Intercalation-enhanced electric polarization and chain for- mation of nano-layered particles

– Microscopy observations show that suspensions of synthetic and natural nanolayered smectite clay particles submitted to a strong external electric field undergo a fast and extended structuring. This structuring results from the interaction between induced electric dipoles, and is only possible for particles with suitable polarization properties. Smectite clay colloids are observed to be particularly suitable, in contrast to similar suspensions of a nonswelling clay. Synchrotron X-ray scattering experiments provide the orientation distributions for the particles. These distributions are understood in terms of competing (i) homogenizing entropy and (ii) interaction between the particles and the local electric field; they show that clay particles polarize along their silica sheet. Furthermore, a change in the platelet separation inside nano-layered particles occurs under application of the electric field, indicating that intercalated ions and water molecules play a role in their electric polarization. The resulting induced dipole is structurally attached to the particle, and this causes particles to reorient and interact, resulting in the observed macroscopic structuring. The macroscopic properties of these electrorheological smectite suspensions may be tuned by controlling the nature and quantity of the intercalated species, at the nanoscale. In this letter we study colloidal suspensions of electrically-polarizable particles in nonconducting fluids. When such suspensions are subjected to an external electric field, usually of the order of 1kV/mm, the particles become polarized, and subsequent dipolar interactions are responsible for aggregating a series of interlinked particles that form chains and columns parallel to the applied field. This structuring occurs within seconds, and disappears almost instantly when the field is removed [1–5]. It coincides with a drastic change in rheological properties (viscosity, yield stress, shear modulus, etc.) of the suspensions [6], which is why they are sometimes called electrorheological fluids (ERFs). This makes the mechanical behavior readily controllable by using an external electric field [1–7]. Particle size has a quite diverse impact on the behavior of ERFs [8]. The nature of the insulating fluid and of the colloidal particles determines the electrorheological behavior of the suspensions. The mechanism is not fully understood yet, but it is mainly triggered by the so-called interfacial polarization, © EDP Sciences 2 EUROPHYSICS LETTERS and requires electric anisotropy of the particles [9]. Consequently, particle shape [10] and surface properties [11] can also be critically important, as dielectric properties largely depend on them. Clays as traditional material have played an important role throughout human history. Their common modern uses include nano-composites, rheology modification, catalysis, paper filling, oil well -drilling and -stability, etc [12]. Smectite (or 2:1) natural clay particles dispersed in salt solutions have been studied for decades [12], and recently there has been a growing activity in the study of complex physical phenomena in synthetic smectites [13]. Much effort has gone into relating the lamellar microstructure of smectite clay-salt water suspensions to their collective interaction and to resulting macroscopic physical properties, such as phase behavior and rheological properties [13–19]. Nematic liquid crystalline-like ordering in smectite systems have been characterized by the observation of birefringent domains with defect textures [14, 16, 18, 19] or by Small-Angle X-Ray Scattering [16, 18]. Wide Angle X-Ray Scattering (WAXS) studies of the well-characterized synthetic smectite clay fluorohectorite, in water suspensions [17], show that fluorohectorite particles suspended in water consist of ”decks of cards” like crystallites containing about one hundred 1 nanometer-thick platelets. Fluorohectorite has a rather large surface charge of 1.2 e-/unit cell, as compared to synthetic smectites such as laponite (0.4 e-/unit cell) [20]; this high surface charge explains why clay stacks stay intact when suspended in water, unlike laponite or the natural clay montmorillonite. The natural smectite illite, on the other hand, behaves much like fluorohectorite in this respect [21]. One or more mono-layers of water may be intercalated into such ”deck of cards” clay particles depending on temperature and relative humidity. For fluorohectorite, the dependence on those two parameters has already been mapped for hydration and dehydration by means of synchrotron X-ray scattering techniques [22]. However, the spatial configuration for the intercalated water molecules, with respect to the silica sheets and to the intercalated cations, is not precisely known yet. We show in this letter that the strong electro-rheological behavior exhibited by suspensions of smectite clay particles in silicon oil can be attributed to the intercalated species. We studied four types of smectite suspensions in oil: Firstly three suspensions based on fluorohectorite (see [17] for its origin and chemical formula), and secondly a mixed natural quick clay from the Trondheim region in Norway. The three types of fluorohectorite samples differ by the nature of the exchangeable cation, which is either mono(Na), di(Ni) or tri-valent (Fe). These synthetic samples are polydisperse with wide distributions of sizes (diameter up to a few micrometers, stack thicknesses around 100 nm) and aspect ratios. The quick clay is far less well-characterized than the synthetic clays, although from preliminary X-ray diffraction analysis we know that this mixed natural clay contains considerable amounts of illite and other natural smectite particles, in addition to the non-smectite clay kaolinite. The samples were initially prepared at ambient temperature by adding 1.5% by weight of clay particles to the silicon oil Rotitherm M150 [23] (viscosity 100cSt at 25°C). The experiments were performed using the supernatant of the oil suspensions, after sedimentation of the heavier particles. When placed between copper electrodes between which a sufficiently large electric field is applied (field strengths above 500 V/mm), the four types of samples exhibit the dipolar chain formation characteristic of electrorheological fluids (see Fig. 1). The sample cell used for these observations consisted of two parallel and identical 1/2 mm thick copper electrodes separated by a gap of 2 mm and glued onto a transparent quartz glass microscope slide. The gap between the electrodes was closed at its ends by a non-conducting plastic material. The top part of the cell was open, and the sample cell was mounted horizontally, with the microscope slide flat down. A small volume (< 1 ml) of the prepared sample was added and studied at J. O. Fossum et al.: Electric polarization and chain formation of clay particles3 Fig. 1 – Microscope images of electrorheological chain formation in oil suspensions of smectite clays. (a) Na-fluorohectorite (b) Ni-fluorohectorite (c) Fe-fluorohectorite (d) Natural quick clay. ambient temperatures. The sample was illuminated from below, and observed from above in a stereomicroscope. An electric field E ∼ 500 V/mm was applied between the copper electrodes, and the changes in the sample were recorded by means of a digital camera connected to a PC. The process resulted in all clay particles being part of the electrorheological chain bundles after 10 to 20 s, and no motion being visible within the sample in less than 1 min. The critical electric field necessary to trigger the electrorheological behavior was found to be Ec ≃ 400 V/mm. The procedure was then repeated using a suspension of kaolinite, a 1:1 natural clay which, in contrast to smectite clays, does not spontaneously intercalate cations and water molecules in-between its silica sheets. Kaolinite particles in their natural state have been reported in the literature to exhibit a weak electrorheological behavior when suspended in a silicon oil [24]. Microscopy observations of kaolinite suspensions with the same density as the smectite suspensions exhibited electrorheology, but only for E & 2 kV/mm. They formed with characteristic time 10 to 100 times larger than that of the smectite suspensions. Furthermore, the bundle structure formed by the kaolinite suspensions appeared much less ordered than those observed with the smectites. Relative orientations of the smectite particles inside the electrorheological chains were determined using synchrotron X-ray scattering experiments: chain and column formations were observed by means of a video camera, while simultaneously recording X-rays scattered by the clay crystallites. These scattering experiments were performed at the Swiss-Norwegian Beamlines (SNBL) at ESRF (Grenoble, France), using the WAXS setup with a 2D mar345 detector at beamline BM01A. The sample cells used for these experiments differ from those described above in that the electrodes were placed vertically, the cells being closed at the bottom and with their top open, which allows to partly fill the cell with sample from above (see a sketch of the experiment in Fig. 2). Fig. 3 shows three different two-dimensional diffractograms. Fig. 3(a) is obtained from a suspension of Na-fluorohectorite prior to the application of an electric field. Each lamellar clay particle may be regarded as a single crystallite, and the particles are randomly oriented inside the sample, so the diffractogram is isotropic. The broad outermost ring is due to scattering from the silicon oil (characteristic length d ∼ 6.9-7.9 Å), whereas the narrow symmetric ring at lower scattering angles is the (001) Bragg peak from the lamellar clay stacks with 1 water layer intercalated (d ∼ 12.3 Å). In the presence of an electric field (Fig. 3(b)), in contrast, the (001) Bragg peak has become anisotropic due to particle orientation in the field. In addition, the number of water layers intercalated was determined directly from the positions of the Bragg peaks in reciprocal space (as in [25]). Fig. 3(c) shows the diffractogram of a suspension of natural quick clay, for E > Ec. Three anisotropic scattering rings are visible: the 1st order 4 EUROPHYSICS LETTERS Fig. 2 – Sketch of the synchrotron X-Ray scattering experiments. Corresponding diffractograms are shown in Figs. 3(b) and 3(c). The magnified area shows a single nano-layered clay-particle inside a dipolar chain, with an arrow indicating the direction of the dipole moment induced by the external