Polar surface are

The second property of importance for bioavailability is the polar surface area (PSA) that is associated with intestinal absorption and cell membrane penetration by passive transport. Compounds with a high polar surface are less likely to penetrate the lipophilic environment of the cell membranes by passive transport. Like the logP, PSA can be computed by summing up fragment contributions (8) with H-bonding fragments as the main contributor. 

According to classical electrostatics, polar surfaces are thus unstable. However, it can be shown [2,94] that the macroscopic dipole moment can be cancelled out by modifications of the charge densities on the outer layers. Assuming that m outer layers have a charge density aj different from the bulk ( cTj 7 (7 for 1 < j < m) and that the layer spacing is alternatively equal to Ri and R2, the electrostatic condition for surface stability reads ... 

Since charge compensation requires a modification of the charge density, changes of covalency at the surface are often assumed to heal the polarity. With the help of the bond transfer model, one can show that this statement is incorrect, as far as semi-infinite polar surfaces are concerned. It is useful to make a distinction between weakly polar surfaces, in which the dipole moment in the repeat unit is entirely due to covalent effects, and truely polar surfaces whose dipole moment contains an integer contribution. As already said, in the fully ionic limit, the first ones are considered as nonpolar, while the second ones are recognized as polar. 

Weakly polar surfaces are met each time that the dipole moment in... 

Obviously, NiO(l 11) is terminated and stabilised by p(2x2) reconstructions with different internal configurations. Importantly the electrostatic criterion is always fulfilled showing that it is very important when polar surfaces are considered. Interestingly, the octopolar reconstruction prediction and the observation of the p(2x2) surface cell were far not enough to understand the polar NiO(lll) surface. Note also that oxide thin films may not exhibit the same surface structure as their bulk counterpart. The studies carried out on thin oxide films may thus only be carefully extrapolated to the bulk surface. 

For metal particles on ionic substrates one can differentiate between interactions with non-polar (stoichiometric) and polar surfaces. The former surfaces are represented by essentially unrelaxed (001) facets of oxides with rock-salt structure, such as MgO(OOl) which is widely utilized as metal support and was addressed in the previous subsection. Oxide supports exhibiting polar surfaces are also common. Clean polar surfaces are unstable and thus difficult to prepare unreconstructed, dehydroxylated, and free of defects [88]. Corundum, a-Al203, is a prototype of such metal oxides. Its most stable Al-terminated (0001) surface... 

The work of adhesion (see Chapter 1,1) reflects the degree to which unsaturated molecular interactions between solids and liquids in contact are balanced. The value of cos 9, which is symbatic to the work of adhesion, is also a measure of the degree of similarity between the solid surface and a liquid (liophilicity). Polar surfaces that are wetted by water well are hydrophilic, while those poorly wetted (solid hydrocarbons, and particularly fluororinated polymers) are hydrophobic. Since the value of 0 is determined by both the work of adhesion and the work of cohesion, a comparison of the contact angles formed by different liquids at the same solid surface does not allow one to compare the works of adhesion (the degree of similarity in the nature of the liquid and solid) directly. For example, polar surfaces are equally wetted well by both water and hydrocarbons. 

ZnO surfaces are more complex than those of the rock-salt type oxides Uke MgO and NiO. ZnO crystalhzes in the wurtzite structure in which each Zn cation is tetrahedrally coordinated to four O anions and vice versa [105]. This crystal structure has no inversion center. The most important low-index surface planes are two polar planes, the Zn-terminated ZnO(OOOl) and 0-termi-nated ZnO(OOO-l) plane, and two neutral planes, ZnO(lO-lO) and ZnO(l 1-20). According to Nosker et al. [106] and Tasker [107], the two polar surfaces are thermodynamically unstable, however, they can be easily prepared and characterized experimentally, and do even show rather regular (1x1) LEED patterns [108]. This indicates that they are not stabilized by major reconstructions or other modifications. Therefore, it was believed for a long time that both polar surfaces exist in an unreconstructed bulk-Hke trimcation. Several contradicting proposals have been made to explain how the stability of the polar un-... 

FIGURE 17.10. The critical surface tension of wetting, ctc, is usually found to be consistent for a homologous series of hydrocarbon liquids (curve 1), a series of alkyl halides (curve 2), and miscellaneous polar liquids (curve 3) on a nonpolar surface such as Teflon. Results on polar surfaces are less unequivocal, but still can be useful. 

In LMO (110) surface calculations [851] the 02-terminated slab consisting of seven planes was taken, i.e. four O2 planes and three LaMnO planes. Such a symmetrical slab is nonstoichiometric, i.e. it does not consist of an integer number of formula units. To restore the stoichiometry of the 7-plane slab, one oxygen atom has been removed from both 02-planes terminating the slab, i.e. the slab of three bulk primitive unit cells with periodically repeated surface oxygen vacancies was used. Such an approach is justified since it is weU known that the polar surfaces are stabilized by surface defects and surface-atom relaxation. The smface energy Eg (per surface unit cell) for such a slab equals ... 

For solids with a similar surface chemistry, as for the activated carbons and CMS, without a developed surface functionality, the measurement of heats of immersion into liquids with different molecular sizes also allows for the assessment of PSD of the carbons. When polar surfaces are analyzed, both the surface accessibility of the immersion liquid, and the specific interactions between the solid surface and the molecules of the wetting agent account for the total value of the heat of immersion. The interpretation of the heat of immersion values for these latter systems is less straightforward, and care has to be taken with the selection of the liquids chosen for the measurements. 

Polar surfaces are type 3 surfaces, according to Tasker s (1979a) classification. They present distinctive features because of the presence of a net charge and a net dipole moment in the repeat unit, in a direction perpendicular to the surface. 

Many questions related to the electronic and atomic structures of oxide surfaces remain unanswered. For example, the respective roles of bond-breaking and structural distortions in the interpretation of surface densities of states and gaps are not well elucidated. Similarly, for complex crystal structures, it is difficult to relate the observations to models, since one does not know precisely the surface terminations. More systematic studies of stoichiometry in surface layers, and of the electronic and atomic features of polar surfaces are needed to better disentangle the origin of the deep surface states which may result either from dangling bonds or from structural and stoichiometric defects. Finally, the modification of correlation effects on oxide surfaces represents a completely virgin field for future investigations. 

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