Statistical thermodynamics of straight rigid rods with nonadditive lateral interactions: Theory and Monte Carlo simulations

PINTO O. A; NIETO F. D.; A. J. RAMIREZ-PASTOR

PHYSICAL REVIEW E

AMER PHYSICAL SOC

Año: 2011 vol. 84 p. 1 - 11

1539-3755

The statistical thermodynamics of straight rigid rods of length k (k-mers) with nonadditive lateral interactionswas developed on a generalization in the spirit of the lattice-gas model and the classical Bragg-Williamsapproximation (BWA) and the quasichemical approximation (QCA). The new theoretical framework is obtainedby combining (i) the exact analytical expression for the partition function of noninteracting linear k-mers adsorbedin one dimension and its extension to higher dimensions, and (ii) a generalization of BWA and QCA in whichthe adsorbate can occupy more than one adsorption site. The traditional assumption of a strictly pairwise additivenearest-neighbors interaction is replaced by a more general one, namely that the bond linking a certain atomwith any of its neighbors depends considerably on how many of them are actually present (or absent) on thesites in the first coordination shell of the atom. The coverage and temperature dependence of the Helmholtz freeenergy, chemical potential, configurational entropy, and differential heat of adsorption are given. The formalism(i) reproduces the classical results for monomers, (ii) leads to the exact statistical thermodynamics of nonadditiveinteracting k-mers adsorbed in one dimension, and (iii) provides a close approximation for two-dimensional1 systems, taking into account multisite occupancy and nonadditive lateral interactions. Comparisons with MonteCarlo simulations are performed in order to test the validity of the theoretical model. Significant quantitativedifferences are shown and discussed. In all cases, the QCA appears to be the more accurate approach.