Electric birefringence spectroscopy of montmorillonite particles

Abstract

Electric birefringence (EB) of suspensions of anisotropic particles can be considered an electrokinetic phenomenon in a wide sense, as both liquid motions and polarization of the electrical double layer (EDL) of the particles participate in the process of particle orientation under the applied field. The EB spectrum can be exploited for obtaining information on the dimensions, average value and anisotropy of the surface conductivity of the particles, and concentration and Maxwell-Wagner polarization of the EDLs. It is thus a highly informative technique, applicable to non-spherical particles. In this paper, we investigate the birefringent response of plate-like montmorillonite particles as a function of the frequency and amplitude of the applied AC electric field, for different compositions (pH, ionic strength, particle concentration) of the suspensions. The transient electric birefringence (i.e., the decay of the refractive index anisotropy with time when the field is switched off) is used for estimating the average dimensions of the particle axes, by modeling it as an oblate spheroid. The obtained values are very similar to those deduced from electron microscopy determinations. The frequency spectra show a very distinct behaviour at low (on the order of a few Hz) and high (up to several MHz) frequencies: the α and Maxwell-Wagner-O'Konski relaxations, characteristic of EDLs, are detected at frequencies above 10 kHz, and they can be well explained using electrokinetic models for the polarization of EDLs. At low frequencies, in contrast, the birefringence changes to negative, an anomalous response meaning that the particles tend to orient with their symmetry axis parallel to the field. This anomaly is weaker at basic pHs, high ionic strengths and low concentrations. The results can be explained by considering the polydispersity of real samples: the fastest particles redistribute around the slowest ones, inducing a hydrodynamic torque opposite to that of the field, in close similarity with results previously described for mixtures of anisometric particles with small amounts of spherical nanoparticles.

Optical and X-ray scattering studies of the electric field-induced orientational order in colloidal suspensions of pigment nanorods

“…Therefore, for f < f ch , where the MW contribution, A MW , is large, the field applied to the particle is strongly attenuated and the electro-optic signal is weak. We note that, in a similar way, the contributions of other low-frequency electro-kinetic processes, like the -relaxation [17], can be neglected in our case.…”

“…Aqueous suspensions of anisometric particles are well known for their strong electro-optic response [13][14][15][16][17][18][19], due to a combination of two factors: the strong coupling of the particle orientation with the applied electric field E, and the strong optical anisotropy of the nanoparticles, which is either intrinsic or related to their anisometric shape [9,20].…”

Electric Field‐Driven Control of Cholesteric Cellulose Nanocrystal Orientation and Morphology

“…[ 17 ] An anomalous negative alignment is most likely caused by the accumulation of charge on the CNC surface. [ 18 ] According to the results, the anomalous negative birefringence regime is either at high E or low f , where the suspension has a higher conductivity. The transport of ions in a high conductivity regime induces an accumulation of charges flowing either in the direction of the electric field or in the opposite direction.…”

Determination of the size distribution of non-spherical nanoparticles by electric birefringence-based methods

“…From 320 particles, a value of 1.65 μ m for the average diameter was obtained. For the calculations, the geometry was modelled by oblate spheroids with a constant aspect ratio of 3.3 46 , an approximation already discussed in the goethite case. The results are presented in Table 1 and Fig.…”

“…The electric birefringence data presented in this work have been measured in different laboratories using the standard device, based on the measurement of the changes in light transmission though an optical setup containing the suspension, which is subjected to electric field pulses 46 . The DLS measurements have been performed with a commercial apparatus (Zetasizer, Malvern Instruments (UK)) which provides the diameter distribution of the particles assuming they are spherical.…”