Thermodynamic study of C2H2 thin films adsorbed on graphite (0001)

Thermodynamic properties of C2H2 thin films adsorbed on (0001) surfaces of an exfoliated graphite substrate have been investigated by means of volumetric adsorption isotherms technique. A complete isotherm of C2H2 adsorbed on graphite shows five steps with different height. Such steps represent a transition from a (2D) dilute phase to a (2D) dense one. For each step, a set of adsorption isotherms is measured. Clapeyron equations P:f(1/T) are then determined and thermodynamic quantities are calculated and compared to the literature available ones. The second step is divided into two sharp steps at a given range of temperature which means that the condensed matter undergoes two phase transitions. The nature of such phases is discussed in light of thermodynamic quantities. The molecules orientation in the suggested structures seems to be governed by quadrupole-quadrupole interaction. Recent neutron diffraction measurements support thermodynamic conclusions.