Infiltration of liquid water in an acid-leached zeolite
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Through acid-leaching treatment, the degree of hydrophobicity of a zeolite  can be controlled in a broad range. As the treatment time increases, the nanopore surface can change from hydrophilic to hydrophobic, leading to the formation of an infiltration plateau in sorption isotherm curve. The infiltration pressure and hysteresis are dependent on the testing temperature. As temperature rises, the effective degree of hydrophobicity is lowered, while the defiltration is much more pronounced.
I. INTRODUCTION
Controlling liquid motion in nanochannels, nanotubes, and nanopores is of important relevance to drug delivery,1 purification and mixture separation,2 and adsorption,3 as well as energy absorption and volume control.4,5 Over the years, a number of techniques were developed based on electrical,6 thermal,7 or mechanical8 methods. For instance, by maintaining a pressure difference, a liquid can penetrate through a nanoporous membrane9 or a carbon nanotube cluster.10 Recently, pressure-induced infiltration technique was investigated to actuate liquid flow in energetically unfavorable nanoenvironment.11,12 In a nanoporous material, if the nanopore wall is hydrophobic, a high quasi-static pressure needs to be applied on the liquid phase; otherwise, the liquid cannot overcome the capillary effect. As the pressure is lowered, the confined liquid may either be “locked” inside13,14 or defiltrate out of the nanopores.15,16 The former phenomenon can be used in advanced protective systems, such as car bumpers and blast-resistant containers, and the latter provides a promising mechanism for developing liquid springs or actuators. One of the most commonly applied nanoporous materials is zeolite.17 Zeolites or zeolitelike materials can be of various degrees of crystallinity. The pore size is smaller than 2 nm, since the framework channel is usually composed with 8 to 20 oxygen member rings. Very often, the porosity is around 0.2 cm3/g and the nanopores are interconnected. A dominant factor affecting its surface property is the surface defect density. The surface defects can be formed by silanol groups or Lewis acid sites. They are catalytically active and/or chemically reactive. If there are a relatively large number of defects,
the material is quite polar and thus is wettable to water. Under this condition, as the zeolite is immersed in water it can be soaked spontaneously. If the defect density is relatively small, the surface tends to be nonpolar, and hence the material can be hydrophobic. Note that as the zeolite is immersed in liquid water, its nominal degree of hydrophobicity is lower than in vapor. In a gas absorption analysis, when the silica-to-alumina (Si/Al) ratio is higher than 18, the material is usually regarded as hydrophobic; while in a pressure-induced infiltration experiment, the silica-to-alumina ratio must be much larger; otherwise no infiltration can be detected.18 Synthesizing low-defect-density zeolite is a nontrivial task.19 To decrease the density of surface defects, aluminum content in the framework nee
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