  The statistical thermodynamics of straight rigid rods of length $k$ on
triangular lattices was developed on a generalization in the spirit of the
lattice-gas model and the classical Guggenheim-DiMarzio approximation. In this
scheme, the Helmholtz free energy and its derivatives were written in terms of
the order parameter $\delta$, which characterizes the nematic phase occurring
in the system at intermediate densities. Then, using the principle of minimum
free energy with $\delta$ as a parameter, the main adsorption properties were
calculated. Comparisons with Monte Carlo simulations and experimental data were
performed in order to evaluate the reaches and limitations of the theoretical
model.
