A Fluorescence Probe Investigation of the Effect of Alkali Metal Ions on the Micellar Properties of a Crown Ether Surfactant

essentially constant over the complete sorption range. A&, changes very little once most of the pore space is filled, as the environment of the sorbed molecules remains nearly constant. Consequently, AC, is almost invarient from about 5 to 6.5 molecules sorbed per unit cell (Figure 4). This could clearly lead to sorption hysteresis being observed in isotherm experiment^.^ The reason this hysteresis is only found with low A1 content materials is linked with the fact that the number of strong sorption sites is determined by the number of A1 atoms present. The effect has little to do with anything unusual about p-xylenelp-xylene interactions in the sorbed state, which are weak, and nothing to do with the rate of transport of p-xylene within the crystal environment becoming slow at a critical sorbed molecular concentration, as the kinetic experiment described in the Results section shows. ACm for benzene sorption on silicalite also changes rather slowly as saturation is approached, and this too is manifest in a somewhat unusual isotherm shape, although no hysteresis has been reported. However, the AHm and G,,, curves show that the effect arises in a quite different way, as both these functions change appreciably with amount sorbed. The molecular reasons for this have been discussed earlier in the paper. This work therefore helps to confirm the view that the properties of ZSM-5 zeolites do vary in a systematic and regular way with aluminum ~ o n t e n t , ~ and that the apparently very strange behavior observed in the sorption isotherms does not have an obscure origin.