We present new experimental data and molecular simulation results for the adsorption of nitrogen in MCM-41. We show that models in which the solid-fluid interaction potential is homogeneous cannot reproduce the form of the adsorption isotherm, particularly at low pressures. A new heterogeneous interaction model is shown to give very good agreement with the data. Simulations based on this new model for pores of various diameters are used to obtain a relation between capillary condensation pressure and pore diameter. This relation can be used to estimate pore size distributions from experimental adsorption data.