Physics of Complex Fluids Group and MESA+ Institute, Faculty of Science and Technology , University of Twente , P.O. Box 217, 7500 AE Enschede , The Netherlands.
Department of Theoretical Biophysics , Max Planck Institute of Biophysics , Max-von-Laue-Straße 3 , 60438 Frankfurt (Main) , Germany.
Langmuir. 2019 Apr 30;35(17):5737-5745. doi: 10.1021/acs.langmuir.9b00520. Epub 2019 Apr 22.
Hydration forces play a crucial role in a wide range of phenomena in physics, chemistry, and biology. Here, we study the hydration of mica surfaces in contact with various alkali chloride solutions over a wide range of concentrations and pH values. Using atomic force microscopy and molecular dynamics simulations, we demonstrate that hydration forces consist of a superposition of a monotonically decaying and an oscillatory part, each with a unique dependence on the specific type of cation. The monotonic hydration force gradually decreases in strength with decreasing bulk hydration energy, leading to a transition from an overall repulsive (Li, Na) to an attractive (Rb, Cs) force. The oscillatory part, in contrast, displays a binary character, being hardly affected by the presence of strongly hydrated cations (Li, Na), but it becomes completely suppressed in the presence of weakly hydrated cations (Rb, Cs), in agreement with a less pronounced water structure in simulations. For both aspects, K plays an intermediate role, and decreasing pH follows the trend of increasing Rb and Cs concentrations.
水合作用力在物理、化学和生物学的广泛现象中起着至关重要的作用。在这里，我们研究了云母表面在与各种碱金属氯化物溶液接触时的水合作用，这些溶液的浓度和 pH 值范围很广。使用原子力显微镜和分子动力学模拟，我们证明水合作用力由单调衰减和振荡两部分组成，每一部分都与阳离子的特定类型有独特的关系。单调衰减的水合作用力随着体相水合能的降低而逐渐减弱，导致从整体排斥（Li、Na）到吸引（Rb、Cs）的转变。相比之下，振荡部分表现出二元特性，几乎不受强水合阳离子（Li、Na）的影响，但在弱水合阳离子（Rb、Cs）存在时完全被抑制，这与模拟中不太明显的水结构一致。对于这两个方面，K 都起到了中间作用，而降低 pH 值的趋势与增加 Rb 和 Cs 浓度一致。
