Mechanistic origin of transient electric birefringence anomaly of clay mineral dispersion

The transient electric birefringence (TEB) anomaly is known for more than 40 years. Despite many experimental results, there is up to now no model describing the TEB anomaly on a molecular level. In this paper, we will present a systematic analysis of the effect of external parameters such as adsorption, solid or ion concentration on the anomaly. For the experiments, we use a special inversed charge clay mineral. This material shows, compared to pure clay minerals, the advantage of enhanced insensitivity to salt-induced aggregation. The results reveal that there is no indication of multiparticle processes. For the existence of the anomaly, both presence of salt and suitable surface conditions are necessary. The disappearance of the anomaly upon adsorption of nonionic substances is due to a change of permanent-like dipole direction from perpendicular to parallel to the clay mineral surface. The explanation model proposed is based on permanent-like dipole moments originating from statistical ion fluctuations leading to different numbers of condensed counterions in the Stern layers of the two sides of the clay disk. Calculated values agree well with experimental quantities. When the mobility of the counterions is high, the mechanism produces a dipole with a dominant perpendicular component. A decrease in counterion mobility leads to a larger parallel component.