Adsorption thermodynamics of interacting particles on diffusion-limited aggregates

Grand canonical Monte Carlo simulations have been performed in order to study adsorption thermodynamics of pairwise interacting particles on fractal surfaces. Diffusion-limited aggregates (DLA) have been used as a substrate where interacting particles are adsorbed. In order to obtain aggregates with different morphologies, DLA clusters are generated on different strongly correlated surfaces. Adsorption isotherm, adsorption energy and differential heat of adsorption were calculated for attractive and repulsive nearest-neighbor (NN) lateral interactions. For the case of repulsive couplings and low temperatures, four novel ordered phases has been found in the adsorbate, each one corresponding to the formation of a chessboard-like structure on sites with one, two, three and four NN sites, respectively. The values of coverage at which these ordered phases emerge are not symmetrical around θ=0.5. This is a consequence of the non-equivalence between vacancy and particle in the case of adsorption on fractal structures. The influence of ordered structures on thermodynamic quantities associated to the adsorbed monolayer has been analyzed and discussed in the context of the Lattice-Gas model.