Adsorbed molecules, geometric arrangement

The simple oxide and hydroxide minerals exhibit important characteristics common to all minerals. The relative numbers of various component atoms are given by chemical stoichiometry. These atoms may be arranged in three-dimensional space in different ways. Geothite and lepidocrocite, for example, share the same stoichiometry (FeOOH), but Fe-O and Fe-OH bond lengths and geometric relationships between octahedrally coordinated Fe atoms are different. As a consequence, the coordinative environment and accessibility of Fe atoms to incoming adsorbate molecules are different for the two minerals. In other situations, minerals comprised of different metals, such as hematite (FeiOj) and corundum (AI2O3), have similar three-dimensional structures. In situations where surface structure is more important than the nature of the incorporated metal, hematite and corundum should exhibit similar behavior. 

The conformation of adsorbed molecules may be different from that of molecules in solution. Constraints imposed by the nature of bonding to the surface and the geometric arrangement of mineral surface sites may introduce strain. These changes may elevate or lessen the reactivity of adsorbed species. 

When growth stops there naturally must be edges, corners, steps, and kinks formed on the particle surface as shown in Fig. 3. Metal atoms that find themselves in such positions are much more chemically reactive, due to their accessibility and their coordinative unsaturation. These sites are believed to be very important in some catalytic processes. Likewise many catalysis experts believe that only certain geometrical arrangements of metal atoms are capable of carrying out a specific chemical (catalytic) transformation of an adsorbed molecule (the so-called ensemble effect). We will deal with these subjects in more detail later. 

The less positive feature of this is that the STM image will depend on the composition and the geometrical arrangement of atoms at the tip. It will change contrast in some impredictable way if an atom or molecule is adsorbed onto the active part of the tip [141]. 

Solid surface will have molecules arranged at the surface in a very well-defined geometrical arrangement. This will give rise to surface forces, which will determine the adsorption of a particular substance. On any solid surface, one can expect a certain number of possible adsorption sites per gram (N, ). This is the number of sites where any adsorbate can freely adsorb. There will be a... 

In contrast to bridging flocculation, depletion flocculation occurs when the added polymer cannot adsorb to the surfaces of the droplets. Under these conditions a layer around the droplets exists where the polymer concentration is depleted as compared to the bulk solution (see the depleted volume in Figure 4.3). The origin of the depleted volume lies with a geometrical restraint imposed by the finite voliune of the polymer molecule. This may be understood by modelling the polymer molecule as a sphere of radius r, whose position is defined by its centre. Since the polymer may not adsorb to the droplet surface, the closest it may approach the droplet is to touch the surface. In this arrangement the centre of the polymer will be a distance rg from the droplet surface. It follows that polymer centres cannot get closer than this to the droplet surface and consequently do not contribute to the segment density there. 

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