Water interlayer, characterization

Characterization of Interlayer Water. X-ray diffraction studies of the 10A hydrate show no hkl reflections indicating a lack of regularity in the stacking of the kaolin layers. In addition to the 10A hydrate, two other less hydrated kaolinites were synthesized. Both have one molecule of water for each formula unit in contrast to the 10A hydrate which has two. These less hydrated clays consequently have smaller d(001) spacings of 8.4 and 8.6 A. The synthesis conditions for these two hydrates are described in (22.). By studying the interlayer water in the 8.4 and 8.6A hydrates, it was possible to formulate a model of the water in the more complicated 10A hydrate. 

By comparison with many other silicate minerals, isotope studies of natural clays are complicated by a number of special problems related to their small particle size and, hence, much larger specific surface area and the presence of interlayer water in certain clays. Surfaces of clays are characterized by 1 or 2 layers of adsorbed water. Savin and Epstein (1970a) demonstrated that adsorbed and interlayer water can exchange its isotopes with atmospheric water vapor in hours. Complete removal of interlayer water for analysis with the total absence of isotopic exchange between it and the hydroxyl group, may not be possible in all instances (Lawrence and Taylor 1971). 

The expanded or expandable 2 1 clay minerals vary widely in chemical composition and in layer charge. These minerals are characterized by the presence of loosely bound cations and layers of water or polar organic molecules between the silica sheets. The interlayer width is reversibly variable. The interlayer water can be driven off at temperatures between 120° and 200°C. Sodium, calcium, hydrogen, magnesium, iron, and aluminum are the most common naturally occurring interlayer cations. 

FTIR characterization of ZnAl-LDH is presented in Figure 20.3. The ZnAl-LDH spectrum reveals absorption bands that are characteristic for the LDHs synthesized by coprecipitation method (3445, 2975,1637,1439,1370, and 800-400 cm ). The appearance of the broad absorption band at around 3445 cm is due to O-H stretching of hydroxyl groups (v jj) of LDH, both in the brucite-like layers and from the interlamel-lar water molecules [14,15]. This broad band is usually followed by adsorption bands at around -2980 and -1630 cm indicating the presence of stretching vibrations of the interlayer water molecules that are connected to the carbonate anions [14, 16]. Also observed is the appearance of a band at 1370 cm that can be assigned to asymmetric stretch vibrations, V3, of interlayer carbonate anions, whereas the band at 1439 cm is attributed to the splitting of v of the carbonate [14,17, 20]. The lattice vibration bands... 

Changes in X-ray diffraction pattern after controlled heat treatment greatly assist the characterization of clay minerals by that technique, and it seems likely that heat treatment will also assist infrared investigations of clays. Changes in water absorption bands during dehydration of the montmorillonites and vermiculite have been previously discussed. Tettenhorst [1962] reported changes, following loss of interlayer water, in the lattice vibra-... 

The reaction scheme of Bode [11] was derived by comparison of the X-ray diffraction patterns of the active materials with those for the model compounds. How the 8-Ni(OH)2 in battery electrodes differs from the model compound is discussed in Section 5.3.I.3. In recent years, the arsenal of in situ techniques for electrode characterization has greatly increased. Most of the results confirm Bode s reaction scheme and essentially all the features of the proposed a/y cycle. For instance, recent atomic force microscopy (AFM) of o -Ni(OH)2 shows results consistent with a contraction of the interlayer distance fiom 8.05 to 7.2 A on charge [61-63]. These are the respective interlayer dimensions for the model a-Ni(OH)2 and y-NiOOH compounds. Electrochemical quartz crystal microbalance (ECQM) measurements also confirm the ingress of alkali metal cations into the lattice upon the conversion of a-Ni(OH)2 to y-NiOOH [45,64,65]. However, in situ Raman and surface-enhanced Raman spectroscopy (SERS) results on electrostretching modes that are consistent with a weakening of the O-H bond when compared with results for the model a- and 8-Ni(OH)2 compounds [66]. This has been ascribed to the delocalization of protons by intercalated water and Na ions. Similar effects have been seen in passive films on nickel in borate buffer electrolytes [67]. 

If in the three phases a, p, y, the same chemical formula has been established by chemical analysis, differences clearly appear in the Mossbauer and infrared spectra (Figure 5) (Bujoli B. Eur, J, Sol, State Inorg, Chem, in press). The three structures would then differ probably in their interlayer structural arrangement, that is in the location of the water molecule. A confirmation of this hypothesis has been given by a thermogravimetric study. The same compound HFe(C H5P03)2 is obtained after the first loss of water at 200 C, characterized by same ii ftared, X-Ray and Mossbauer spectra, independent of the starting material (a, p, y),... 

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