Faces of crystals

Figure 3 Cross-section depicting the smallest steps present on the surface of each face of crystals with aspect ratios 1.5. Where 1 i) shows the position of the cross-section, lii) shows the cross-section and the corresponding heights and 1 iii) shows a schematic of the possible cancrinite unit of attachment onto the hexagonal face. Where 2i) shows the position of the cross-sections, 2ii) and iv) show the cross-sections and the corresponding heights and 2iii) and v) show schematics of the possible cancrinite unit of attachment onto the side walls.

Figure 12. (a), (b) Scanning electron micrographs of the (010) faces of crystals of asparagine-H20 partially dissolved in the presence of (5)-aspartic acid (a) ( )-asparagine crystal (b) (fl)-asparagine crystal. 

The faces of crystals, both when they grow and when they are formed by cleavage, tend to be parallel either to the sides of the unit cell or to planes in the crystal that contain a high density of atoms, it is useful to be able to refer to both crystal faces and to the planes in the crystal in some way—to give them a name—and this is usually done by using Miller indices. 

We now have to consider the faces of crystals and their relation to the geometry of the precisely patterned assemblage of atoms which constitutes the solid material, This subject is best approached by thinking about the manner in which crystals grow. Crystals usually have plane faces, firstly because they do not grow at the same rate in all directions, and secondly as a result of the specific manner in which solid material is deposited. 

It is the regularity of arrangement of the atoms in a crystal which gives to the crystal its characteristic properties, in particular the property of growing in the form of polyhedra. The faces of crystals are defined by surface layers of atoms, as shown in Figures 3-S and 34. These faces lie at angles to one another which have definite charac-... 

Reactions may occur on the surfaces (faces) of crystals, and the different presentation of the molecules on different faces leads to differing reactivities of these faces. 

The dehydration of nickel sulfate heptahydrate [13] resulted in nuclei being formed on the (110) face of crystals. These nuclei were fairly uniformly distributed half-eUpses. The number of nuclei increased widi time according to a square law and the initial relatively slow rate of growth increased up to a constant value. The catalytic effect resulting from the addition of monohydrate to the heptahydrate was again attributed [53] to changes in gas composition and not to reactant-product contacts. 

Also electrolytes and polyelectrolytes affect the -potential. Studies on mont-morillonite clays showed that an excess of Na ions in solution does not produce changes in -potential although it is known that Na ions react with the edges of clay. Thus, only interaction with the face of crystal affects -potential. Ca " ions can replace sodium counterions on the montmorillonite face and this replacement causes a shift towards negative values of -potential. When ions replace sodium counterions on the montmorillonite face they cause deflocculation and an in- 108 crease m viscosity. 

The equilibrium state for various faces of crystal is determined by the condition of Ap=const., which yields the Curie-Wulff expression, stating that the ratio of the free surface energy of a particular face to its distance from the crystal center is constant for all faces in equilibrium state, i.e. ... 

As the molecular bond lengths became known, correlations were sought between the geometry of the surface and catalytic activity. There developed the multiplet theory of Balandin which was applied successfully to dehydrogenation catalysts. It also provided an adequate explanation of the work of Maxted and others on catalytic poisons and of the behavior of the different plane faces of crystals. There is no inherent conflict between the interpretations based on geometry and those based on the electronic potential of the surface. The two effects are probably complementary. More knowledge is, however, required about the influence of the electronic potential on the decomposition of complex molecules, before a decision can be made on their relative significance. 

Powder x-ray diffraction spectra could show distinct bands for enantiomers and corresponding racemates if the latter is a racemic compound. The nonracemic character was reported for cryptochiral samples of triglycerides and 1-lauro dipalmitin [38,39]. Natural triglycerides had little or no optical activity (cryptochiral) while the synthetic racemic triglycerides showed optical activity. This property could be further characterized by inducing piezoelectricity (creation of charges on opposite faces of crystals... 

The notation used for identifying planes and faces of crystals is that of W.H. 

Crystals of phthalocyanine tend to form needles or rods parallel to the stacking direction. The side faces of crystals are mainly covered by aromatic hydrogen atoms, while the basal faces expose the jc system and the copper atom. The lateral surfaces exhibit nonpolar character, while the basal surfaces have relatively polar character. 

A theory, such as the atomic theory, usually involves some idea about the nature of some part of the universe, whereas a law may represent a summarizing statement about observed experimental facts. For example, there is a law of the constancy of the angles between the faces of crystals. This law states that whenever the angles between corresponding faces of various crystals of a pure substance are measured they are found to have the same value. The law simply expresses the fact that the angles between... 

The selectivity varies with catalysts (improved with zeolites, that are acid-base aluminosilicate catalysts) and surfaces (faces of crystals). 

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