Bulk atoms, definition

The term atomic layer is used here to indicate, in general, a layer of atoms on the surface, where all the atoms are in contact with the surface. The term atomic layer does not specify a coverage, just that the layer is no more then one atom thick, probably less then a ML, relative to the number of substrate surface atoms. There can be several structures formed at different coverages, all under a ML, but all are one atom thick, and all would correspond to an atomic layer. Thus a statement that an atomic layer was formed suggests only that no bulk atoms were deposited. Where as, the statement that a monolayer was formed suggests a coverage, dependent on the ML definition in use. 

Figure 1.3. Definition of the fractions m2 and 2m of nearest neighbours lying respectively in the same plane and in adjacent planes for a bulk atom lying in a symmetry plane of a crystal and having Z...

The topic of this book is forces acting between interfaces. There is no dear, unique definition of an interfacial force. One possible definition is as follows Interfacial forces are those forces that originate at the interface. For example, electrostatic double-layer forces are caused by surface charges at the interface. Such a definition would, however, not include van der Waals forces. For van der Waals interaction, the surface atoms do not have a distinct role compared to the bulk atoms. Still, van der Waals forces substantially contribute to the interaction between small particles. One could define surface forces as all interactions that increase proportional to the interfacial area. Then, for certain geometries gravitation should also be induded. Gravitation is, however, not described here. On the other hand, hydrodynamic interactions would be excluded because thqr depend on the specific shape of interacting interfaces and not only on the interfadal area. 

The explicit definition of water molecules seems to be the best way to represent the bulk properties of the solvent correctly. If only a thin layer of explicitly defined solvent molecules is used (due to hmited computational resources), difficulties may rise to reproduce the bulk behavior of water, especially near the border with the vacuum. Even with the definition of a full solvent environment the results depend on the model used for this purpose. In the relative simple case of TIP3P and SPC, which are widely and successfully used, the atoms of the water molecule have fixed charges and fixed relative orientation. Even without internal motions and the charge polarization ability, TIP3P reproduces the bulk properties of water quite well. For a further discussion of other available solvent models, readers are referred to Chapter VII, Section 1.3.2 of the Handbook. Unfortunately, the more sophisticated the water models are (to reproduce the physical properties and thermodynamics of this outstanding solvent correctly), the more impractical they are for being used within molecular dynamics simulations. 

This chapter has given an overview of the structure and dynamics of lipid and water molecules in membrane systems, viewed with atomic resolution by molecular dynamics simulations of fully hydrated phospholipid bilayers. The calculations have permitted a detailed picture of the solvation of the lipid polar groups to be developed, and this picture has been used to elucidate the molecular origins of the dipole potential. The solvation structure has been discussed in terms of a somewhat arbitrary, but useful, definition of bound and bulk water molecules. 

Eirst of all, what is meant by a solid surface Ideally the surface should be defined as the plane at which the solid terminates, that is, the last atom layer before the adjacent phase (vacuum, vapor, liquid, or another solid) begins. Unfortunately such a definition is impractical because the effect of termination extends into the solid beyond the outermost atom layer. Indeed, the current definition is based on that knowledge, and the surface is thus regarded as consisting of that number of atom layers over which the effect of termination of the solid decays until bulk properties are reached. In practice, this decay distance is of the order of 5-20 nm. 

By a fortunate coincidence, the depth into the solid from which information is provided by the techniques described here matches the above definition of a surface almost exactly. These techniques are, therefore, surface-specific, in other words, the information they provide comes only from that very shallow depth of a few atom layers. Other techniques can be surface sensitive, in that they would normally be regarded as techniques for bulk analysis, but have sufficient sensitivity for certain elements that can be analyzed only if they are present on the surface only. 

At any interface between two different phases there will be a redistribution of charge in each phase at the interface with a consequent loss of its electroneutrality, although the interface as a whole remains electrically neutral. (Bockris considers an interface to be sharp and definite to within an atomic layer, whereas an interphase is less sharply defined and may extend from at least two molecular diameters to tens of thousands of nanometres the interphase may be regarded as the region between the two phases in which the properties have not yet reached those of the bulk of either phase .) In the simplest case the interface between a metal and a solution could be visualised as a line of excess electrons at the surface of the metal and an equal number of positive charges in the solution that are in contact with the metal (Fig. 20.2). Thus although each phase has an excess charge the interface as a whole is electrically neutral. 

In the absence of definitive information about the structure of the active site theoretical modeling of enzyme catalyzed reactions is difficult but not impossible. These difficulties are caused by the extremely large size of the enzyme-substrate-solvent system which typically comprises thousands or tens of thousands of atoms so that direct theoretical treatment at the microscopic quantum mechanical level is not yet practical. The computational demand is simply too enormous. As a compromise, a scheme generally referred to as QM/MM (quantum mechanics/molecular mechanics) has been devised. In QM/MM calculations, the bulk of the enzyme-solvent system (i.e. most of the atoms) is treated at a low cost, usually at the molecular mechanics (MM) level, while the more nearly correct and much more expensive quantum level (QM) computation is applied only to the reaction center (active site). 

If we assume above / = 1 excess fluorine atoms over those required to fill the available surface are in a bulk solid phase of definite composition, then the second moment of the line is the weighted sum of the second moments of fluorine in the bulk phase ((AH ))b and fluorine on the filled surface That is, if n, is the mole fraction on the filled surface and... 

The van der Waals forces scale up from atomic distances to colloidal distances undiminished. How the molecular forces scale up in the case of large objects, expressions for such forces, definition of the Hamaker constant, and theories based on bulk material properties follow in Sections 10.5-10.7. 

We call this Pt(100) surface reconstructed. Surface reconstruction is defined as the state of the clean surface when its LEED pattern indicates the presence of a surface unit mesh different from the bulklike (1 x 1) unit mesh that is expected from the projection of the bulk X-ray unit cell. Conversely, an unreconstructed surface has a surface structure and a so-called (1 x 1) diffraction pattern that is expected from the projection of the X-ray unit cell for that particular surface. Such a definition of surface reconstruction does not tell us anything about possible changes in the interlayer distances between the first and the second layers of atoms at the surface. Contraction or expansion in the direction perpendicular to the surface can take place without changing the (1 x 1) two-dimensional surface unit cell size or orientation. Indeed, several low Miller index surfaces of clean monatomic and diatomic solids exhibit unreconstructed surfaces, but the surface structure also exhibits contraction or expansion perpendicular to the surface plane in the first layer of atoms (9b). 

Nanomaterials are commonly defined as materials with morphological features on the nanoscale (commonly below 100 nm in one, two, or all three dimensions), but a more precise definition would limit the scope to only those materials showing unique properties different from individual atoms or molecules as well as the bulk as a result of their nanoscale dimensions. 

---