PFAS表面活性剂进行了建模，即长链（全氟辛酸（PFOA））和短链（全氟丁酸（PFBA）以及2,3,3,3-四氟-2-（七氟丙氧基）丙酸（GenX））。对这些不同大小和结构的代表性PFAS进行系统比较，揭示了影响它们缔合行为、表面张力降低机制以及作为表面覆盖率函数的界面迁移率的因素。
与长链（PFOA）PFAS相比，短链PFAS表面活性剂（GenX或PFBA）在达到相同表面张力降低程度时需要更低的表面覆盖率。然而，由于GenX和PFBA在水中的溶解度较高，因此在本体水溶液中需要更高的浓度才能达到与PFOA相同的表面覆盖率。发现PFAS在界面处的分子取向和迁移率受疏水碳氟链的长度和结构影响很大。在高表面浓度下，阴离子头基附近的水偶极矩有明显的有序排列。建立了PFAS界面性质与PFAS-PFAS、PFAS-抗衡离子和PFAS-水相互作用之间的直接关联。
