Hydration and dewetting near graphite-CH3 and graphite-COOH plates

COMP 181

Jingyuan Li, quengwan@yahoo.com.cn1, Ting Liu, l_aki@sohu.com2, X. Li, xinli@chem.columbia.edu3, Lei Ye, yelei@zju.edu.cn2, Huajun Chen, huajunsir@vip.sina.com2, Haiping Fang, fanghaiping@sinap.ac.cn4, Zhaohui Wu, wzh@zju.edu.cn2, and Ruhong Zhou, ruhongz@us.ibm.com5. (1) Department of Physics, Zhejiang University, 38 Zheda Road, Hangzhou, China, (2) Department of Computer Science, Zhejiang University, Hangzhou, China, (3) Department of Chemistry, Columbia University, New York, NY 10027, (4) Shanghai Institute of Applied Physics, Shanghai, China, (5) IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598
The dynamics of water near the nanoscale hydrophobic (graphite-CH3) and hydrophilic (graphite-COOH) plates has been studied in detail with molecular dynamics simulations in this paper. It is shown that these designed surfaces (by growing a layer of methyl or carboxyl groups on top of graphite) can have significant impact on the neighboring water dynamics, with the hydrophilic carboxyl surface having even more profound effects. The water hydrogen bond lifetime is much longer near both types of surfaces than that in the bulk, while on the other hand the water diffusion constant is much smaller than that in the bulk. The difference in the diffusion constant can be as large as a factor of eight and the difference in the hydrogen bond lifetime can be as large as a factor of two, depending on the distance from the surface. Furthermore, the water molecules in the first solvation shell of surface atoms show a strong bias in hydroxyl group orientation near the surface, confirming some of the previous findings. Finally, the possible water dewetting transition between two graphite-CH3 plates and the effect of the strength of the solute-solvent attractions on the water drying transition are investigated. The relationship among the dewetting transition critical distance, van der Waals potential well depth, and the water contact angle on graphite-CH3 surface is also analyzed based on a simple macroscopic theory, which can be used to predict the dewetting transition critical distance.