On the transition from two- to three-dimensional behavior in adsorbed films

Citation

Day, Peter Kenneth (1993) On the transition from two- to three-dimensional behavior in adsorbed films. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/qhp1-et19. https://resolver.caltech.edu/CaltechETD:etd-11152002-104611

Abstract

Argon and krypton films adsorbed on graphite foam have been studied in detail using vapor pressure and high resolution, heat capacity measurements. Heat capacity features near the bulk triple point temperature, previously associated with the surface melting of the uniform film, are shown to be due to the melting of bulk material condensed in pores in the substrate. The melting curve of the capillary condensate agrees with the prediction of a modified Clausius-Claperon equation. The second and third layers in argon and the second layer in krypton have a triple point at which two-dimensional solid, liquid, and gas phases coexist atop a solid lower layer. Commensurate-incommensurate transitions are found in the first two layers of argon and in the second layer of krypton, so that monolayer argon melts from a registered phase, but the second layers of both systems melt from incommensurate bilayer phases. The melting of the second and third layers in both systems are likely to be first order, but the data are not conclusive. At coverages starting with 3 1/2 layers, heat capacity features that are due to reentrant layering-transitions are seen in both systems, confirming the result of recent ellipsometry studies. Further heat capacity peaks suggest phase transitions that join the newly observed reentrant layering-transitions with the well studied layering-transitions at low temperature. These heat capacity peaks may be related to the recently proposed preroughening transition. A mean field theory is developed that reproduced the reentrant layering behavior for ratios of nearest and next nearest neighbor interaction energies greater than a critical value. The mean field theory gives an explanation for the appearance of reentrant layering-transitions at different film thicknesses depending on the substrate-adsorbate interaction parameter. Multilayer phase diagrams are drawn from the data that suggest a crossover from two-dimensional behavior in the second layer to bulk interfacial behavior in the higher layers. In an appendix, layering and melting transitions in CF4 on graphite are analysed.

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