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Seminar Talk: Do ionically charged anisometric particles have a permanent dipole?

Prof. H. Hoffmann, University of Bayreuth

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06.Feb.2014 14:00
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06.Feb.2014 15:00
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Seminarraum A1-A3, Building 04.6

The alignment of anisometric structures in electric fields was investigated with the electric birefringence method. Originally, the measurements were carried out to determine the orientation time t of the particles in order to calculate their lengths or widths (1). Surprisingly, it was observed that the electric birefringence signals showed not only the process for the alignment of the particles but a somewhat longer effect with opposite sign. This effect is called the electric birefringence anomaly. Depending on the strength of the electric field the effect can even have a larger amplitude as the first one and the sign of the total birefringence signal can be reversed (1). The two effects have a different field strength dependence. The cause for the second effect was not clear for a long time. Meanwhile it was shown that some of the plate or rodlike particles align perpendicular during the second process. This could be demonstrated by SANS measurements in electric fields (2).

The reason for the perpendicular orientation seems to be a dipole perpendicular to the particles. The dipole is probably due to the fact that the center of the fixed ionic charges on the particles deviates from the center of the counter-ions (3).

The permanent dipole can easily be detected by the reverse double pulse method. While the induced dipole along the axis of the particles follows instantaneously the electric fields and does not change the birefringence the permanent dipole has to turn around when the field is reversed. The birefringence therefore decreases first, passes through a maximumand then approaches the stationary value that is due to the first pulse.

When amphiphilic compounds are adsorbed on the surface of the particles, the anomalous signal disappears. It is likely that the disappearance is due to the reduced mobility of the counter-ions on the particles.

                                  

 (1) The anomaly of the electric birefringence, M. Angel, H. Hoffmann, H. Thurn, U. Krämer, Ber. Bunsenges. Phys. Chemie 93, 184 – 89

(2) Transient SANS studies on p-Tetrafluorethylen (p-TFE), J. Baumann, H. Hoffmann, K. Ibel, J. Kalus, G. Neubauer, Ber. Bunsenges. Phys. Chemie 93 (1989), 874 – 878

(3) Mechanistic origin of transient electric birefringence anomaly of clay mineral dispersion, S. Holzheu, H. Hoffmann, J. Phys. Chem. B 2002, 106, 4412-4418

 

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