Electron microscopy, distances

Cationic quaternary ammonium compounds such as distearyldimethylammonium-chloride (DSDMAC) used as a softener and as an antistatic, form hydrated particles in a dispersed phase having a similar structure to that of the multilayered liposomes or vesicles of phospholipids 77,79). This liposome-like structure could be made visible by electron microscopy using the freeze-fracture replica technique as shown by Okumura et al. 79). The concentric circles observed should be bimolecular lamellar layers with the sandwiched parts being the entrapped water. In addition, the longest spacings of the small angle X-ray diffraction pattern can be attributed to the inter-lamellar distances. These liposome structures are formed by the hydrated detergent not only in the gel state but also at relatively low concentrations. 

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

Fig. 12.8 Electron microscopy study of a PDMS-Si02-CaO ormosil (A) Original HRTEM image of the amorphous matrix, (B) filtered HRTEM image and (C) Fourier transform pattern. Distances up to 0.53 nmfor (Si04)4-can be observed in the filtered image, indicating the presence of Ca2+ between the tetrahedra.

We shall also outline how bond distances and information pertaining to the distribution of electron density may, in principle, be extracted from measurements carried out using electron microscopy. Finally we touch upon future possible lines of development likely to be of value to the inorganic, surface and analytical chemist. 

Electron probe and X-ray fluorescence methods of analysis are used for rather different but complementary purposes. The ability to provide an elemental spot analysis is the important characteristic of electron probe methods, which thus find use in analytical problems where the composition of the specimen changes over short distances. The examination of the distribution of heavy metals within the cellular structure of biological specimens, the distribution of metal crystallites on the surface of heterogeneous catalysts, or the differences in composition in the region of surface irregularities and faults in alloys, are all important examples of this application. Figure 8.45 illustrates the analysis of parts of a biological cell just 1 pm apart. Combination of electron probe analysis with electron microscopy enables visual examination to be used to identify the areas of interest prior to the analytical measurement. 

FIG. 4. Ultrastructure of vascular smooth muscle of the rabbit inferior vena cava revealed with electron microscopy. Serial cross-sections of VSMCs are shown in series 1 (panel A—D) and series 2 (panel E—G). Series 1 illustrates the close spatial apposition between the superficial SR sheet and the PM with the apices of the caveolae perforating through the superficial SR sheets to come into contact with the bulk cytoplasm. The membranes of the PM (dotted line) and the SR (solid line) in panel A-D are outlined to the right of the respective panels. The close apposition between the superficial SR sheet, the PM and the neck region of the caveolae creates a narrow and expansive restricted space. Series 2 illustrates the perpendicular sheets of SR, which appear to arise from the superficial SR sheets. Mitochondria also come into close contact with the perpendicular SR sheets. Panel H contains a stylized illustration of the close association between the superficial SR sheet, which is continuous with the perpendicular sheet, the perforating caveolae (C), the PM and a mitochondrion (M). Panel I shows calyculin-A mediated dissociation of the superficial SR sheets from the PM (see arrows). The black scale bar indicated represents 200 nm of distance. 

To avoid sintering, the nanoparticles are isolated from each other by a small loading. For a support of circa lOOm g" , and metal loading of about 1 wt%, with ruthenium particles of about 2 nm in diameter one obtains a statistical repartition of each particle every 30 nm. This means that such metallic particles of ruthenium particles are at a random distance of about 30 nm from each other. Electron microscopy indicates that this is frequently the case. 

The convenience of using X-rays for stmcture determination stems from the nature of their interactions with matter the wavelengths of radiation in the X-ray region of the electromagnetic spectmm are comparable to the sizes of atoms and interatomic distances that are to be analyzed. Although, in principle, interatomic distances can be determined by electron microscopy, unlike electron microscopic... 

Relaxation of CT samples also depends strongly on Sn content. For the most Sn-abundant sample x-ray analysis showed the presence of almost completely oriented Sn crystallites of 100-500 nm size, and electronic microscopy revealed that metal particles were separated by distances less than their size. It means that in this case we have practically metal film. The time response of such a sample appeared to be the same as that for pure Sn film. We attribute pulsewidth-limited rise of negative transmission and reflection (curves 10,11) to excitation of electrons in metal. Subsequent 5 ps rise reflects electron-phonon relaxation and further long decay is due to lattice cooling. Contribution of this dynamics is observed for the... 

---