Surface pressure, marker permeability

Dependence of Marker Permeability on Surface Pressure The dependence of the cyclic voltammogram area on the surface pressure applied to the cyclodextrin monolayer was examined. By increasing the surfece pressure from 10 to 50 mN m or, in other words, by decreasing the molecular area from 260 to 210 molecule the voltammogram areas for all three markers decreased almost linearly. However, an important point to be emphasized here is that the magnitudes of the decrease were different the decrease for p-quinone was much smaller than that for [Co(phen)3]2+ and [Mo(CN)g]. This is also consistent with the capability of p-quinone and incapability of the other two bulky markers to sterically pass tiirough the P-cyclodextrin cavity. 

Another point to be noted is the fact that a dramatic increase in the voltammogram area upon a decrease in the surface pressure was observed for both of the two bulky markers, regardless of whether the marker is hydrophilic ([Mo(CN)g] ) or hydrophobic ([Co(phen)3]2+). These results indicate that the main factor controlling the permeability through e intramolecular channel is the steric bulkiness rather than the hydrophobicity of the marker. Such an aspect seems to be characteristic of the cyclodextrin monolayer and contrasts to the properties of monolayers of simple alkane derivatives, in which the permeability (through intermolecular voids) is controlled mainly by the hydrophobicity and not the steric bulkiness of the marker (30). 

Figure 5. Position of subsurface numerical markers during an atmospheric pressure cycle. Simulation is of a 60-m unsaturated zone with air permeability of 10-" m2. Note that soil gas initially deeper than 1,75 m never reaches the surface while soil gas at shallower depths would be flushed from the system. Reprinted from Auer et al. (1996), Copyright 1996, pg. 152, with permission from Elsevier Science.

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