Orientational defects

Fig. 2 Mechanically oriented bilayer samples as a membrane model for ssNMR. (a) Illustration of the hydrated lipid bilayers with MAPs embedded, the glass supports, and the insulating wrapping, (b) A real sample consists of 15 stacked glass slides, (c) Schematic solid-state 19F-NMR lineshapes from an oriented CF3-labelled peptide (red), and the corresponding powder lineshape from a non-oriented sample (grey), (d) Illustration of typical orientational defects in real samples - the sources of powder contribution in the spectra...

The kinetic nature of mixed potentials is, in most cases, responsible for the lack of reproducibility. In a given solution under the same conditions, the same metal with different surface characteristics may adopt a different corrosion potential and, even at a given polycrystalline electrode, the corrosion potential is an average of different local regions of different properties crystal orientation, defects, and chemical heterogeneities. 

In addition to the hole defects in a liquid with H-bonds the angle dependence of the H-bond interaction induces an orientation defect mechanism. Figure 4 gives indications on co-operative mechanism of orientation defects as well. 

The first part of the right side of Eq. (1) gives the portion of the H-bonded OH groups with the concentration (1-Of) the second part gives the portion of the non H-bonded OH groups with the concentration 0F. The partial molar volume of H-bonded groups and the coefficient of thermal expansion is taken as ice like datas. Both properties of the orientation defects are adjusted. Spectroscopy cannot give informations on these constants. Therefore, the proof of the orientation defects assumption by the density is not very accurate. 

The density maximum at 4 °C is induced by the sum of two effects. The normal lowering of density with increasing T by the increase of the volume of thermal vibrations and the rise of p with T by an increase of the content of 0F. The structure near the orientation defects is more like non-polar liquids with their densed package and higher coordination number. 

The summary of our simple approximation model of water is water consists at T< 100 °C of a network of H-bonded molecules. The co-operative mechanism of the angle dependence of H-bond energy has the consequence that the complete orientation defects of non H-bonded OH groups are not distributed statistically, they are concentrated by fissure plains of defects. The content of orientation defects at room T is about 12%. The number of molecules per idealized aggregate at room T is about 300 molecules (about 7 in one dimension). 

There are some ions which effect the water spectra like a -increase. Ions with the largest structure-breaker effect can have salt-in effects on organic molecules. This can be understood as follows the water becomes more hydrophilic because the content of orientation defects of OH groups increases. Structure breakers are mainly large mono-valent anions. 

The smallness of the ion effect on the water spectra is contradictry to the large effects by ions on the solubility of organic molecules in water. The apparent paradox can be easily understood by the simple water model. At room T the content of orientation defects is not very sensitive to the expansion of H-bond systems12,49 ... 

At higher T the expansion of the system of H-bonded molecules decreases one can calculate that the sensitivity of the content of orientation defects on a change of the... 

The diameter of 24 A of the glass capillaries corresponds to the diameter of the water aggregates in the simple model of bulk water. From the point of this model one can assume that the strong interactions of the glass wall or the cell walls prevent the flickering process of the orientation defects. 

The situation is quite similar to that of ice. A dielectric measurement on a KOH-doped THE showed that the relaxation time t for the reorientational motion was dramatically shortened by the dopant, possibly by creating a pair of the orientational defects proposed by Bjemim. Not only the absolute value of t but also the activation energy for the process decreased by the dopant, as shown in Fig. 3. The value of x at 70 K is 10- times smaller than that for pure (undoped) sample. This is the reason why the ordering transition has escaped from observation for a pure sample by a kinetic reason appeared now at 62 K in the doped sample by a catalytic action of the dopant within a reasonable time. Also given in the figure is a Cole-Cole plot of the dielectric permittivity of the KOH-doped THF hydrate. The distribution of dielectric relaxation times is much wider in the doped sample than in the pure sample. 

Iron, nickel and cobalt were selected as catalysts for the generation of nanocarbons in the present study. Structural peculiarities shape, orientation, defects of catalyst particles and fillings of as-prepared specimens were studied using HRTEM analysis. 

The Hamiltonian (480) of orientational oscillations of ionic groups in the hydrogen-bonded chain can be related to the model of easy-axis ferromagnetic in transversal external field 2Trot. The Hamiltonian (480) resembles the Hamiltonian (451) in outward appearance, and this means that we can reduce the problem to the previous one. However, we are interested in the explicit form of parameters Or<)l and UIot. For this purpose we should start from the appropriate classical Hamiltonian that describes the motion of an oriental defect in the hydrogen bonded chain [325] ... 

A particularly important question involves the understanding of the role of crystal defects in the peculiar electrical behaviour of ice 4. Upon the application of an electric field, the solid becomes polarized by the thermally activated reorientation of the molecular dipoles. Niels Bjerrum postulated the existence of orientational defects, which represent local disruptions of the hydrogen-bond network of ice 4, to explain the microscopic origin of this phenomenon. 

FIRST-PRINCIPLES CALCULATION OF STRUCTURE AND DYNAMICAL PROPERTIES OF ORIENTATIONAL DEFECTS IN ICE... 

In general, chiral nematic polymer liquid crystals (LCP) cannot form monodomains in which the rodlike polymers have a spatially uniform orientation within the sample. Typically, because of the high density of orientational defects, the LCPs are textured, with a distribution of polymer orientation. Microscopically, the polymer chains have a preferred orientation with a relatively narrow distribution around the average orientation. Macroscopically, the variation in space of the orientation results in a domain structure. Defects and orientational variations give rise to the polydomain texture and the overall LCP sample may be randomly ordered (Fig. 3). 

We must point out two related limitations of the LE theory. First, it applies to small-molecule LCs and to LCPs in the limit of vanishing strain rate. This is because the LE theory uses a vector n to represent the orientation state of the fluid, tacitly assuming that the molecular orientation distribution stays at its equilibrium state. This is reasonable when the molecular relaxation time is much shorter than the characteristic time of the flow. Second, the theory does not allow orientational defects, which would be singularities in the n field. In reality, LCs and LCPs tend to have a high density of defects. ° Near the defect core, large spatial gradients distort the molecular orientation distribution, thus invalidating the LE theory. 

The inability of the LE theory to describe orientational defects has motivated efforts to generalize it. 

Nevertheless in polymeric liquid crystals the same types of orientational defects and thus the same types of textures as present in the low mass counterparts have been observed. The textures often formed by polymers are the threaded texture, the schlieren texture and the focal conic texture of smectics. As is for low mass liquid crystals, the texture is a consequence of defects (disclinations and dislocations, refer to Chapter 1) present in the liquid crystal and is characteristic of a specific type of the phase. The texture examination has become a very useful tool in the determination of the type and nature of the polymeric liquid crystals. 

Fig. 7.3. (a) Formation of an orientational defect pair, v and L, by an oblique proton jump or, equivalently, by rotation of a water molecule by znis about one of its bonds. (6) Separation of these two orientational defects by further oblique proton jumps. 

This information allows us to make tentative estimates of the concentration of orientational defects in pure ice, using an equation like (7.1), and of their mobility. It is clear from the energies involved that they should be much more numerous in pure ice than are the ion states. The energy barrier to proton motion is comparable in height to that for ion states but twice as wide, so that it is possible, and indeed turns out to be the case, that the anomalously high mobility of ionic states does not extend to orientational defects. Experimental information, derived from studies of the electrical properties of ice, is summarized for convenience in table 7.3. 

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