Francisco Hung M.S., Chemical Engineering Universidad Simón Bolívar, Venezuela (1999) B.S., Chemical Engineering Universidad Simón Bolívar, Venezuela (1996)

Project Title:

Freezing and Melting Behavior within Porous Materials

Abstract:

The goal of this project is to use molecular simulation and experimental techniques to understand freezing and melting phenomena for phases confined within porous materials such as carbon nanotubes, silica glasses, templated mesoporous materials (MCM-41, SBA-15) and activated carbons. Confinement within a porous material is known to lead to a number of novel phenomena on the phase behavior. The freezing temperature can increase or decrease relative to the bulk value, and new phases may appear depending on factors such as the size and shape of the pores and the interactions between the confined fluid and the porous material. In addition to the fundamental scientific interest, such phenomena can have practical applications in the fabrication of nanostructured materials, lubrication, nanotribology, weathering of man-made materials, frost heaving and material science in general. Freezing in porous media has also been widely employed in the characterization of porous materials using the method of thermoporometry. Experimental research in this area is complicated by the lack of well-characterized porous materials with appropriate pore sizes, ambiguities in attempting to determine the nature of the confined phase and the prevalence of long-lived metastable states. Molecular simulation studies do not suffer from any of these difficulties. By using advanced sampling techniques, it is possible to overcome metastability and estimate the free energies of the confined phases, and thus identify the thermodynamic equilibrium states and the order of the phase transitions. Nevertheless, simulations face other difficulties, such as uncertainties in intermolecular potentials and pore characterization, as well as limitations due to the speed of current supercomputers. The difficulties found in experiments and simulations make the two approaches complementary, so that combined experimental-simulation studies can lead to a molecular-level understanding of the problem. This research involves a collaboration with the group of Dr. Malgorzata Sliwinska-Bartkowiak, from Adam Mickiewicz University in Poznan, Poland.

The first system we have investigated is the freezing and melting of carbon tetrachloride within carbon nanotubes. Parallel tempering Monte Carlo simulations in the grand canonical ensemble were performed for such a system, and snapshots of typical configurations of the adsorbed phase at 290 K and 192 K are presented in Figure 1. Front views of the same snapshots depicting only the centers of mass of the adsorbed CCl4 molecules are also provided, to help visualize the formation of concentric layers as the temperature is reduced. The pore walls are not shown for clarity. Our results indicate that the outer layers of adsorbate solidify at temperatures slightly higher than the bulk freezing point, whereas the inner layers freeze at lower temperatures. The simulation results are in good agreement with dielectric relaxation spectroscopy measurements.

We have recently begun an investigation of freezing of simple fluids confined within silica pores and templated mesoporous materials. Figure 2 shows front and lateral views from snapshots of CCl4 confined within a model MCM-41 pore (from data provided by Dr. Flor Siperstein).

Publications:

• F. R. Hung, G. Dudziak, M. Sliwinska-Bartkowiak and K. E. Gubbins, “Freezing/melting behavior within carbon nanotubes”, Mol. Phys. (submitted, 2003)
• E. A. Müller, F. R. Hung and K. E. Gubbins, “Adsorption of water vapor-methane mixtures on activated carbons”, Langmuir 16, 5418-5424 (2000)

Resume (pdf)

North Carolina State University
Department of Chemical Engineering
113 Riddick Labs
Raleigh, NC 27695

Phone: (919) 513-2051
Fax: (919) 513-2470
Send e-mail to: frhung@unity.ncsu.edu

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