  In the present work we introduce a generalized lattice-gas model to study the
multilayer adsorption of interacting polyatomics on heterogeneous surfaces.
Using an approximation in the spirit of the well-known Brunauer--Emmet--Teller
(BET) model, a new theoretical isotherm is obtained in one- and two-dimensional
lattices and compared with Monte Carlo simulation. In addition, we use the BET
approach to analyze these isotherms and to estimate the monolayer volume. In
all cases, we found that the use of the BET equation leads to an underestimate
of the true monolayer capacity. However, significant compensation effects were
observed for heterogeneous surfaces and attractive lateral interactions.
