Thermodynamic study of ethane thin films adsorbed on MgO(100) powder

Abstract

Stepped adsorption isotherms of C 2H 6 adsorbed on a MgO(100) surface have been measured in the temperature range 102–154 K. A layer by layer growth mode is observed at least up to a 5 layer thick film. Presumably ethane wets the MgO(100) surface of square symmetry. We have deduced from our adsorption isotherm network the thermodynamic properties of the first and the second ethane layers. The adsorption heat Q 1 adsof the C 2H 6 condensed 2D phasee on the MgO(100) surface is significantly lower (=20%) than that measured on the hexagonal graphite basal plane surface, on the other hand the 2D critical temperatures T (1) 2c of the C 2H 6 monolayers adsorbed on both substrates are quite similar, T (1) 2c = 133 ± 3K. Such results indicate that the surface electric field influence of the ionic (100) MgO surface is weak. Preliminary neutron diffraction experiments point out that in the monolayer range the ethane film absorbed on Mg(100) is characterized by a short range order. This result is in agreement with the large differential entropy of the C 2H 6 2D film, which is likely in a 2D liquid phase.