Recent Research

Study of hydrophobic/water interfaces and ordering of gases confined in a small space

Post Date:2015-01-07

The interfaces between water and hydrophobic solids are under highly debate among researchers. There have been numerous reports that gases dissolved in water can accumulate at this type of interfaces. However, many questions, such as why gases segregate to the interfaces, the nature of the interfacial gas structures, and the high stability of the gas structures, remain unanswered. With advanced AFM techniques, the research group led by Dr. Ing-Shouh Hwang resolved various structures of oxygen and nitrogen at an interface between water and a hydrophobic solid (graphite). These gas structures can be in two-dimensional ordered (solid gas), two-dimensional disordered, or three-dimensional liquid-like states under ambient conditions, which contradicts the conventional concept based on the phase diagrams of bulk gases. This observation is consistent with previous observations of solid or liquid nanoprecipitates of inert gases in solids at room temperature. These results clearly show that the ordering of gases confined in a small space cannot be understood with the phase diagrams of bulk gases. A new concept, interface-induced ordering, is proposed to explain this special thermodynamic phenomenon. This concept provides a new direction in developing techniques for high-density gas storage. The results were published in a Nature-series journal, Scientific Reports.

The group also proposes that gas molecules dissolved in water tend to adsorb at hydrophobic solid surfaces, because this type of surfaces provide low-chemical-potential sites. Thus hydrophobic surfaces can effectively catch gases dissolved in water, which may also have implications regarding the breathing of aquatic lives and many phenomena in water. Since there are only weak van der Waals interaction among gas molecules, the liquid-like interfacial gas structures can act as a low-viscosity liquid to explain the boundary slip for water flow over hydrophobic solid surfaces. This long-standing puzzle in fluid dynamic may simply result from the special properties of the interfacial gas structures.


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