Döpke Max F, Hartkamp Remco
Process & Energy Department, Delft University of Technology, Leeghwaterstraat 39, 2628 CB Delft, The Netherlands.
J Chem Phys. 2021 Mar 7;154(9):094701. doi: 10.1063/5.0038161.
Molecular Dynamics (MD) simulations are uniquely suitable for providing molecular-level insights into the Electric Double Layer (EDL) that forms when a charged surface is in contact with an aqueous solution. However, simulations are only as accurate in predicting EDL properties as permitted by the atomic interaction models. Experimental ζ-potential values and surface charges could provide a potentially suitable reference to validate and tune the interaction models, if not for the fact that they themselves are a product of imperfect models used to interpret the raw measurement data. Here, we present an approach to tune an interaction model by comparing Electro-Osmotic Flow (EOF) MD simulations against experimental Streaming Current (SC) measurements while minimizing potential modeling errors arising from both approaches. The point that is least susceptible to interpretation and modeling errors is argued to be at the concentration for which zero flow velocity is observed in EOF simulations and a net zero electric current is measured in SC experiments. At this concentration, the ζ-potential is also zero. We were able to match the experimental concentration at which ζ = 0 in MD simulations for a CaCl solution at pH 7.5 in contact with fused silica by tuning the ion-surface Lennard-Jones cross interactions. These interactions were found to greatly affect the ion distribution within the EDL and particularly the formation of inner-sphere surface-complexes, which, in turn, affects the electrokinetic flow. With the ion distribution determined explicitly, a series of properties can be calculated unambiguously, such as the capacitance needed for surface complexation models.
分子动力学（MD）模拟非常适合从分子层面深入了解当带电表面与水溶液接触时形成的双电层（EDL）。然而，模拟在预测双电层性质方面的准确性仅取决于原子相互作用模型所允许的程度。如果不是因为实验测得的ζ电位值和表面电荷本身就是用于解释原始测量数据的不完美模型的产物，那么它们可能会为验证和调整相互作用模型提供一个合适的参考。在此，我们提出一种方法，通过将电渗流（EOF）MD模拟与实验测得的流动电流（SC）进行比较来调整相互作用模型，同时尽量减少这两种方法可能产生的建模误差。在EOF模拟中观察到流速为零且在SC实验中测得净电流为零的浓度点，被认为是最不易受到解释和建模误差影响的点。在此浓度下，ζ电位也为零。通过调整离子 - 表面的 Lennard - Jones 交叉相互作用，我们能够使MD模拟中与熔融石英接触的pH 7.5的CaCl溶液中ζ = 0时的实验浓度相匹配。发现这些相互作用极大地影响了双电层内的离子分布，特别是内球表面络合物的形成，进而影响了电动流。明确确定离子分布后，就可以明确计算一系列性质，例如表面络合模型所需的电容。
