Cluster approximation applied to multisite-occupancy adsorption: configurational entropy of the adsorbed phase for dimers and trimers on triangular lattices

DE LA CRUZ FELIZ, NORIS M.; LONGONE, PABLO JESÚS; SANCHEZ VARRETTI, FABRICIO ORLANDO; BULNES, FERNANDO; RAMIREZ-PASTOR, ANTONIO

PHYSICAL CHEMISTRY CHEMICAL PHYSICS

ROYAL SOC CHEMISTRY

Año: 2023 vol. 25 p. 14942 - 14954

1463-9076

The adsorption of dimers and trimers on triangular lattices is studied by combining theoreticalmodeling and Monte Carlo (MC) simulations. The thermodynamic process is analyzed through thebehavior of the configurational entropy per site of the adsorbed phase as a function of the coverage.MC calculations, supplemented by thermodynamic integration method, are performed in the grandcanonical ensemble. The theoretical model used in the present study is called Cluster Approximation(CA), and it is based on exact calculation of states on finite cells. An efficient algorithm allows usto determine the detailed structure of the configuration space for m = l1 ×l2 cells. From there, thethermodynamic properties can be obtained. Five systems are investigated, according to the size andshape of the molecule in the adsorbed state: (i) dimers, (ii) linear trimers, (iii) triangular trimers,(iv) 60◦-angular trimers and (v) 120◦-angular trimers on triangular lattices. Dimer and trimer arethe simplest cases of a polyatomic adsorbate, contain all the properties of the multisite-occupancyadsorption and can be used to model several experimental systems. CA solutions are tested bycomparison with MC simulations and previous data in the literature. A special interest is devotedto the calculation of the configurational entropy per site in the limit case of θ →1 (full coverage),where some exact results are available. The theoretical formalism is also applied to model CH4 andCO2 clathrate hydrates. In these systems, a triangular lattice is used to simulate the substrate,and methane(carbon dioxide) molecules can be well represented by triangular(linear) trimers. Thegood qualitative agreement between simulation and analytical data supports the validity of the CAscheme to predict the behavior of a wide variety of multisite-adsorption models, for which theoreticalsolutions are very difficult to obtain.