Electric-Field-Induced Perfect Anti-Nematic Order in Isotropic Aqueous Suspensions of a Natural Beidellite Clay

Abstract

We study the electric-field-induced birefringence and orientational order in the isotropic phase of aqueous suspensions of exfoliated natural beidellite clay particles, thin (L = 0.65 nm) flat charged sheets with high aspect ratio, D/L ≈ 300. Our electric birefringence experiment is optimized for aqueous suspensions of colloidal particles, with a high frequency a.c. electric field, ν ≈ 1 MHz, applied by two external electrodes to a thin flat sample, sealed in an optical capillary. In isotropic and biphasic samples, we observed strong field-induced birefringence Δn(E), saturating at moderate E(sat) field to a plateau Δn(sat) proportional to the volume fraction ϕ. The field-induced order parameter S(E) is negative and saturates to S(sat) = -0.5 above E(sat). This corresponds to a perfect "anti-nematic" order, i.e. the normals of the beidellite particles are perpendicular to the field, without any preferred azimuthal direction. The measured specific excess polarizability ΔA(sp) is among the highest data reported for other strongly anisometric dielectric and metal particles. We explain the high ΔA(sp) value with the strong induced polarization of the electric double layer of counterions at the charged particle/electrolyte interface. The estimated equivalent conductivity of the beidellite particle K(eq) = 2 K(σ)/L is several orders of magnitude larger than the bulk conductivity of the electrolyte K(e), resulting in a metal-like behavior of the beidellite disks under field. In the isotropic regions of biphasic nematic/isotropic samples, the excess polarizability is further enhanced by an order of magnitude, indicating collective reorientation of the particles. We propose that this enhancement might be due to pretransitional fluctuations of the spontaneous nematic order S(N) of the colloidal suspension and/or formation of chains of particles, with antinematic order of the beidellite disks in the chains.

Electric birefringence spectroscopy of montmorillonite particles

“…At larger fields, deviation from this law due to saturation of the birefringence is observed, in agreement with the data reported elsewhere 22 where it was found that linearity with E 2 0 is lost above 20 V/mm for similar clays. Hereafter all experiments are performed with an electric field of 10 V/mm and hence, the Kerr constant can be properly used.…”

“…For example, at 1 MHz, the value of the electrical polarizability is α e a = −5.49 × 10 −28 Fm 2 along the symmetry axis and α e b = 2.41 × 10 −27 Fm 2 along the particle surface. In this case, the polarizability anisotropy per unit volume, ∆α e /V p = 3.7 × 10 −9 F/m, is comparable to values obtained elsewhere 14,22 for similar materials.…”

“…41 , where a mixture of kaolinite, illite and smectite was analysed and a surface conductivity of K σ = 0.25 × 10 −8 S was reported. In addition, Dozov et al 22 calculated K σ = 0.5 × 10 −8 S for beilledite particles. This corresponds to a larger value for K e f f (15 S/m in the latter case), that is, to highly conductive particles.…”

“…[19][20][21][22] Moreover, it has been found that suspensions of sodium montmorillonite and similar clays exhibit a negative (anomalous) birefringence, 14,23,24 sparking interest in the phenomenon. The mentioned anomaly consists in the tendency of the particles to orient under some conditions with their major axis perpendicular to the external field, instead of parallel, as in "normal" EB.…”

How the hydro-gel flocculation microstructure changes

“…This effect was recently described in clay suspensions [47][48][49][50][51][52], in which the liquidcrystalline character is evidenced by polarised light microscopy and small-angle X-ray scattering. These experiments on clay-based gel samples reveal strong positional and orientation orders of the particles, proving unambiguously the nematic character of the clay mineral-rich gel.…”