Electric birefringence and streaming-electric birefringence of synthesized imogolite: The anisotropy of electric polarizability

Imogolite synthesized from sodium orthosilicate and aluminum trichloride was fractionated into four fractions by centrifuging at 12,000 × g (1 h). The supernatant, which did not deposit by three times centrifugations, was used for all measurements. The signal of birefringence under a reversing electric pulse showed that the permanent dipole moment is negligibly small at low fields. The electric birefringence under a rectangular electric pulse is positive. The saturated value is proportional to the concentration of imogolite in the range of 0–0.1 mg/ml and decreases rapidly with an increase of added salt concentration for NaCl and AgNO3. It slightly depends on the pH of solution and is biggest in pure water. Then we have determined the anisotropy of electric polarizability (Δα) for imogolite in pure water at 0.05 mg/ml. Δα we obtained from the method decreases rapidly at low fields and slowly at high fields as shown in references [M. Matsumoto, Colloids Surf. A 148 (1999) 75, M. Matsumoto, Biophys. Chem. 58 (1996) 173]. It is approximately shown by the following expression, Δα = Δα∞ + (Δα0 − Δα∞)/(1 + KE), (Δα0:Δα at E = 0, Δα∞:Δα at E = ∞). Using this relation we can draw the curve of electric birefringence as a function of electric field and compare it with experimental values. The results, when Δα0 = 1.17 × 10−28 F m2, Δα∞ = 0.005 × 10−28 F m2 and K = 0.00031 m/V, are in good agreement with each other. In order to explain the reason why the anisotropy of electric polarizability rapidly decreases with an increase of electric field we propose that the difference of electrophoretic mobility between parts of colloidal particle causes the orientation of a rod like particle. The theoretical electric birefringence as a function of electric field we obtained is considerably in good agreement with the experimental values.