hydration and phase behavior of swelling clays

Gel Phases: Probing Nematic Order

With ~3 weight % clay in NaCl solution, several strata form (sediments, gels, sols). We measure the scattered x-ray intensity in two different orientations at different positions in the sample tube, obtaining "powder" diffraction patterns that yield information about particle alignment. This is a direct, powerful technique which is sensitive to liquid crystalline order.

Peak positions and widths:

From the position of the (00L) peaks we find c = 15 Å, the same swollen state obtained in our solid samples at T < 20°C. The peak widths indicate particle thicknesses of 300--1600 Å. Slight broadening of the higher order peaks suggests some strain (dc/c~1.5%). Unlike some other clays, Na-Fh evidently does not hydrate further or exfoliate in solution.

1. Diffraction pattern through gel samples, showing the clay Bragg peaks and the glass/water scattering background. 2. The dependence of peak width on scattering angle suggests a modest strain (Williamson and Hall, Acta Met. 1 (1953) 22).

Particle anisotropy observed:

The clay (001) Bragg peak is perpendicular to the plane of the flat platelet. Scattered intensity from particles which are lying flat can be compared to that of vertical plates. For example, we can see that between the "gel" and "sol/gel" phases identified above, the population of horizontal platelets is greater and that of vertical platelets, smaller. This is a direct observation of particle reorientation.

This phase diagram summarizes the distinct regions found in the Na-Fh/NaCl system. At low salt concentrations, an isotropic sol exists which is unstable at higher ionic strength. The stable gel phases have nematic order, which our experiments observe directly. The figures below show the detailed scattering data that provide information about each phase, including the evidence for anisotropy that allows us to identify the gels as nematic phases.

3. Comparison of raw intensity at the (001) peak position for two different particle orientations: vertical (solid line) and horizontal (dashed line). Scans are shifted for clarity. 4. Phases identified through x-ray scattering. The isotropic sol/gel was found to be unstable, flocculating and condensing over a five month period.

5a. When the particles are oriented randomly, intensity does not depend on the scattering configuration. An example of such an isotropic region is the sol/gel (a liquid that slowly became more viscous) found only at low salt concentrations. 5b. In anisotropic phases such as these gels, intensity differs in the two geometries. Note that the only vertical plates seen in this experiment are those with the correct azimuthal orientation: therefore the anisotropy is probably greatly underestimated. 5c. The 0.008 M solution appears to be unique, with a much smaller and position dependent anisotropy. This suggests that aligned domains are small. An advantage of x-ray diffraction over optical methods is that the overall homogeneity can be assessed. 5d. We also obtain information from the peak widths in each phase. The narrower peaks (FWHM~0.04°, or particles 1600 Å thick), are found for the gels at high ionic strength. At lower salt concentrations, thinner particles predominate in the suspensions.

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

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


		With ~3 weight % clay in NaCl solution, several strata form (sediments, gels, sols). We measure the scattered x-ray intensity in two different orientations at different positions in the sample tube, obtaining "powder" diffraction patterns that yield information about particle alignment. This is a direct, powerful technique which is sensitive to liquid crystalline order.
	Peak positions and widths: From the position of the (00L) peaks we find c = 15 Å, the same swollen state obtained in our solid samples at T < 20°C. The peak widths indicate particle thicknesses of 300--1600 Å. Slight broadening of the higher order peaks suggests some strain (dc/c~1.5%). Unlike some other clays, Na-Fh evidently does not hydrate further or exfoliate in solution.
1. Diffraction pattern through gel samples, showing the clay Bragg peaks and the glass/water scattering background.			2. The dependence of peak width on scattering angle suggests a modest strain (Williamson and Hall, Acta Met. 1 (1953) 22).



5a. When the particles are oriented randomly, intensity does not depend on the scattering configuration. An example of such an isotropic region is the sol/gel (a liquid that slowly became more viscous) found only at low salt concentrations.	5b. In anisotropic phases such as these gels, intensity differs in the two geometries. Note that the only vertical plates seen in this experiment are those with the correct azimuthal orientation: therefore the anisotropy is probably greatly underestimated.	5c. The 0.008 M solution appears to be unique, with a much smaller and position dependent anisotropy. This suggests that aligned domains are small. An advantage of x-ray diffraction over optical methods is that the overall homogeneity can be assessed.	5d. We also obtain information from the peak widths in each phase. The narrower peaks (FWHM~0.04°, or particles 1600 Å thick), are found for the gels at high ionic strength. At lower salt concentrations, thinner particles predominate in the suspensions.