Chemistry Department, University of California , Berkeley, California 94720, United States.
Langmuir. 2015 Jan 13;31(1):215-25. doi: 10.1021/la503813d. Epub 2014 Dec 26.
Ion binding to acidic groups is a central mechanism for ion-specificity of macromolecules and surfaces. Depending on pH, acidic groups are either protonated or deprotonated and thus change not only charge but also chemical structure with crucial implications for their interaction with ions. In a two-step modeling approach, we first determine single-ion surface interaction potentials for a few selected halide and alkali ions at uncharged carboxyl (COOH) and charged carboxylate (COO(-)) surface groups from atomistic MD simulations with explicit water. Care is taken to subtract the bare Coulomb contribution due to the net charge of the carboxylate group and thereby to extract the nonelectrostatic ion-surface potential. Even at this stage, pronounced ion-specific effects are observed and the ion surface affinity strongly depends on whether the carboxyl group is protonated or not. In the second step, the ion surface interaction potentials are used in a Poisson-Boltzmann model to calculate the surface charge and the potential distribution in the solution depending on salt type, salt concentration, and solution pH in a self-consistent manner. Hofmeister phase diagrams are derived on the basis of the long-ranged forces between two carboxyl-functionalized surfaces. For cations we predict direct, reversed, and altered Hofmeister series as a function of the pH, qualitatively similar to recent experimental results for silica surfaces. The Hofmeister series reversal for cations is rationalized by a reversal of the single-cation affinity to the carboxyl group depending on its protonation state: the deprotonated carboxylate (COO(-)) surface group interacts most favorably with small cations such as Li(+) and Na(+), whereas the protonated carboxyl (COOH) surface group interacts most favorably with large cations such as Cs(+) and thus acts similarly to a hydrophobic surface group. Our results provide a general mechanism for the pH-dependent reversal of the Hofmeister series due to the different specific ion binding to protonated and deprotonated surface groups.
离子与酸性基团的结合是大分子和表面离子特异性的核心机制。根据 pH 值的不同，酸性基团可以质子化或去质子化，因此不仅会改变电荷，还会改变化学结构，这对它们与离子的相互作用有至关重要的影响。在两步建模方法中，我们首先通过带有显式水的原子模拟 MD 从几个选定的卤化物和碱离子在不带电的羧基（COOH）和带电的羧酸盐（COO（-））表面基团上确定单离子表面相互作用势。我们特别注意扣除由于羧酸盐基团的净电荷而产生的裸库仑贡献，从而提取非静电离子-表面势。即使在这个阶段，也观察到明显的离子特异性效应，并且离子表面亲和力强烈取决于羧基是否质子化。在第二步中，离子表面相互作用势用于泊松-玻尔兹曼模型中，以自洽方式计算取决于盐类型、盐浓度和溶液 pH 的溶液中表面电荷和电势分布。根据两个羧基功能化表面之间的长程力，推导出 Hofmeister 相图。对于阳离子，我们预测了 pH 值的直接、反转和改变的 Hofmeister 序列，与最近关于二氧化硅表面的实验结果定性相似。阳离子 Hofmeister 序列反转是由于羧基基团的单阳离子亲和力取决于其质子化状态的反转：去质子化的羧酸盐（COO（-））表面基团与小阳离子（如 Li（+）和 Na（+））相互作用最有利，而质子化的羧基（COOH）表面基团与大阳离子（如 Cs（+））相互作用最有利，因此类似于疏水性表面基团。我们的结果提供了一个普遍的机制，即由于与质子化和去质子化表面基团的不同特定离子结合，Hofmeister 序列的 pH 依赖性反转。
