Structural analysis of clay particles in water under shear and the effect of additives
National Institute for Materials Science, Hiroshi Sakuma

1. Introduction
Clay minerals have a high affinity with organic molecules and these minerals can be a good adsorbent of toxic
molecules in water. The toxic molecules adsorbed on clay minerals are recovered by filtration, however, small
clay nanoparticles make a clogging problem. Improving the water permeability in filtration can be realized byforming microparticles of clay minerals. The morphology of microparticles alters the water permeability andtherefore the control of the morphology can improve the permeability. Clay microparticles have been fabricatedby spray drying method. We found that “Doughnut”-like clay microparticles can be fabricated by adding
tetrasodium pyrophosphate in the clay dispersion, while spherical microparticles are formed without the additive[1]. The mechanism of morphological alteration is, however, unclear. The network structure of clay particles inwater before spray drying may be a clue for understanding the reason why the morphology of microparticles
changed by the presence of additives.
The network structure of clay (Laponite RD, LRD) dispersion without any additives was studied by small angle
neutron scattering (SANS) coupled with rheological measurements (Rheo-SANS) and scanning electron-assisted
dielectric microscopy (SE-ADM) [2]. The SE-ADM revealed that 100-300 nm scale periodic structure which canbe interpreted by the periodic aggregation of clay particles. No significant difference of SANS profiles wasobserved under high shear rate of 1000 s−1, indicating that structure from 10 to 100-nm scale was preserved under
the shear rate. These properties of clay dispersion may be altered by the presence of additive. In this study, thestructure of hectorite dispersion with tetrasodium pyrophosphate was analyzed by the Rheo-SANS method toreveal the difference of network and microscopic structure of clay dispersed in water with or without the additive.
2. Experiment
The network structure of clay dispersion was studied by Rheo-SANS installed on BL15 (TAIKAN) at Materials
and Life Science Experimental Facility (MLF) in Japan Proton Accelerator Research Complex (J-PARC).
Commercially available synthetic hectorite with additive of sodium pyrophosphate, Laponite-RDS® (LRDS), wasdispersed in deuterated water (D2O) using a magnetic stirrer over 24 hours. The SANS profiles of 2 and 11 wt%
LRDS dispersions were measured under shear. The experimental apparatus and conditions were the same as ourprevious experiments [2].
3. Results
Rheology of LRDS dispersions: The flow curves of 2 and 11wt% LRDS dispersions were shown in Fig. 1. No yield point
was observed for both curves. The viscosity of 11 wt%
dispersion slightly decreased at high shear rate. The curve of2 wt% LRDS dispersion was similar to that of Newtonianfluid. The viscosity of 11 wt% LRDS was larger than that of 2wt% LRDS; however, the clay particles in both dispersions
seem not to form aggregations.
Effects of shear rate and shear direction: The SANS profiles
1Fig. 1. Shear stress as a function of shear rate
for 2 and 11 wt% LRDS dispersion. The
arrows indicate the direction of shear rate.

of 11 wt% LRDS dispersion were plotted in Fig. 2. No significant difference was observed among shear rates,
shear directions, and sector-averaged two-dimensional (2D) SANS profiles for horizontal (Ix) and vertical (Iy)directions. The difference at high Q > 0.1 Å−1 depending on the shear directions is ascribed to the low S/N ratio oftangential measurements owing to the small beam size (0.5mm × 10 mm) compared with that of radial measurements(10 mmΦ).
Effects of clay concentrations: The SANS profiles
depending on the clay concentrations were plotted in Fig. 3.
To compare the shape of the SANS profiles between 2 and11 wt%, the intensities were normalized by the clay
concentrations. No significant difference was observed forthe gradient at Q > 0.03 Å−1. The well-fitted theoreticalcurve to the profile of 2 wt% dispersion indicates that thepresence of monodispersed circular plate of 125 Å inradius and 10 Å in thickness which are similar with that of
LRD dispersion [2]. The difference at low Q (<0.03 Å−1)
would reflect the strong excluded volume effect in 11 wt%
dispersion due to the clay particle-particle interaction.
Further analysis is required for revealing the precise

Fig. 2. SANS profiles of 11 wt% LRDS
depending on the shear rates (0 and 1000 s−1),
shear

directions(lines:radial;symbols:tangential), and sector-averaged 2D profiles(horizontal: Ix, vertical: Iy).

interparticle interactions.
4. Conclusion
The profiles at low Q would reflect the interparticle
interactions depending on the concentration. These profiles
seem to be different from the highly concentrated LRD
dispersion reported by our previous study [2]. In future
works, the SANS profiles of LRDS dispersion as afunction of clay concentrations would be useful forunderstanding the difference of interparticle interactions inwater depending on the clay species.
Fig. 3. SANS profiles of LRDS depending on theconcentrations (2 and 11 wt%) at the radial shear
5. References[1]H.

Sakuma,direction. The effect of incoherent scattering
K.

Tamura,K.

Minagawa,“Doughnut”-like clay microparticles fabricated using afrom hydrogen included in the clays wereexcluded from the experimental SANS profiles.

hybrid method of spray drying and centrifugal discatomization, Chemistry Letters, 47 (2018) 68-70.
[2] H. Iwase, T. Ogura, H. Sakuma, K. Tamura, Y. Fukushima, Structural investigation of hectorite aqueous
suspensions by dielectric microscopy and small-angle neutron scattering coupling with rheological measurement,
Applied Clay Science, 157 (2018) 24-30.

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