The electric birefringence anomaly in colloidal and micellar systems

Abstract

Rod‐like micelles of perfluorosurfactants in aqueous solution were examined by using the transient and dynamic electric birefringence method. Tetraethylammoniumperfluoroctanesulfonat shows in the overlap region of the rods between 5 mM and 15 mM up to three different relaxation times τ1, τ2 and τ3 between a few μs and several ms. A forth very long relaxation time τ4 (seconds) appears at higher concentrations. The relaxation processes with the time constants τ1, τ3 and τ4 have a negative birefringence, while τ2 has a positive sign. In the anomaly region the total birefringence can change sign as a function of the total surfactant concentration and of the field strength. The anomalous behaviour of the time and frequence dependence of the electric birefringence is explained on the basis of intermicellar interaction. It is assumed, that the external electric field induces an additional anisotropic Lorentz‐field, which changes dramatically the local Lorentz‐field around the micelles. As a result both the correlation of the center of masses and the correlation of the rodlike micelles changes. This gives rise to the processes τ1, τ2 and τ3. The largest relaxation time τ4 seems to be due to a deformation and relaxation of a temporary network, which is built by the rodlike micelles.

Electric birefringence anomaly of solutions of ionically charged anisometric particles

“…In the dilute region, only one relaxation time was observed. Surprisingly, a second effect with opposite sign appeared at the concentration at which the viscosity of the solution was still small and began to increase from the value of the water viscosity [7]. No sensible explanation could be offered for the effect.…”

“…The group of the author of this article observed the electric birefringence anomaly in 1985 in studies of viscoelastic micellar solutions which are formed from entangled wormlike micelles [7]. The anomaly effect had already been observed before on other colloidal systems.…”

“…The micelles grow in length with increasing surfactant concentration, and at some critical concentration begin to overlap. In the overlap region it is difficult to determine the length of the micelles [7].…”

“…When the structures in the investigated solutions were ionically charged, usually a second effect with opposite sign, and with a rotation time that is a factor of three to ten longer than the first one appeared. This signal is called the electric birefringence anomaly [7,[11][12][13].…”

“…Electric birefringence measurements have been made on different colloidal systems like on polymer and polyelectrolyte solutions, on micellar and clay solutions, on microemulsions and on fibre dispersions [5][6][7][8][9][10] and on virus solutions [11]. When the structures in the investigated solutions were ionically charged, usually a second effect with opposite sign, and with a rotation time that is a factor of three to ten longer than the first one appeared.…”