弱键合离子簇介导的极化致欠屏蔽效应

中文摘要 AI 辅助

我们探究以下假说：在水溶液中高离子强度条件下，离子会形成松散连接的簇，且这些簇与实验观测到的“异常欠屏蔽”现象相关。簇的形成会使有效离子强度低于其标称值，从而减慢屏蔽长度的衰减，但不会逆转衰减趋势。本研究聚焦于额外的贡献项——簇极化效应，我们证明该效应会在浓盐溶液中产生同电荷表面或粒子间更长程的排斥相互作用。我们推导得到了一个解析体相关系，其中簇的整体结构通过单一量（分子内电荷结构因子的电荷加权二阶矩）体现，该量可量化任意簇拓扑结构的极化响应。我们还采用经典聚合物密度泛函理论（cDFT），对基于星型簇的模型进行数值计算，该模型中卫星离子通过简谐弹簧弱键合于共同的中心离子。由于高电荷簇的形成会伴随显著的自能成本，假设卫星离子的晕区呈电中性。利用该模型，我们计算了不同总盐浓度下的表面相互作用与屏蔽长度。结果表明，在“属于簇的离子分数随盐浓度增加而上升”的假设下，可定性得到“异常欠屏蔽”效应，即有效屏蔽长度在总单价盐浓度约1 M处呈现最小值。定量来看，我们注意到，在该阈值以上，屏蔽长度的预测增长弱于实验表面力测量通常得到的结果。

英文摘要

We explore the hypothesis that ions form loosely connected clusters at high ionic strength in aqueous solutions, and that these clusters have relevance to the experimentally observed phenomenon usually referred to as "anomalous underscreening". Cluster formation lowers the ionic strength below its nominal value, slowing the decay of the screening length but not reversing it. Here we focus on an additional contribution, cluster polarisation. We demonstrate that this effect produces longer-ranged repulsive interactions between like-charged surfaces or particles in concentrated salt solutions. We derive an analytical bulk relation in which the entire architecture of a cluster enters through a single quantity, the charge-weighted second moment of its intramolecular charge structure factor, which quantifies the polarisation response of an arbitrary cluster topology. We also make numerical calculations using classical polymer Density Functional Theory, cDFT, for a model based on star-like clusters in which satellite ions are weakly bonded by a harmonic spring to a common central ion. The corona of satellite ions is assumed to be net neutral, since the formation of highly charged clusters would be accompanied by a significant self-energy cost. Using this model, we calculate surface interactions and screening lengths at various overall salt concentrations. We show that, under the assumption that the fraction of ions belonging to clusters increases with salt concentration, one may qualitatively arrive at "anomalous underscreening", i.e., an effective screening length that displays a minimum at an overall (monovalent) salt concentration of about 1 M. Quantitatively, we note that the predicted growth of the screening length beyond this threshold value is weaker than typically found by experimental surface force measurements.

发表机构

• Lund University（隆德大学）
• Heidelberg University, Faculty of Chemistry and Earth Sciences（海德堡大学化学与地球科学学院）
• University College, University of New South Wales（新南威尔士大学学院）

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