Treating electrostatic shielding at the surface of silica as discrete siloxide center dot cation interactions
Treating electrostatic shielding at the surface of silica as discrete siloxide center dot cation interactions
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DOI：
10.1021/ja9638115
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发表时间：
1997-04-16
影响因子：
15
通讯作者：
Whitesides, GM
中科院分区：
文献类型：
--
作者：
Mammen, M;Carbeck, JD;Whitesides, GM
本研究利用毛细管电泳研究了溶液中离子对熔融石英毛细管内电渗透的影响；因此，它研究了带电界面的“屏蔽”。回顾了将离子溶液作为连续介质模型的理论：泊松-玻尔兹曼方程的非线性形式产生了简化的、更常用的德拜-哈克（DH）方程。毛细管电泳（CE）用于测量水溶液中不同单价和二价阳离子浓度的电渗透流动速率。这些数据被用来测试DH理论的三个具体预测：该理论没有充分描述这些数据。DH理论不足的主要原因是它无法解释单个离子水平上的细节，而只能通过平均场静电学来解释：也就是说，离子的化学特性——例如，极化性、水合大小、水合能、通过螯合协调其他离子的能力——没有得到解释。描述了一种模型（“解离模型”），该模型根据具有特征解离常数K-d（eff）的离散关联平衡来处理溶液中的阳离子与表面上带负电荷的基团之间的相互作用。然后将CE用作测量不同阳离子的K-d（eff）值的工具。这些解离常数遵循的模式与我们在溶液研究中所熟悉的模式一致。
This work examines the influence of ions in solution on electroosmosis inside a fused silica capillary using capillary electrophoresis; it thereby examines ''shielding'' at charged interfaces. Theories are reviewed that model ionic solutions as continuous dielectrics: the nonlinear form of the Poisson-Boltzmann equation gives rise to the simplified, more commonly used Debye-Huckel (DH) equation. Capillary electrophoresis (CE) is used to measure the rate of electroosmotic flow as a function of the concentration of different monovalent and divalent cations in aqueous solution. These data are used to test three specific predictions of DH theory: this theory does not describe these data adequately. The central reason behind the inadequacy of DH theory here is its inability to account for details at the level of individual ions other than by mean-field electrostatics: that is, chemical characteristics of ions-for example, polarizability, hydrated size, energy of hydration, ability to coordinate other ions by chelation-are not accounted for. A model (the ''dissociation model'') is described that treats the interactions between cations in solution and negatively charged groups on a surface in terms of discrete association equilibria with characteristic dissociation constants, K-d(eff). CE is then used as a tool to measure values of K-d(eff) for different cations. These dissociation constants follow patterns that are consistent with ones that are familiar from studies in solution.