Vermiculite swelling

Gjems, O., 1963. A swelling dioctahedral clay mineral of a vermiculite-smectite type in the weathering horizons of podzols. Clay Miner., 5 183-193. 

Clays are aluminosilicates with a two-dimensional or layered structure including the common sheet 2 1 alumino- and magnesium- silicates (montmorillonite, hectorite, micas, vermiculites) (figure 7.4) and 1 1 minerals (kaolinites, chlorites). These materials swell in water and polar solvents, up to the point where there remains no mutual interaction between the clay sheets. After dehydration below 393 K, the clay can be restored in its original state, however dehydration at higher temperatures causes irreversible collapse of the structure in the sense that the clay platelets are electrostatically bonded by dehydrated cations and exhibit no adsorption. 

Jinnai et aL [18] measured by neutron-scattering the interlayer separation between the butylammonium vermic-ulite platelets as a function of the poly(vinyl methyl ether) (PVME) volume fraction in an aqueous butylammonium chloride solution. The n-butylammonium vermiculite swelled in the n-butylammonium chloride solution, and the interlayer separation of the platelets increased from 2 to 12 nm at 8<7 <20 °C. When the PVME was added to this... 

The compression of the interlayer water between the unit layers of the vermiculite clay is probably a consequence of the large attractive force between the negatively charged unit layers and a densely populated layer of sodium ions midway between the unit layers. This large attractive force also keeps the unit layers from swelling beyond a two-layer complex. 

The 2 1 layer clays (i.e. with three-sheet layers) include a group of expanding or swelling clays, which comprise the smectites (e.g. montmorillonite, saponite and hectorite) and the vermiculites. The basic structure of a smectite is shown in Figure... 

FIGURE 1.3 Butylammonium vermiculite (Kenya) crystals before and after swelling in water (lateral dimensions of the crystals approximately 2.5 x 2.5 mm). (Reproduced with kind permission of the Clay Minerals Society, from Garrett, W.G. and Walker, G.F., Clays Clay Min., 9, 557, 1962.)... 

In principle, it would be desirable to study the swelling for many different vermiculites and many different cations. However, as we shall see, the study of even one swelling system becomes a complicated many-variable problem. The swelling seems to be most pronounced and homogeneous for vermiculites with x = 1.3 and M = C4H9NH3, so we choose to investigate this ideal clay colloid system as rigorously as possible and focus our studies on the Eucatex minerals in hand. 

FIGURE 1.4 Schematic illustration of the swelling of n-butylammonium vermiculite. (a) shows the unexpanded in a 1.0 M butylammonium chloride solution, (b), (c) and (d) show the gels formed in 0.1 M, 0.01 M and 0.001 M solutions, respectively. Note how the extent of swelling is suppressed by an increase in the salt concentration. V represents the volume occupied by the clay, with V the volume of the whole condensed matter system (clay plus excess soaking solution.)... 

The samples for the diffraction experiments were prepared as follows. A crystal of the n-butylammonium vermiculite was cut to a thickness of about 0.5 mm and an area of about 5x5 mm and soaked in the appropriate solution of protonated n-butylammonium chloride in D20. DzO was used because it gave a lower incoherent-scattering background than H20. The gel was allowed to come to its equilibrium swelling distance for 48 hours in a cold room at a temperature of 7°C. A slice of this swollen gel about 1 mm thick was then transferred to a quartz cell of internal dimensions 30 x 5 x 2 mm. The remainder of the quartz cell was filled with some of the original solution. The cell was sealed with Parafilm to prevent loss of solution by evaporation, clamped into an aluminum block, as shown in Figure 1.5, and... 

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. 

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... 

FIGURE 1.21 Phase diagram for swelling of n-butylammonium vermiculite in water as a function of temperature T and pressure P, at c = 0.1 M. The swollen gel phase lies in the lower half of the diagram the gel phase is the high-density, low-temperature phase. 

It is clear from this chapter that the coulombic attraction theory potential is much better adapted to explain the experimental phenomena described in Chapter 1 than the DLVO theory potential (Equation 1.2). Of course, if you predict an interaction potential, you predict force-distance curves along the swelling axis. There have been a lot of arguments about how direct measurements of forces between spherical colloidal particles refute the coulombic attraction theory. Let us get the facts first. We now examine the experimental curves for the n-butylammonium vermiculite system. 

FIGURE 2.3 The phase transition in n-butylammonium vermiculite swelling is thermodynamic, so (a) the primary and secondary minima must be of equal depth, irrespective of the shape of the total interaction potential VT. This cannot be accounted for by the DLVO potential, shown in (b). Part (c) shows VT in the coulombic attraction theory (dashed curve), composed of an electrostatic part (solid curve) and a short-range potential (dotted curve) that includes the weak contribution from the van der Waals potential. The vertical dashed lines indicate the positions of the two minima. 

For these experiments, some of which were also conducted on the ILL D17 small-angle diffractometer, crystals of n-butylammonium vermiculite exhibiting the fewest obvious structural defects were selected and trimmed to a rectangular cross section with a razor blade. This enabled the surface area, and therefore the applied pressure, to be measured accurately. The samples were then immersed in a dilute solution of n-butylammonium chloride of the desired concentration and allowed to swell freely. After equilibration for at least two days at 7°C, the swollen (or colloidal) gel phase samples were placed into the uniaxial pressure cell shown in Figure 3.1. 

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... 

TABLE 4.3 Sol Concentration Effect in n-Butylammonium Vermiculite Swelling in 0.001-M, 0.01 Mand 0.1 M Soaking Solutions c= 0.001 M ... 

FIGURE 5.5 The salt fractionation effect in n-butylammonium vermiculite swelling. The salt fractionation factor s = ceJc x is plotted as a function of cex, the salt concentration in the supernatant fluid. The solid line shows the coulombic attraction theory prediction, s = 2.8. 

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