Robertson Hayden, Elliott Gareth R, Nelson Andrew R J, Le Brun Anton P, Webber Grant B, Prescott Stuart W, Craig Vincent S J, Wanless Erica J, Willott Joshua D
College of Science, Engineering and Environment, University of Newcastle, Callaghan, NSW 2308, Australia.
Australian Centre for Neutron Scattering, ANSTO, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia.
Phys Chem Chem Phys. 2023 Sep 20;25(36):24770-24782. doi: 10.1039/d3cp02206d.
Hypersaline environments are ubiquitous in nature and are found in myriad technological processes. Recent empirical studies have revealed a significant discrepancy between predicted and observed screening lengths at high salt concentrations, a phenomenon referred to as underscreening. Herein we investigate underscreening using a cationic polyelectrolyte brush as an exemplar. Poly(2-(methacryloyloxy)ethyl)trimethylammonium (PMETAC) brushes were synthesised and their internal structural changes and swelling response was monitored with neutron reflectometry and spectroscopic ellipsometry. Both techniques revealed a monotonic brush collapse as the concentration of symmetric monovalent electrolyte increased. However, a non-monotonic change in brush thickness was observed in all multivalent electrolytes at higher concentrations, known as re-entrant swelling; indicative of underscreening. For all electrolytes, numerical self-consistent field theory predictions align with experimental studies in the low-to-moderate salt concentration regions. Analysis suggests that the classical theory of electrolytes is insufficient to describe the screening lengths observed at high salt concentrations and that the re-entrant polyelectrolyte brush swelling seen herein is consistent with the so-called regular underscreening phenomenon.
高盐环境在自然界中无处不在，并且存在于众多技术过程中。最近的实证研究表明，在高盐浓度下，预测的筛分长度与观察到的筛分长度之间存在显著差异，这种现象被称为筛分不足。在此，我们以阳离子聚电解质刷为例研究筛分不足。合成了聚（2-（甲基丙烯酰氧基）乙基）三甲基氯化铵（PMETAC）刷，并通过中子反射仪和光谱椭偏仪监测其内部结构变化和溶胀响应。随着对称单价电解质浓度的增加，这两种技术都显示出刷状结构单调塌陷。然而，在所有多价电解质中，在较高浓度下观察到刷厚度的非单调变化，即所谓的再入式溶胀；这表明存在筛分不足。对于所有电解质，数值自洽场理论预测与低至中等盐浓度区域的实验研究结果一致。分析表明，经典电解质理论不足以描述在高盐浓度下观察到的筛分长度，并且本文中观察到的再入式聚电解质刷溶胀与所谓的常规筛分不足现象一致。
