Phase behavior of attractive k-mers on two-dimensional lattices: a study from quasi-chemical approximation

Abstract

The phase behavior of attractive linear rigid k-mers on two-dimensional lattices was studied by theoretical and simulation calculations in the framework of the lattice-gas model. Combining (i) the analytical expression for the partition function of non-interacting k-mers, and (ii) a generalization of the classical quasi-chemical approximation (QCA) in which the adsorbate can occupy more than one adsorption site, the main thermodynamic functions of the system were explicitly obtained. It was found that, for temperatures below a certain condensation temperature, the system undergoes a first-order phase transition which is observed as a clear discontinuity in the adsorption isotherms. The transition was studied in detail by calculating the temperature–density phase diagrams. Comparisons with analytical data from Bragg–Williams approximation and Monte Carlo simulations were performed in order to test the validity of the theoretical model. The results obtained allowed us not only to analyze the effect of introducing the lateral interactions by following the configuration-counting procedure of the QCA, but also to discuss the consequences of choosing the configurational factor associated to adsorption of non-interacting k-mers from different models developed to treat the multisite occupancy adsorption problem.