State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology , Beijing 100029, China.
Physics of Fluids Group, Department of Science and Technology, Max Planck Center Twente for Complex Fluid Dynamics, Mesa+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente , P.O.Box 217, 7500 AE Enschede, The Netherlands.
Langmuir. 2017 Aug 15;33(32):8090-8096. doi: 10.1021/acs.langmuir.7b01231. Epub 2017 Aug 3.
The solvent exchange procedure has become the most-used protocol to produce surface nanobubbles, while the molecular mechanisms behind the solvent exchange are far from being fully understood. In this paper, we build a simple model and use molecular dynamics simulations to investigate the dynamic characteristics of solvent exchange for producing nanobubbles. We find that at the first stage of solvent exchange, there exists an interface between interchanging solvents of different gas solubility. This interface moves toward the substrate gradually as the exchange process proceeds. Our simulations reveal directed diffusion of gas molecules against the gas concentration gradient, driven by the solubility gradient of the liquid composition across the moving solvent-solvent interface. It is this directed diffusion that causes gas retention and produces a local gas oversaturation much higher near the substrate than far from it. At the second stage of solvent exchange, the high local gas oversaturation leads to bubble nucleation either on the solid surface or in the bulk solution, which is found to depend on the substrate hydrophobicity and the degree of local gas oversaturation. Our findings suggest that solvent exchange could be developed into a standard procedure to produce oversaturation and used to a variety of nucleation applications other than generating nanobubbles.
溶剂交换程序已成为产生表面纳米气泡最常用的方法，而溶剂交换背后的分子机制还远未被完全理解。在本文中，我们构建了一个简单的模型，并使用分子动力学模拟来研究产生纳米气泡的溶剂交换的动态特性。我们发现，在溶剂交换的第一阶段，存在着不同气体溶解度的溶剂之间的界面。随着交换过程的进行，这个界面逐渐向基底移动。我们的模拟揭示了气体分子在液体组成的溶解度梯度跨越移动的溶剂-溶剂界面时，沿着浓度梯度的定向扩散。正是这种定向扩散导致了气体的滞留，并在靠近基底的地方产生了比远离基底的地方高得多的局部气体过饱和度。在溶剂交换的第二阶段，局部气体过饱和度很高，导致气泡在固体表面或在体相溶液中形成，这取决于基底的疏水性和局部气体过饱和度的程度。我们的发现表明，溶剂交换可以被开发成一种产生过饱和度的标准程序，并应用于除了产生纳米气泡以外的各种成核应用。
