The adsorption isotherm of argon on the zeolite MFI at liquid nitrogen temperature exhibits a sub-step at high loading before saturation that, in spite of much theoretical and experimental effort, is still lacking a definitive microscopic interpretation. In this work, we try to get insight into this peculiar behaviour by investigating the adsorption of argon on MEL, a zeolite that is structurally very similar to the MFI. First, we performed volumetric experiments that confirm that the adsorption of argon on MEL presents the same qualitative behaviour as on the MFI, again a sub-step appearing at high loading before saturation. Subsequently, the microscopic origin of this behaviour was investigated by means of molecular simulation. The simulations indicate that, for loads lower than that of the experimental sub-step, argon atoms can accommodate at low energy positions within the zeolite pores, whereas, above this point, some reordering of the adsorbate is needed to host further argon atoms. Moreover, the flexibility of the zeolite can have a significant impact on the shape of the adsorption isotherm, although the magnitude of this change depends on the zeolite model potential.

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