Layer neighbouring

Molecular adsorbates usually cover a substrate with a single layer, after which the surface becomes passive with respect to fiirther adsorption. The actual saturation coverage varies from system to system, and is often detenumed by the strength of the repulsive interactions between neighbouring adsorbates. Some molecules will remain intact upon adsorption, while others will adsorb dissociatively. This is often a frinction of the surface temperature and composition. There are also often multiple adsorption states, in which the stronger, more tightly bound states fill first, and the more weakly bound states fill last. The factors that control adsorbate behaviour depend on the complex interactions between adsorbates and the substrate, and between the adsorbates themselves. 

Figure Bl.23.11. Above selected time-resolved SARIS images of 4 keV Ar scadering from Pt l 11 ] along (I 12). Below view of Pt 111 ] surface along (112) showing Ar scadering from a first-lay er Pt atom (1) and spliding into two focused beams by an atomic lens fonned by neighbouring first-layer Pt atoms (2, 3, 4).

Figure Bl.23.14. Schematic illustration of the Pt 111 ] -(1 x 1) surface. Arrows are drawn to indicate the nearest-neighbour first-first-, second-first-, and third-first-layer interatomic vectors.

Black phosphorus is formed when white phosphorus is heated under very high pressure (12 000 atmospheres). Black phosphorus has a well-established corrugated sheet structure with each phos phorus atom bonded to three neighbours. The bonding lorces between layers are weak and give rise to flaky crystals which conduct electricity, properties similar to those ol graphite, it is less reactive than either white or red phosphorus. 

The molecules in an adsorbed layer interact not only with the solid, hut also with their neighbours within the layer. The effect is negligible when the fractional coverage 0 of the surface is small and the adsorbed molecules are therefore far apart, but it becomes increasingly significant as the monolayer becomes more and more crowded. A densely occupied monolayer will act in some degree as an extension of the solid, and will be able to attract further molecules from the gas phase in the manner already described, though more... 

An alternative way of deriving the BET equation is to express the problem in statistical-mechanical rather than kinetic terms. Adsorption is explicitly assumed to be localized the surface is regarded as an array of identical adsorption sites, and each of these sites is assumed to form the base of a stack of sites extending out from the surface each stack is treated as a separate system, i.e. the occupancy of any site is independent of the occupancy of sites in neighbouring stacks—a condition which corresponds to the neglect of lateral interactions in the BET model. The further postulate that in any stack the site in the ith layer can be occupied only if all the underlying sites are already occupied, corresponds to the BET picture in which condensation of molecules to form the ith layer can only take place on to molecules which are present in the (i — l)th layer. 

A second criticism is that the model restricts attention to the forces between the adsorbent and the adsorbate molecules—the vertical interactions—and neglects the forces between an adsorbate molecule and its neighbours in the same layer—the horizontal interactions. From the nature of intermolecular forces (p. 5) it is certain that these adsorbate-adsorbate interactions must be far from negligible when a layer is approaching completion and the average separation of the molecules is therefore small in relation to their size. 

Indium clusters have also recently been characterized, notably in intermetallic compounds. Thus, the Zintl phase, Rbzinj, (prepared by direct reaction between the two metals at I530°C) has layers of octahedral closo-lnf, clusters joined into sheets through exo bonds at four coplanar vertices. These four In atoms are therefore each bonded to five neighbouring In atoms at the comers of a square-based pyramid, whereas the remaining two (Irans) In atoms in the Ine cluster... 

Figure 8.24 Schcmaiic diagram of the layer structure of CuCN.NH showing the tridentate CN groups each Cu is also bonded to 1 NH molecule at 207 pm. Note also the unusual 5-coordination of Cu including one near neighbour Cu at 242 pm (13 pm closer than Cu-Cu in the metal). The lines in the diagram delineate the geometry and do not represent pairs of electrons.

Figure 13.1 Puckered layer structure of As showing pyramidal coordination of each As to 3 neighbours at a distance rj (252 pm). The disposition of As atoms in the next layer (r2 312 pm) is shown by dashed lines.

Fig. 16.13b (Se-Fax 180 pm, Se-Feq 167 pm, with axial and equatorial angles subtended at Se of 169.3° and 96.9°, respectively).However, these pseudo-tbp molecules are arranged in layers by weaker intermolecular interactions to neighbouring molecules so as to form an overall distorted octahedral environment with two further Se F at 266 pm (Fig. 16.13b) somewhat reminiscent of the structure found earlier for Tep4 (see Fig. 16.13c and below).

The above statements are valid for monomolecular layers only. In the case of polymer films with layer thickness into the p-range, as are usually produced by electropolymerization, account must also be taken of the fact that the charge transport is dependent on both the electron exchange reactions between neighbouring oxidized and reduced sites and the flux of counterions in keeping with the principle of electroneutrality Although the molecular mechanisms of these processes... 

Here the summation is over molecules k in the same smectic layer which are neighbours of i and 0 is the angle between the intermolecular vector (q—r ) projected onto the plane normal to the director and a reference axis. The weighting function w(rjk) is introduced to aid in the selection of the nearest neighbours used in the calculation of PsCq). For example w(rjk) might be unity for separations less than say 1.4 times the molecular width and zero for separations greater than 1.8 times the width with some interpolation between these two. The phase structure is then characterised via the bond orientational correlation function... 

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