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We present isotherms for the adsorption of n-butane on the external and internal surfaces of silicalite, in the temperature range of 320-400 K and at pressures up to 530 kPa. The isotherms were calculated using molecular dynamics simulations. A modified thermostatting algorithm, Soft-HEX, was developed for this purpose. The results for the external as well as the internal surface were fitted to Langmuir models, and equilibrium constants are reported. Two surface structures were considered: a flat surface having only one adsorption site and a surface with a zigzag texture, characterized by two adsorption sites. The surface excess concentrations were computed. Negative surface excesses were found in some cases, indicating that the surface can serve as a barrier to transport. We show that Henry's law can be used to calculate adsorbate concentration-gas pressure relationships, and that the surface can be regarded as a thermodynamic system. These findings are important for studies of the surfaces as barriers to transport.