Ion repelling effect of nanopores in a hydrophobic zeolite
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Aijie Han Department of Chemistry, University of Texas—Pan American, Edinburg, Texas 78539
Hyuck Lim Program of Materials Science and Engineering, University of California—San Diego, La Jolla, California 92093
Yu Qiaoa) Department of Structural Engineering, University of California—San Diego, La Jolla, California 92093-0085 (Received 9 December 2010; accepted 22 February 2011)
By measuring the ion concentration in a pressure-induced infiltration experiment on a hydrophobic Zeolite Socony Mobil-5, it is found that the nanopore wall has a strong ion repelling effect. When the initial ion concentration is relatively low, only water molecules can enter the nanopores. Once the initial ion concentration is relatively high, ions can infiltrate into the nanopores, but the effective ion concentration of the confined liquid is much lower.
I. INTRODUCTION
Recently, behaviors of ions confined in nanoenvironment became an active research area. It is of both important technological relevance and prime scientific interest.1 At a large solid–liquid interface, as a result of surface polarization, solvated ions can form a doublelayered structure. Outside the interface layer, the ions are thermally disordered.2 However, once liquid molecules and ions are confined in a nanopore or a nanotube, their structures can be entirely different. A confined water phase may exhibit multiple layers, with the effective layer thickness, molecular density, and preferred molecular orientation strongly affected by the solid atoms from all directions.3 The vital factors include solid species, nanochannel size, temperature, and pressure. Many basic properties, such as density and viscosity, must be redefined. 4,5 The distribution of confined ions is a function of nanopore/nanotube radius.6 Moreover, because of the confinement effect of nanopore walls, the responses of ions to environmental factors, such as temperature and/or electric field, can be different.7 In molecular-sized pores below 1 nm, the confined liquid/ion behaviors are more complicated. Water molecules would directly interact with the solid wall, resulting in a “column resistance.”8 As the confined liquid moves, the effective viscosity is much smaller than its bulk counterpart and becomes size dependent.9 Computer simulation indicates that the sequence of infiltration of cations, anions, and water molecules in a nanopore may not be random.10 In a nanopore, the ions can no longer be fully solvated. They may form a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.73 1164
J. Mater. Res., Vol. 26, No. 9, May 14, 2011
http://journals.cambridge.org
Downloaded: 13 Mar 2015
crystalline-like ion couples, which considerably changes the system free energy. Although these investigations revealed many unique nanometer-scale phenomena, the study in this area is still at its early stage. Experimental data on a number of important issues are still unavailable, imposing tremendous challenges to developing counterparts of conventional surface and interface theories for nanoe
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