A thermodynamic study of layering and capillary condensation of methane adsorbed on graphite foam
Multilayer films of methane adsorbed on graphite foam have been studied using heat capacity and vapor pressure measurements. An automated, high resolution differential calorimeter was designed and constructed that yielded far better data in much less time. It was found that capillary condensate in t...
| Summary: | Multilayer films of methane adsorbed on graphite foam have been studied using heat capacity and vapor pressure measurements. An automated, high resolution differential calorimeter was designed and constructed that yielded far better data in much less time. It was found that capillary condensate in the pores of the graphite foam coexists with uniform films as thin as 1.1 layers. Heat capacity features near the triple point previously thought to be the melting of the uniform film are identified as the melting of the capillary condensate. The latent heat of melting of the capillary condensate was measured to be as small as half the bulk value when it was confined inside the smallest pores. The melting temperature of the capillary condensate confirms a simple model for the melting of bulk matter in cylindrical pores. This model explains why the melting temperature is the same for systems with the same chemical potential but on different branches of the hysteresis curve. The model indicates that the capillary condensate may undergo surface melting if the solid does not wet graphite and the condensate undergoes substrate freezing. The phase diagram of the layer closest to the substrate is altered slightly by the finding that this layer melts from the commensurate phase when the uniform film is thicker than 1.1 layers. Heat capacity signals from phase transitions in the uniform films map out complicated phase diagrams in the second, third and fourth layers, including a 2-D triple point and liquid vapor coexistence region for each layer. The layering critical temperatures indicate that the bulk solid-vapor interface may roughen at about 81 K. |
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