N 2 Monolayers on graphite: Specific heat and vapor pressure measurements — thermodynamics of size effects and steric factors
Abstract
Calorimetric and vapor pressure measurements of N 2 monolayers on Grafoil have been made in the regions recently explored by neutron scattering. The phase transition at T = 48 K involves approximately symmetric specific heat anomalies of 2∼ K FWHM, and the peak position shifts about 1 K from {1}/{3} to {2}/{3} of one monolayer. In spite of the peak width and temperature shift the transition is intrinsically first order, due to melting of the √3 registered structure into a disordered phase in the presence of 2D vapor. The shift and broadening are shown to be a consequence of size effects primarily due to large edge-to-area ratios of the ordered phase domains. A formula relating the domain size to the shift in transition temperature gives r = 50' A for the mean radius of the domains. At film densities above {2}/{3} monolayer the peak broadens and shifts to a higher temperature until a critical region is reached near T= 85 K. The combined heat capacity and vapor pressure data indicate that there is no two phase region at coverages near monolayer completion and that the transition near T = 85 K is higher than first order. Effects of molecular orientation and surface-normal vibration appear in certain regions of temperature and density. A phase diagram is constructed which is consistent with both calorimetric and neutron scattering results.
- Publication:
-
Surface Science
- Pub Date:
- September 1977
- DOI:
- 10.1016/0039-6028(77)90038-3
- Bibcode:
- 1977SurSc..66..559C