Montmorillonite sodium form

The peculiar layer structure of these clays gives them cation exchange and intercalation properties that can be very useful. Molecules, such as water, and polar organic molecules, such as glycol, can easily intercalate between the layers and cause the clay to swell. Water enters the interlayer region as integral numbers of complete layers. Calcium montmorillonite usually has two layers of water molecules but the sodium form can have one, two, or three water layers this causes the interlayer spacing to increase stepwise from about 960 pm in the dehydrated clay to 1250, 1550, and 1900 pm as each successive layer of water forms. 

Montmorillonite, especially the sodium form, swells in low-ionic-strength solutions consequently, centrifugation in moderately high fields is needed to separate the clay particles and the solutions. Clay suspensions were centrifuged with a Du Pont Sorvall RC-5 refrigerated super-speed centrifuge at 15,000 rpm (28,000 g) for 15 minutes, and the supernatant was then withdrawn for analysis. No correction was made for possible ion exclusion... 

Figure 1. Effect of loading on distribution coefficients of Cs(I) on the sodium form of Wyoming montmorillonite (0.5M NaCl + 0.07 M NaOAc, pH 5, equilibration for 74 hr)...

Sr(II), and Ba(II), for the sodium form of montmorillonite. The Sr(II) results are from a different set of measurements than those in Figure 2, but are in good agreement with them. Effects of loading are small up to the several percent of ion-exchange capacity covered. Values of distribution coefficients for these three ions fall in a narrow range. 

Sodium Form of Montmorillonite Alkali metal ions Distri-bution coefficients of Cs as a function of sodium concentration are summarized in Figure 8 for clays from four different sources. 

Figure 8. Adsorption of Cs(I) on the sodium form of montmorillonites from several sources (Loading 2 X lO — 4 X 10 mol Cs(I)/kg, equilibration for 24 hr.).

Figure 7. Adsorption of Eu(III) on the sodium form of purified Wyoming montmorillonite (trace loading, equilibration for 96 hr).

Figure 10.5 Schematic illustration of the use of wide angle x-ray diffraction for studying the dispersion of montmorillonite clay plates, (a) sodium form, (b) quaternary ammonium form and (c) quaternary ammonium form dispersed in polymer...

Zielke and Pinnavaia [106] compared the adsorption of several chlorinated phenols by pillared montmorillonite and Laponite (Laponites are synthetic hectorite-like materials). Pentachlorophenol, which is the strongest Brensted acid, is best adsorbed by the polyoxoaluminum derivative and less by the polyhydroxo form. The adsorption capacity decreases strongly with increasing solution pH (from pH = 4.7 to 7.4), showing that this pollutant is adsorbed in undissociated form. The effectivity of the smectites (at pH = 4.7) is polyoxoaluminum laponite > polyhydroxoaluminum laponite > polyoxoduminum montmorillonite > polyoxochromium montmorillonite. Sodium laponite and montmorillonite show no tendency to adsorb the pollutant from aqueous solution. 

Bentonite and the other clay minerals have considerable ion exchange capability, and their physical characteristics change dramatically depending upon the content of the salt solution in their pore space, and whether the clay is in the calcium or sodium form. As examples of publications considering the influence of calcium chloride on clays Sjoblom et al. (1999) noted the slowness of water to penetrate compacted bentonite (Na-montmorillonite is its major constituent), but that a calcium chloride solution could rapidly penetrate the bentonite and allow it to be washed away. Fresh water causes compacted bentonite to swell and to produce free-surface particles by exfoliation. These particles form a gel which further closes the pores to water uptake, while the CaCl2 solution causes the exfoliated material to shrink (or at least swell less). This allows more solution to enter the pores, causing differential expansion and a lower gel strength so that the clay may be more easily washed away. 

The morphology of rubber-based nanocomposites also seems to change in the presence of compounding ingredients [89, 90]. HNBR, when melt-compounded with organo-modified sodium montmorillonite clays (o-MMTs) prior to sulfur curing, resulted in the formation of nanocomposites with exfoliated or intercalated structures. In stark contrast, under similar conditions HNBR compounded with unmodified sodium montmorillonite clays (NA) formed microcomposites [90]. This was traced to its reactivity with the sulfur in the presence of amine-type organomodifiers. 

If we refer to the MR -2R -3R diagram, the pole MR (feldspar) would become sodic as temperature increases while the mixed layered phase becomes potassic. If we consider calcium in this system, it will not form a feldspar as does sodium and must enter into solution or be precipitated as carbonate when montmorillonite layers decrease in the mixed layered phase. In either event the net effect is to reduce the potassium part... 

The adsorbability of monovalent and divalent ions on either the sodium or calcium form of montmorillonite decreases with supporting electrolyte concentration approximately as expected from... 

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