Perkin Susan, Goldberg Ronit, Chai Liraz, Kampf Nir, Klein Jacob
Department of Physical and Theoretical Chemistry, University of Oxford, Oxford, UK.
Faraday Discuss. 2009;141:399-413; discussion 443-65. doi: 10.1039/b805244a.
Prompted by the recent discovery that water and aqueous monovalent Na+ solutions remain fluid-like when confined to films of a few molecular layers between mica surfaces,[Raviv et al., Nature, 2001, 413, 51-54; and Raviv and Klein, Science, 2002, 297, 1540-1543] we now extend the previous study by comparing the shear- and normal-force properties of 0.1 M Na+, Cs+ and Ni2+ salt solutions which demonstrate a diverse range of behaviours under confinement. In the case of hydrated Na+ we extend the previous study to higher pressures, up to approximately 10 atmospheres, and record similar fluidity of the hydration layers at these elevated pressures. Aqueous Cs+ films under confinement between mica sheets have been found to be unable to support an applied load--that is to say they do not demonstrate any hydration repulsion regime--as a result of their lower hydration energy [see Goldberg et al., Phys. Chem. Chem. Phys., 2008, 10, 4939-4945] which contrasts with the properties of Na+. We show that 0.1 M Ni2+ solution remains close to its bulk viscosity down to nanometre thin films, but does not demonstrate a hydration repulsion. By comparing the properties of this range of cations, with differing valency and hydration, we aim to examine the conditions under which ions serve as effective lubricants and what we call the 'hydration lubrication' mechanism.
受近期一项发现的启发，即当水和单价钠离子水溶液被限制在云母表面之间的几个分子层的薄膜中时仍保持类似流体的状态[拉维夫等人，《自然》，2001年，413卷，51 - 54页；以及拉维夫和克莱因，《科学》，2002年，297卷，1540 - 1543页]，我们现在通过比较0.1 M的Na⁺、Cs⁺和Ni²⁺盐溶液的剪切力和法向力特性来扩展之前的研究，这些盐溶液在受限情况下表现出多种不同的行为。对于水合Na⁺的情况，我们将之前的研究扩展到更高的压力，高达约10个大气压，并记录了在这些升高的压力下水化层类似的流动性。已发现云母片之间受限的Cs⁺水溶液薄膜由于其较低的水合能[见戈德堡等人，《物理化学化学物理》，2008年，10卷，4939 - 4945页]而无法承受施加的负载——也就是说它们没有表现出任何水合排斥机制——这与Na⁺的特性形成对比。我们表明，0.1 M的Ni²⁺溶液在薄膜厚度达到纳米级时仍接近其本体粘度，但没有表现出水合排斥。通过比较这一系列具有不同化合价和水合作用的阳离子的特性，我们旨在研究离子作为有效润滑剂的条件以及我们所称的“水合润滑”机制。
