COLL 441 |
| Tommaso G. Bellini, Dpt Chimica, Biochimica e Biotecnologie per la Medicina, University of Milano, via Cervi 93, Segrate (MI), 20090, Italy and Francesco Mantegazza, Dpt Medicina Sperimentale, University of Milano "Bicocca", via Cadore 48, Monza (MI), 20052, Italy. |
| For over 60 years encompassing about 100 reports, "anomalous birefringence" has been observed in concentrated aqueous solutions of nonspherical polyelectrolytes: their electro-optic susceptibility (e.g., electric birefringence) changes its sign upon increasing the particle concentration. This indicates that at high concentration the particles align, on average, their longest axis perpendicular to the electric field – without satisfactory explanation. In this work we present an experimental and theoretical investigation of the phenomenon aimed at giving a common denominator to the whole body of recent and older observations. At variance with previous understanding, we show that the origin of this effect can be described in terms of single particle mechanisms, and we propose a theoretical model to interpret the results. Anomalous birefringence is a pseudo-collective electrokinetic phenomenon: the high concentration of polyelectrolytes effectively modifies the mobility of co-ions and counter-ions, in turn changing the anisotropy of the polarizability of each single particle. |
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Colloidal and Molecular Electro-Optics
Division of Colloid and Surface Chemistry |