Effects of surface charging treatment on outer and inner surfaces of a nanoporous carbon
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Aijie Han Department of Chemistry, University of Texas–Pan America, Edinburg, Texas 78539
Taewan Kim and Hyuck Lim Program of Materials Science & 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; and Program of Materials Science & Engineering, University of California–San Diego, La Jolla, California 92093 (Received 3 February 2009; accepted 24 March 2009)
As the outer surface of a nanoporous carbon is treated with 16-mercaptohexadecanoic acid, the carbon particles can form a stable suspension in water. When the water phase is compressed, the liquid infiltration behavior in the nanopores becomes significantly different from that of untreated material, suggesting that the inner surface is also modified. After the treatment, the infiltration pressure does not decrease. Therefore, the chain configurations at the inner and outer surfaces must be different, which explains the variations in infiltration pressure and volume.
I. INTRODUCTION
In the past, nanoporous materials were mainly used in the biological and chemical fields for catalysis, absorption and adsorption, separation and purification, and so on.1–3 One of their most attractive properties is the ultra-large surface area, typically at the level of 102–103 m2/g,4,5 which can greatly amplify beneficial surface chemical reactions (e.g., for catalysis) and/or surface physical processes (e.g., for absorption). Recently, the application of nanoporous materials was extended to mechanical systems, such as nanoporous energy absorption systems (NEAS).6–8 A NEAS contains two phases: a nanoporous phase, which is often in the form of micrometer (mm) sized particles, and a liquid phase in which the nanoporous particles are immersed. The properties of the inner surfaces of nanopores must be appropriately controlled so that the effective solidliquid interfacial tension, gsl, is larger than gs + gl, where gs and gl are the effective surface tensions of the nanopore wall and the liquid, respectively; that is, work needs to be done to expose the nanopore surface to the liquid. Thus, at rest the repelling effect of the nanopore surfaces would keep the liquid phase out of the nanopores. As a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0296 J. Mater. Res., Vol. 24, No. 8, Aug 2009
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external pressure is applied, the liquid can be compressed into the nanopores, accompanied by a large increase in system free energy, the amount of which is proportional to the total surface area. Such a system is effectively a compressible liquid, which is relevant to advanced damping and protection devices, such as vehicle bumpers, liquid armors, and damping stages, among others.9,10 Among the large number of nanoporous materials that have been investigated for NEAS, nanoporous carbons have received wide attention.11 Unlike many other nanoporous materials, such
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