Montmorillonite alignment induced by magnetic field: Evidence based on the diffusion anisotropy of water molecules
Graphical abstract
Alignment of Na-montmorillonite particles in magnetic fields () was revealed based on the diffusion anisotropy of water detected by pulsed-field gradient spin–echo (PGSE) NMR method.
Introduction
Na-montmorillonite (Na-mon) is an interesting inorganic material, because its nanolayered structure and platelike morphology are substantially useful for synthesizing inorganic–organic hybrid materials and nanocomposites [1]. Recently, much attention has been paid in colloidal science to behavior of clay particles in aqueous media [2], [3], [4], [5], [6]. This is due to clay minerals generally showing characteristics such as liquid crystal phases classified into nematic, smectic, and columnar depending on the degree of structural order [2]. Mineral liquid crystals have some advantages that are not obtained from conventional liquid crystals synthesized using organic substances [3]. Clay alignment in the nematic phase is of great interest and has been studied by a synchrotron X-ray scattering method [4] and by polarized light spectroscopy [5].
Nuclear magnetic resonance (NMR) spectroscopy has also been used as a useful tool to understand the orientational ordering produced in liquid crystal droplets [7]. Among various kinds of NMR measurements, the pulsed-field gradient spin–echo (PGSE) method is a reliable technique for determining the self-diffusion coefficients of water molecules, cations, and anions in various liquid phases and porous media [8], [9]. Determination of diffusion coefficients in three orthogonal directions makes it possible to infer the microstructure in materials or in tissues in in vivo systems [10], [11]. The PGSE method is also effective for studying microstructures in clay suspensions, because the diffusion distance of water molecules observed by the PGSE method is in the micrometer order region, and the observed diffusion phenomenon reflects porous structures formed by clay particles. In recent years, the PGSE method was successfully applied to understanding the alignment of clay particles in the nematic phase [12], [13]. On the other hand, the space arrangements of clay particles in dilute suspensions have not been studied sufficiently, because it has been believed that clay particles are randomly distributed in dilute suspensions [2].
However, it is expected that random arrangement should be depressed in a strong magnetic field, because layer silicates have been known to show magnetic alignment under approximately 2 T [14], [15], [16], [17]. Although it has been theoretically proposed that the anisotropy in the magnetic susceptibility of the material controls its alignment in magnetic fields [16], the alignment of Na-mon in the magnetic field has not been reported. This is probably due to the swelling of Na-mon with water adsorption. As shown by Kuchel et al. [18], the PGSE method should be powerful for estimating the alignment of particles even in swelling samples.
Therefore, in this study, the PGSE method was applied to the observation of diffusion behavior of water molecules in relatively dilute Na-mon suspensions. Effects of the static magnetic field and temperature on diffusion behavior were also examined. Based on these results, we discuss the alignment of Na-mon particles exposed to magnetic fields.
Section snippets
Sample preparation and equipment
Na-mon was supplied by the Clay Science Society of Japan (JCSS-3101). Na-mon suspensions were prepared by putting the Na-mon particles and ion-exchange water in bottles under atmospheric conditions. These bottles were quickly sealed. To examine the effect of adsorption of CO2, the pH values for suspensions were determined. As a result, no distinct decrease in pH values was observed even after 72 h.
The Na-mon particles in the suspensions were dispersed, using an ultrasonic processor (27 kHz, 150
Diffusion anisotropy of water molecules determined by the PGSE method
The echo attenuations for the x and z directions of three representative samples are shown in Fig. 2. The variation for the y direction was identical with that for the x direction. The linear relationship of the plots in Fig. 2 indicates the existence of one diffusion component of water in the observational timescale of the PGSE method. Usually, one experiment was completed within 5 min. In order to confirm the uniformity in the Na-mon suspensions during PGSE experiments, the echo attenuations
Conclusion
The diffusion anisotropy was observed by the PGSE method in relatively dilute Na-mon suspensions; that is, the diffusion in the z direction is explicitly faster than that in the x and y directions. This result indicates that Na-mon particles exposed to strong magnetic fields are aligned with their basal planes parallel to the static magnetic field and their nanoorder crystalline c-axes perpendicular to it. The PGSE method provided crucial and definite evidence for the alignment of Na-mon
Acknowledgements
We thank Professor Chiaki Uyeda (Osaka University) and Professor Katsuyuki Kawamura (Tokyo Institute of Technology) for their kind advice and helpful discussions with respect to magnetic alignment of sheet silicates. We also deeply appreciate the reviewers, whose comments were very useful in improving our manuscript.
References (33)
- et al.
Curr. Opin. Colloid Interface Sci.
(2005) Physica A
(1999)- et al.
Electrochim. Acta
(2004) - et al.
Magn. Reson. Imaging
(2003) - et al.
Colloids Surf. A Physicochem. Eng. Asp.
(1999) - et al.
Physica B
(1992) - et al.
J. Magn. Reson.
(2000) - et al.
J. Magn. Reson.
(1999) - et al.
J. Colloid Interface Sci.
(2004) - et al.
Nano Lett.
(2002)