Since the channels of this zeolite are somewhat wider, the PCE molecules are displaced from the center of the zigzag channels, mostly interacting with one side of the pore wall. This lessthan-optimal fit explains why the heat of adsorption in IFR at low coverages is much lower than in the zeolites discussed so far. The strong guest-guest interactions that are responsible for the large increase of qst(PCE) with loading can also be understood on the basis of the MOR channels. In contrast, adsorbed PCE molecules are never found in such a perpendicular orientation, but they always lie more or less in the plane of the elliptical 12MR channels (right panel of Figure ). Apparently, PCE, although being only slightly larger than TCE, is too bulky to assume an orientation perpendicular to the channel axis, resulting in a less efficient packing of the adsorbed molecules. For this reason, the attainable PCE loading is significantly smaller (0.9 mmol g -1 , compared to 1.2 mmol g -1 for TCE), and contribution of attractive guestguest interactions is also reduced considerably.