Electrooptics of beta-FeOOH particle in aqueous media 2.Field-strength dependence of decay and steady-state birefringence,and the hydrodynamic and electrooptic properties

摘要

Steady-state and decay birefringence,expressed in terms of the optical phase retardation per cell length S/d,was measured on beta-FeOOH in aqueous ionic media at 633 nm and at 25 deg C by an electric square-pulse technique over a wide range of field strength E to ca.6 kV/cm.The field-strength dependence of both S/d and field-free rotational relaxation time r was determined at the sample concentrations between 0.0011 and 0.055 g/L and in the 0.02-2.0 mM NaCl concentration range.Extrapolation of both S/d and r values to infinitely high fields(E~2-> infinity)could yield birefringence-and weight-average quantities,respectively.Observed r values were decreased at weak fields but leveled off to ca.0.3 ms at very high fields due to a slight polydispersity regarding the length and volume of particles.The weight-average relaxation time _w was calculated with Perrin's expressions theoretically from the length,width,and volume of beta-FeOOH particles estimated in the dried state from electron micrograph.These quantities were variously averaged.The size distribution was discussed in terms of observed discrete histogram and theoretical(Weibull and Lansing-Kraemer)distribution functions.The sign of observed S/d value was always positive.The infinitely high-field(delta/d)_infinity values and the reduced optical anisotropy factor DELTA g were evaluated by fitting to theoretical orientation functions.The intrinsic birefringence(n_3-n_1)could be estimated with the mean refractive index n_p reported in the literature.For the spindle-shaped particle with an axial ratio of ca.4,the sign of DELTA g is always positive,whereas the quantity(n_3-n_1)was either negative(n_p > 2.35)or positive(n_p < 2.05)in sign or nearly zero(ca.n_p = 2.26),depending critically on the n_p values.