Vermiculite, crystalline

In view of the problems associated with the expanding 2 1 clays, the smectites and vermiculites, it seemed desirable to use a different clay mineral system, one in which the interactions of surface adsorbed water are more easily studied. An obvious candidate is the hydrated form of halloysite, but studies of this mineral have shown that halloysites also suffer from an equally intractable set of difficulties (JO.). These are principally the poor crystallinity, the necessity to maintain the clay in liquid water in order to prevent loss of the surface adsorbed (intercalated) water, and the highly variable morphology of the crystallites. It seemed to us preferable to start with a chemically pure, well-crystallized, and well-known clay mineral (kaolinite) and to increase the normally small surface area by inserting water molecules between the layers through chemical treatment. Thus, the water would be in contact with both surfaces of every clay layer in the crystallites resulting in an effective surface area for water adsorption of approximately 1000 tor g. The synthetic kaolinite hydrates that resulted from this work are nearly ideal materials for studies of water adsorbed on silicate surfaces. 

FIGURE 1.1 Schematic illustration of the structure of a crystalline vermiculite. Within the clay plates, the open circles represent oxygen, the closed circles silicon, and the shaded circles magnesium. The oxygen-oxygen separation of the surface layers is about 7 A. In the interlayer region, the open circles represent water molecules and the shaded circles univalent cations. The dotted lines show the unit cell. 

Humes [4] had already studied the n-butylammonium Eucatex crystals by X-ray diflfaction. At relative humidities (RH) somewhat less than 100%, the n-butylammonium vermiculite exists in two stable crystalline phases having c-axis spacings of about... 

FIGURE 1.6 Neutron diffraction pattern of the fully hydrated crystalline phase of n-butylammonium vermiculite measured in reflection geometry X = 4.52 A. 

FIGURE 1.7 Rocking curve on the 001 peak of a fully hydrated crystalline n-butylammonium vermiculite d = 19.4 A. 

FIGURE 1.14 Neutron diffraction patterns of n-butylammonium vermiculite close to the crystalline-gel phase transition at T = 14°C and an external salt concentration of c — 0.1 M. (a) low-angle region, (b) wide-angle region... 

FIGURE 1.15 Temperature-induced phase transition between the gel and crystalline phases of n-butylammonium vermiculite at an external salt concentration of c = 0.01 M A = 17 A. 

FIGURE 1.17 Heat capacity across the swelling transition of n-butylammonium vermiculite (per gram of crystalline material) in a 0.1 M solution of n-butylammonium chloride. 

Intensity of the 001 Reflection of the Crystalline Phase of n-Butylammonium Vermiculite in 0.1 -M n-Butylammonium Chloride... 

Because the application of hydrostatic pressure caused the vermiculite to swell to its gel phase, the total volume of the gel phase was less than that of the crystalline phase plus the appropriate amount of solution, even though the gel phase itself... 

Let us recall the schematic illustration of the raw phenomenon of the clay swelling in Figure 1.4. In the cases studied in Chapters 1 to 3, V was always much greater than V, the volume occupied by the macroions. We now define Vm to be the volume occupied by the macroions in the coagulated (crystalline) state, as in Figure 1.4a in the vermiculite system. This is an experimentally controlled variable. We define the sol concentration r by... 

But it was not to be. Try as we might, the difference in scattering lengths between the 6Li and 7Li isotopes was too small to permit us to measure the lithium ion distribution in the swollen state. We had to content ourselves with the results for the crystalline phase, where the behavior of the lithium ions is different from that of the larger alkali metal cations [27], Potassium and cesium ions bind directly to vermiculite clay surfaces rather than hydrating fully. Because only lithium-substituted vermiculites of the alkali metal series will swell macroscopically when soaked in water, it seems that interlayer cations must form fully hydrated ion-water complexes if the particles are to expand colloidally. This conclusion has since been supported... 

To estimate the degree of polymer adsorption, we first calculate the total layer surface area of the vermiculite in the crystalline state. The formula for the dry sodium Eucatex sample considered in Chapter 1 can be abbreviated as... 

A definite prediction of DLVO theory is that charge-stabilized colloids can only be kinetically, as opposed to thermodynamically, stable. The theory does not mean anything at all if we cannot identify the crystalline clay state (d 20 A) with the primary minimum and the clay gel state (d 100 to 1000 A) with the secondary minimum in a well-defined model experimental system. We were therefore amazed to discover a reversible phase transition of clear thermodynamic character in the n-butylammonium vermiculite system, both with respect to temperature T and pressure P. These results rock the foundations of colloid science to their roots and... 

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