Electric birefringence anomaly of solutions of ionically charged anisometric particles

Hoffmann, H.; Gräbner, D.

doi:10.1016/j.cis.2014.11.006

Cited by 12 publications

(18 citation statements)

References 49 publications

(55 reference statements)

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“…5a and 6a). Such an effect has been previously reported for a number of materials, 10,[14][15][16][23][24][25][26] always for DC or slowly oscillating fields, and cannot be explained by classical electrokinetic theories. The features of the anomalous effect for sodium montmorillonite samples can be summarized as follows.…”

Section: Low-frequency Anomalous Birefringencementioning

confidence: 86%

“…In the Hz range the birefringence presents a negative value that is not predicted by classical electrokinetics, and as found by other authors working with sodium montmorillonite and similar clays. 14,16,23,24,26 Note that the negative values correspond to an anomalous orientation of the particle, with its axis of symmetry along the electric field. The characteristic frequency of this process (ν c ) is located at around 7 Hz.…”

Section: General Featuresmentioning

confidence: 99%

“…Moreover, the fulfillment of Kerr's law is not compatible with the existence of a saturable induced dipole moment. In order to explain this concentration dependence, a particleconcentration induced mechanism that gives rise to an overall perpendicular dipole moment was proposed in 16 to account for the anomalous orientation of laponite and hectorite. There it was argued that, for high concentrations, the overlapping of the EDLs of the edges of adjacent particles diminishes the polarization along the particle surface, leaving unperturbed the perpendicular component.…”

Section: Low-frequency Anomalous Birefringencementioning

confidence: 99%

“…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. In dilute suspensions, negative birefringence has been recently reported for a variety of materials, 10,15,16,25,26 and in most cases it cannot be explained theoretically inside a classical framework.…”

Section: Introductionmentioning

confidence: 99%

“…2,5,6,[10][11][12][13] However, although the foundations of the electric birefringence have been long-established, the process is hitherto not understood on the whole and cannot yet be considered as a widespread characterization technique. 12,[14][15][16] Thus, microscopic models relating the nanoparticle properties to their macroscopic birefringent response are to date not completely satisfactory. 6,8,12,13 For instance, only simplified geometries are considered; the effect of the polydispersity of the sample and the presence of interparticle interactions are taken into account partially if at all; the effect of the hydrodynamic torque is normally neglected 13 and the contribution of a heterogeneous charge distribution is still an open question.…”

Section: Introductionmentioning

confidence: 99%

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Electric birefringence spectroscopy of montmorillonite particles

et al. 2016

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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.

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