Akaishi Akira, Yonemaru Tomohiro, Nakamura Jun
Department of Engineering Science, The University of Electro-Communications (UEC-Tokyo), 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan.
CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
ACS Omega. 2017 May 18;2(5):2184-2190. doi: 10.1021/acsomega.7b00365. eCollection 2017 May 31.
Although graphitic materials were thought to be hydrophobic, recent experimental results based on contact angle measurements show that the hydrophobicity of graphitic surfaces stems from airborne contamination of hydrocarbons. This leads us to question whether a pristine graphitic surface is indeed hydrophobic. To investigate the water wettability of graphitic surfaces, we use molecular dynamics simulations of water molecules on the surface of a single graphene layer at room temperature. The results indicate that a water droplet spreads over the entire surface and that a double-layer structure of water molecules forms on the surface, which means that wetting of graphitic surfaces is possible, but only by two layers of water molecules. No further water layers can cohere to the double-layer structure, but the formation of three-dimensional clusters of liquid water is confirmed. The surface of the double-layer structure acts as a hydrophobic surface. Such peculiar behavior of water molecules can be reasonably explained by the formation of hydrogen bonds: The hydrogen bonds of the interfacial water molecules form between the first two layers and also within each layer. This hydrogen-bond network is confined within the double layer, which means that no "dangling hydrogen bonds" appear on the surface of the double-layer structure. This formation of hydrogen bonds stabilizes the double-layer structure and makes its surface hydrophobic. Thus, the numerical simulations indicate that a graphene surface is perfectly wettable on the atomic scale and becomes hydrophobic once it is covered by this double layer of water molecules.
尽管石墨材料被认为是疏水的，但最近基于接触角测量的实验结果表明，石墨表面的疏水性源于空气中碳氢化合物的污染。这使我们质疑原始石墨表面是否真的是疏水的。为了研究石墨表面的水润湿性，我们在室温下对单层石墨烯表面的水分子进行了分子动力学模拟。结果表明，水滴会在整个表面铺展，并且表面会形成水分子的双层结构，这意味着石墨表面是可以被润湿的，但只能被两层水分子润湿。没有更多的水层能够附着在双层结构上，但液态水的三维团簇的形成得到了证实。双层结构的表面起到了疏水表面的作用。水分子的这种特殊行为可以通过氢键的形成得到合理的解释：界面水分子的氢键在前两层之间以及每层内部形成。这种氢键网络被限制在双层内，这意味着在双层结构的表面不会出现“悬空氢键”。这种氢键的形成稳定了双层结构并使其表面具有疏水性。因此，数值模拟表明，石墨烯表面在原子尺度上是完全可润湿的，一旦被这两层水分子覆盖就会变成疏水的。
