Dynamical X-Ray Microscopy Studies
of Clay Mineral Particles in Aqueous Media
T. Preis1 and J. Thieme2
1
Institut für Physik, Johannes-Gutenberg-Universität Mainz, Staudingerweg 7,
D-55099 Mainz, Germany
E-mail: preis@dipmza.physik.uni-mainz.de
2
Forschungseinrichtung Röntgenphysik, Georg-August-Universität Göttingen, Geiststraße 11,
D-37073 Göttingen, Germany
E-mail: jthieme@gwdg.de
Abstract. X-ray microscopy allows to investigate colloidal particles in their natural
aqueous environment with high resolution. One example from soil science is the
study of aqueous clay mineral dispersions. The concentration of cations in the
dispersion medium strongly influences the structure of the aggregates formed by
clay particles. This influence has been studied by visualizing the changes in shape,
size and structure of a single aggregate after the addition of cations. Four X-ray
micrographs illustrate the behaviour of Na+-montmorillonite particles after the
addition of Fe3+-ions. Thus aggregation phenomena of colloidal particles have been
observed directly at one single aggregate within its aqueous environment.
1 Introduction
Many chemical reactions in soils are affected by their internal surface. Due to their
high specific surface area, clay minerals are one of the most reactive species in soils.
Concerning the chemical conditions in soils, the clay mineral particles form large
aggregates which are major constituents of a micro pore system. Figure 1 shows an Xray micrograph of a 0.1% dispersion of Na+-montmorillonite in bidistilled water.
Fig. 1. X-ray micrograph of a 0.1% dispersion of Na+-montmorillonite in bidistilled water.
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Shape, size and the internal structure of these aggregates depend on the ionic
concentration of the dispersion medium [1]. In fig. 2 X-ray micrograph of a 0.1%
dispersion of Na+-montmorillonite in bidistilled water after the addition of Fe3+-ions is
shown.
Fig. 2. X-ray micrograph of a 0.1% dispersion of Na+-montmorillonite in bidistilled water
after the addition of Fe3+-ions.
To study this effect, a direct visualization of clay mineral particles in their natural
aqueous environment is desirable. This is impossible both with light and electron
microscopy. Due to the size of the structure formed by clay mineral particles the
resolution of light microscopy is insufficient. For imaging with electron microscopy
the samples have to be dried and stained. This preparation may introduce changes to
the shape and the structure of the clay aggregates.
Due to its principle and setup, X-ray microscopy is well suited to solve this
problem. X-ray microscopy provides a higher resolution than light microscopy. The
radiation above λ = 2.34 nm ( near the K-absorption edge of oxygen ) is weakly
absorbed by water compared to other materials like silicates, and gives a good
amplitude contrast of clay mineral particles in aqueous media.
X-ray micrographs have already been taken to show the ability of X-ray
microscopy to visualize directly structures in clay mineral dispersions. However it is
highly desirable to demonstrate the influence of the chemical conditions of the
dispersion medium on size, shape and the internal structure of these aggregates by
changing the ionic concentration while visualizing one single clay mineral aggregate.
2 Methods
All images are taken with the X-ray microscope of the Forschungseinrichtung
Röntgenphysik installed at the electron storage ring BESSY in Berlin, using
synchrotron radiation with a wavelength λ = 2.34 nm [2].
Dynamical X-Ray Microscopy Studies of Clay Mineral Particles
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The X-ray microscope is operating in amplitude contrast mode, imaging the object
by a high resolution zone plate with an outermost zone width of drn= 38 nm [3]. The
image is recorded by a CCD-camera [4].
The clay dispersion is held in an object chamber (Fig. 3) [5]. A capillary tube
placed into the center of the object chamber was connected to a syringe to inject a
solution next to the clay mineral aggregates under investigation [6].
X-Ray Window +
Monochromator Pinhole
Object Chamber
Syringe 2
Micro Capillary Tube
Micro Zone Plate
CCD-Camera
Condensor
Zone Plate
Clay Mineral
Dispersion
Elastic Foils
on VA-Metal Rings
Monitor
Syringe 1
Fig. 3. Sketch of the object chamber.
A 0.1% dispersion of Na+-montmorillonite ( from Upton, Wyoming ) in bidistilled
water is brought into the object chamber. The thickness of the dispersion layer is
adjusted to about 10 µm by pulling syringe 1.
Figure 4a shows the X-ray image of aggregates within the 0.1% dispersion of Na+montmorillonite before injection of the FeCl3-solution. By pushing syringe 2, a small
amount of a 1% FeCl3-solution is injected next to the aggregates. It was injected into
the object chamber over the first 3 minutes and was again injected 20 minutes later for
a duration of 5 minutes. Figures 4b-4d show the behavior of these aggregates after
injection of the FeCl3-solution. Figure 4b was taken 22 minutes after the second
injection. After another 30 minutes the solution was again injected over 3 minutes.
Figures 4c and 4d were taken 10 minutes and 30 minutes later, respectively.
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T. Preis and J. Thieme
Fig. 4. X-ray micrographs of a single aggregate of Na+-montmorillonite in bidistilled water
before and after the addition of FeCl3 –solution.
3 Results
Elementary montmorillonite layers, tactoids and aggregates are found in the Na+montmorillonite dispersion [7]. The thin elementary silicate layers (≤ 1nm) are not
resolved in the X-ray image. The smallest particles are tactoids consisting of a stack of
elementary silicate layers. The aggregates shown in figure 4a are formed by these
tactoids.
Due to the injection of the FeCl3-solution new tactoids are formed by face to face
aggregation of the silicate layers. These tactoids form new structures or connect the
already existing aggregates (Fig. 4b-4d). Both effects should have a strong influence
on the rheological properties of the dispersion.
In order to prove that changes are caused by the Fe3+-ions, the experiment was
repeated by injecting bidistilled water instead of FeCl3-solution into the object
chamber. In fig.5 a 0.1% dispersion of Na+-montmorillonite in bidistilled water is
shown before (left) and 135 minutes after (right) this injection. Changes in shape and
structure of the aggregate cannot not observed.
Dynamical X-Ray Microscopy Studies of Clay Mineral Particles
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Fig. 5. Na+-montmorillonite aggregate in bidistilled water before (left) and after (right) the
addition of bidistilled water.
Acknowledgements
The study of colloidal particles in their aqueous environment is a collaboration with J.
Niemeyer, University of Trier.
This work has been supported by the Federal Minister for Education and Research
under the contract number 055MGDXB6. We would like to thank the BESSY staff for
excellent working conditions.
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