The Configurational-bias Monte Carlo (CBMC) technique in the grand-canonical (ìVT) ensemble has been used to calculate the pure and binary adsorption equilibrium isotherms of propane and propylene in zeolite 4A (zeolite Na-LTA) at 298.15, 373, 423 and 473 K in the 0-500 kPa pressure range. The influence of sodium cations on the adsorption properties of zeolite 4A was investigated by applying the united atom (UA) force field in order to establish effective Lennard-Jones (LJ) parameters for interactions between the cations and the sp2-hybridized bond of propylene as well as the other cation-carbon interactions present in this system. A cation-free Zeolite LTA framework and the inclusion of sodium cations into the Zeolite framework have been focused in this work. The quality of the parameters describing the carbon-sodium interactions was tested by comparing with published experimental results for the cation-free LTA and by using other published LJ interaction parameters. Surprisingly, it was found that the sodium force field which is good for other Na-contained zeolite frameworks, namely FAU, MOR and MFI, does not work well for Na-LTA system. The reason is the interaction with sodium, since without sodium ions the experimental data for cation-free LTA have been reproduced. The aluminum and silicon partial charges are explicitly distinguished and the new force field is in good agreement with recent experiments which show that the adsorbed amount of propane is much higher than those reported previously. Good agreement of simulations with experiments was also obtained for adsorption of propane/propylene mixtures in zeolite 4A.