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Origin of Interfacial Nanoscopic Gaseous Domains and Formation of Dense Gas Layer at Hydrophobic Solid–Water Interface

机译:疏水固水界面在界面纳米镜气体结构域的起源和致密气体层的形成

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摘要

Interfacial gas enrichment (IGE) covering the entire area of hydrophobic solid-water 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-graphite 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-water interface, -17.5 mV, is significantly closer to 0 than the surface potential, -65 mV, of normal gas bubble-water 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-water interface as compared with the normal gas-water interface. In addition, the contact angle of the cylindrical cap high density nitrogen gas domains is 141. This contact angle is far greater than 85 observed for water on graphite at ambient conditions and much closer to the 150 contact angle observed for nanobubbles in experiments.
机译:最近已经通过原子力显微镜(AFM)检测到覆盖疏水性固体-水界面整个区域的界面气体富集(IGE),并推测是气体纳米气泡的意外稳定性和反常接触角以及从DLVO至非DLVO部队。在本文中,我们通过分子动力学模拟进一步证明了IGE以致密气体层(DGL)的形式存在。使用分子动力学模拟在300 K和1 atm常压下研究了水-石墨界面上的氮气吸附。结果表明形成了密度等于500atm的气体的DGL,并在1atm的常压下达到平衡。通过改变系统中气体分子的数量,我们观察到几种类型的致密气体域:聚集体,圆柱帽和DGL。在模拟过程中会形成球形帽盖气域,但它们不稳定,并且总是恢复为另一种类型的气域。此外,DGL-水界面的计算表面电势-17.5 mV比正常气泡-水界面的表面电势-65 mV显着接近0。该结果支持了我们先前陈述的假设,即当石墨表面被IGE层覆盖时,表面电势的变化会导致AFM尖端从排斥力变为吸引力。与常规的气-水界面相比,表面电势的变化来自DGL-水界面处水分子的结构变化。另外,圆柱形帽的高密度氮气域的接触角为141。该接触角远大于在环境条件下在石墨上观察到的水与85的接触角,并且与实验中对纳米气泡观察到的150接触角非常接近。

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