Dewan Shalaka, Carnevale Vincenzo, Bankura Arindam, Eftekhari-Bafrooei Ali, Fiorin Giacomo, Klein Michael L, Borguet Eric
Department of Chemistry, Temple University , Philadelphia, Pennsylvania 19122, United States.
Langmuir. 2014 Jul 15;30(27):8056-65. doi: 10.1021/la5011055. Epub 2014 Jun 30.
The properties of water molecules located close to an interface deviate significantly from those observed in the homogeneous bulk liquid. The length scale over which this structural perturbation persists (the so-called interfacial depth) is the object of extensive investigations. The situation is particularly complicated in the presence of surface charges that can induce long-range orientational ordering of water molecules, which in turn dictate diverse processes, such as mineral dissolution, heterogeneous catalysis, and membrane chemistry. To characterize the fundamental properties of interfacial water, we performed molecular dynamics (MD) simulations on alkali chloride solutions in the presence of two types of idealized charged surfaces: one with the charge density localized at discrete sites and the other with a homogeneously distributed charge density. We find that, in addition to a diffuse region where water orientation shows no layering, the interface region consists of a "compact layer" of solvent next to the surface that is not described in classical electric double layer theories. The depth of the diffuse solvent layer is sensitive to the type of charge distributions on the surface and the ionic strength. Simulations of the aqueous interface of a realistic model of negatively charged amorphous silica show that the water orientation and the distribution of ions strongly depend on the identity of the cations (Na(+) vs Cs(+)) and are not well represented by a simplistic homogeneous charge distribution model. While the compact layer shows different solvent net orientation and depth for Na(+) vs Cs(+), the depth (~1 nm) of the diffuse layer of oriented waters is independent of the identity of the cation screening the charge. The details of interfacial water orientation revealed here go beyond the traditionally used double and triple layer models and provide a microscopic picture of the aqueous/mineral interface that complements recent surface specific experimental studies.
靠近界面的水分子性质与均匀体相液体中观察到的性质有显著偏差。这种结构扰动持续存在的长度尺度（即所谓的界面深度）是广泛研究的对象。在存在可诱导水分子长程取向有序排列的表面电荷时，情况尤其复杂，这反过来又决定了多种过程，如矿物溶解、多相催化和膜化学。为了表征界面水的基本性质，我们对存在两种理想化带电表面的碱金属氯化物溶液进行了分子动力学（MD）模拟：一种表面电荷密度集中在离散位点，另一种表面电荷密度均匀分布。我们发现，除了水取向无分层的扩散区域外，界面区域还包括紧邻表面的一层“致密层”溶剂，这在经典的双电层理论中并未描述。扩散溶剂层的深度对表面电荷分布类型和离子强度敏感。对带负电的无定形二氧化硅真实模型的水界面模拟表明，水的取向和离子分布强烈依赖于阳离子的种类（Na⁺ 与 Cs⁺），简单的均匀电荷分布模型无法很好地描述。虽然致密层对于 Na⁺ 和 Cs⁺ 显示出不同的溶剂净取向和深度，但取向水扩散层的深度（约 1 nm）与屏蔽电荷的阳离子种类无关。这里揭示的界面水取向细节超越了传统使用的双层和三层模型，提供了一个水/矿物界面的微观图景，补充了近期的表面特异性实验研究。
