hydration and phase behavior of swelling clays -- intercalation vs percolation

Hydration: Intercalation versus Percolation

Diffraction patterns from furnace pressed, partly oriented powders of Na fluorohectorite show that three different hydration states can be obtained. From the (00L) peaks, we obtain the lattice constants of c = 9.5, 12.4, or 15.0 Å, depending upon the external conditions and history. These distinct hydration states are sometimes referred to in terms of the "number of water layers" intercalated.
However, the structure is evidently more complicated during hydration. Diffraction patterns measured while lowering the temperature with the sample in humid air show that regions having different hydration states not only coexist, but change slightly as they give way to each other. For example, the peak at q = 0.5 Å^-1 shifts in position as hydration proceeds.

Intensity between peaks at q = 0.42 and 0.5 Å^-1 suggests an alternating pattern of one- and two- water molecule intercalation.

(001) peaks during hydration, showing the Hendricks-Teller stacking disorder [S. Hendricks and E. Teller, J. Chem. Phys. 10 (1942) 147].

Diffraction pattern along the stacking direction for the three hydrations states previously reported for Na fluorohectorite [P.D.Kavaratna et al, J. Phys. Chem. Solids 57 (1996) 1897]. Peaks marked q are from quartz impurities.

The complete time-dependent crystallographic picture promises to be quite involved.

However, considerable information can be obtained even from a simple interpretation of these data, because the measurements of peak intensity during hydration show how the water occupation in the clay changes with time. The figures below show selected peak intensities as temperature is stepped in 5°C intervals under humid conditions. In particular, the intensity of the "dry" peaks is proportional to the volume of dehydrated clay, and thus shows the rate at which the first hydration step occurs.

A slow cool, measured in transmission through the bulk of the sample, shows that hydration begins at 65°C with a time constant on the order of 3 to 4 hours. Then for T < 45°C, the hydration rate increases sharply. What mechanisms determine these T-dependent hydration rates? To answer this question, we compare the behavior of the surface of the sample. Diffracted intensities measured in reflection under similar conditions show that fast hydration (t <1 hour) begins at 55°C. This temperature and time scale are evidently intrinsic to the microscopic intercalation process. In contrast, hydration of the bulk sample for T > 45°C is evidently rate limited by percolation of water or vapor through the porous medium. A fast bulk cool, with T dropped quickly below 45°C, supports this idea. Our data show that hysteresis observed in temperature cycling is largely due to the percolation effect. An especially interesting direction for these experiments will be to look at clay hydration and phases under pressure, relevant to practical applications such as oil drilling through clay soils and sea beds.
Further in-situ synchrotron x-ray scattering studies will be complemented by small-angle scattering, electron microscopy, crystallography, and optical measurements.

Go back to Introduction; or on to Clay Gels: probing nematic order

_{Brookhaven National Laboratory}_{is supported under USDOE Contract #DE-AC02-98CH10886.

This site updated 10 July 2001 by Elaine DiMasi (dimasi@bnl.gov).}


	Diffraction patterns from furnace pressed, partly oriented powders of Na fluorohectorite show that three different hydration states can be obtained. From the (00L) peaks, we obtain the lattice constants of c = 9.5, 12.4, or 15.0 Å, depending upon the external conditions and history. These distinct hydration states are sometimes referred to in terms of the "number of water layers" intercalated. However, the structure is evidently more complicated during hydration. Diffraction patterns measured while lowering the temperature with the sample in humid air show that regions having different hydration states not only coexist, but change slightly as they give way to each other. For example, the peak at q = 0.5 Å^-1 shifts in position as hydration proceeds. Intensity between peaks at q = 0.42 and 0.5 Å^-1 suggests an alternating pattern of one- and two- water molecule intercalation.

		(001) peaks during hydration, showing the Hendricks-Teller stacking disorder [S. Hendricks and E. Teller, J. Chem. Phys. 10 (1942) 147].
Diffraction pattern along the stacking direction for the three hydrations states previously reported for Na fluorohectorite [P.D.Kavaratna et al, J. Phys. Chem. Solids 57 (1996) 1897]. Peaks marked q are from quartz impurities.



A slow cool, measured in transmission through the bulk of the sample, shows that hydration begins at 65°C with a time constant on the order of 3 to 4 hours. Then for T < 45°C, the hydration rate increases sharply.	What mechanisms determine these T-dependent hydration rates? To answer this question, we compare the behavior of the surface of the sample. Diffracted intensities measured in reflection under similar conditions show that fast hydration (t <1 hour) begins at 55°C. This temperature and time scale are evidently intrinsic to the microscopic intercalation process.	In contrast, hydration of the bulk sample for T > 45°C is evidently rate limited by percolation of water or vapor through the porous medium. A fast bulk cool, with T dropped quickly below 45°C, supports this idea. Our data show that hysteresis observed in temperature cycling is largely due to the percolation effect.	An especially interesting direction for these experiments will be to look at clay hydration and phases under pressure, relevant to practical applications such as oil drilling through clay soils and sea beds. Further in-situ synchrotron x-ray scattering studies will be complemented by small-angle scattering, electron microscopy, crystallography, and optical measurements.