Origin of Interfacial Nanoscopic Gaseous Domains and Formation of Dense Gas Layer at Hydrophobic Solid鈥揥ater Interface
详细信息 查看全文
- 作者：Hong Peng ; Greg R. Birkett ; Anh V. Nguyen
- 刊名：Langmuir
- 出版年：2013
- 出版时间：December 10, 2013
- 年：2013
- 卷：29
- 期：49
- 页码：15266-15274
- 全文大小：482K
- 年卷期：v.29,no.49(December 10, 2013)
- ISSN：1520-5827
文摘
Interfacial gas enrichment (IGE) covering the entire area of hydrophobic solid鈥搘ater interface has recently been detected by atomic force microscopy (AFM) and hypothesized to be responsible for the unexpected stability and anomalous contact angle of gaseous nanobubbles and the significant change from DLVO to non-DLVO forces. In this paper, we provide further proof of the existence of IGE in the form of a dense gas layer (DGL) by molecular dynamic simulation. Nitrogen gas adsorption at the water鈥揼raphite interface is investigated using molecular dynamic simulation at 300 K and 1 atm normal pressure. The results show that a DGL with a density equivalent to a gas at pressure of 500 atm is formed and equilibrated with a normal pressure of 1 atm. By varying the number of gas molecules in the system, we observe several types of dense gas domains: aggregates, cylindrical caps, and DGLs. Spherical cap gas domains form during the simulation but are unstable and always revert to another type of gas domain. Furthermore, the calculated surface potential of the DGL鈥搘ater interface, 鈭?7.5 mV, is significantly closer to 0 than the surface potential, 鈭?5 mV, of normal gas bubble鈥搘ater interface. This result supports our previously stated hypothesis that the change in surface potential causes the switch from repulsion to attraction for an AFM tip when the graphite surface is covered by an IGE layer. The change in surface potential comes from the structure change of water molecules at the DGL鈥搘ater interface as compared with the normal gas鈥搘ater interface. In addition, the contact angle of the cylindrical cap high density nitrogen gas domains is 141掳. This contact angle is far greater than 85掳 observed for water on graphite at ambient conditions and much closer to the 150掳 contact angle observed for nanobubbles in experiments.