SupMat_EPSR_Solution_Partials_SQ.dat from Molecular interactions in short-chain perfluoroalkyl carboxylic acids and aqueous solutions

The presence of short-chain per- and polyfluoroalkyl substances in water poses a major health and environmental challenge. Here, we have performed high-energy small- and wide-angle X-ray scattering measurements on CF₃[CF₂]<i>_n</i>COOH (where <i>n</i> = 1, 2, 3 represents the chain length) and their aqueous solutions at 10% mole concentrations to characterize their molecular interactions at the atomic and nanometer length scales. The experimental wide-angle structure factors have been modelled using Empirical Potential Structural Refinement. The oxygen–oxygen partial X-ray pair distribution functions show that the coordination number between the hydroxyl oxygen on the acid and surrounding oxygen water molecules increases significantly with acid chain length, rising from 3.2 for <i>n</i> = 1 to 4.1 for <i>n</i> = 3. The small-angle scattering is dominated by a sharp, high-intensity peak at <i>Q</i>₁∼0.2 Å⁻¹ and a smaller peak at <i>Q</i>₂= 1.2 Å⁻¹ for <i>n</i> = 3, both of which decrease with decreasing chain length. The <i>Q</i>₂ peak is attributed to groups of adjacent non-bonded acid molecules, and <i>Q</i>₁ has contributions both from correlations between acid molecules and water–water interactions. In all cases, the models show nanoscale aggregation occurs in the form of denser channels of winding hydrogen-bonded chains, approximately 20 water molecules in length, surrounding clusters of acid molecules.This article is part of the theme issue 'XXXX'.

短链全氟及多氟烷基物质（per- and polyfluoroalkyl substances, PFAS）在水环境中的赋存，构成了重大的健康与环境挑战。本研究针对三氟直链全氟烷基羧酸（CF₃[CF₂]ₙCOOH，其中n=1、2、3代表碳链长度）及其10%摩尔浓度的水溶液开展了高能小角与广角X射线散射测量，以在原子及纳米尺度上表征其分子间相互作用。实验所得的广角结构因子采用经验势结构精修（Empirical Potential Structural Refinement, EPSR）方法进行建模分析。氧-氧偏X射线对分布函数结果显示，羧酸分子羟基氧与周围水分子氧之间的配位数随羧酸碳链长度增长显著提升，从n=1时的3.2升至n=3时的4.1。对于n=3的样品，小角散射信号主要由Q₁≈0.2 Å⁻¹处的尖锐高强峰以及Q₂=1.2 Å⁻¹处的弱峰主导，且两类峰的强度均随碳链长度缩短而降低。Q₂峰可归因于相邻非键合羧酸分子的聚集，而Q₁峰则同时来源于羧酸分子间的关联以及水分子间的相互作用。所有建模结果均表明，纳米尺度聚集以缠绕氢键链构成的致密通道为形式，该类通道长度约包含20个水分子，环绕包裹着羧酸分子团簇。本文属于“XXXX”专题栏目文章。