X-ray and neutron scattering studies of methyl iodide and hydrogen confined in porous media
Abstract (Summary)iii X-ray diffraction was used to study the liquid - solid phase transition and structure of methyl iodide confined in porous GelTech© glass, with 25, 50 75 and 200 ? pore diameters and 70 ? Vycor. Confinement of methyl iodide in the 200 ? pores exhibits two phase transitions. The first transition takes place at 203 K, and is a liquid - amorphous solid transition (glass). This transition has never been observed for bulk methyl iodide or any of the studies in confinement. The scattering pattern of the glass is identical to that of the liquid pattern with one exception. At high momentum transfer there is a drop in scattering amplitude correlating to the Debye-Waller factor. The system therefore has short range order like a liquid, but the atoms move on a time scale that is experimentally immeasurable. The second transition appears at 168 K, well below the bulk freezing temperature. This transition is from the thermodynamically unstable glass state to a bulk orthorhombic crystal structure, identical to the bulk crystal structure, with an underlying amorphous layer. Neutron scattering studies in the 200 ? pores are in agreement with the x-ray measurements. The liquid is supercooled by 6 K when the liquid-solid transition takes place. A diffusion constant of 4.2 x 10-9 ± 8.7 x 10-9 cm2/s and an activation energy of 61 K were extracted from the fit to the diffusion data, though these values are uncertain because the data fit was to three closely-spaced points. The quasi-elastic neutron scattering studies (QENS) of methyl iodide within Vycor showed a depression of the freezing transition temperature of at least 17 K. The diffusion in Vycor is an order of magnitude slower than in the 200 ? pores, but the iv activation energy is much smaller than would be expected, 10.1 ± 1.1 K. The fit to these data were also over a small temperature range and must be considered unreliable. Quasi-elastic neutron scattering measurements of the diffusion of H2 adsorbed in porous MCM-48, with an ordered pore network were used to monitor the liquid - solid transitions of the adsorbed fluid. Unlike other porous media of similar pore size, where supercooling of the liquid-solid transition by 3 - 4 K was observed, no depression of the transition was observed for MCM-48. Furthermore, no hysteresis between freezing and melting is observed, as in other porous media. The diffusion constant for the confined liquid has been extracted from the measurements, and the hydrogen adsorbed in the MCM-48 has a slightly higher diffusion constant than bulk liquid H2.
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication: