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

Steady-state and decay birefringence, expressed in terms of the optical phase retardation per cell length δ/dδ/d, was measured on β-FeOOH in aqueous ionic media at 633 nm and at 25 °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 δ/dδ/d and field-free rotational relaxation time τ   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 δ/dδ/d and τ   values to infinitely high fields (E2→∞E2→∞) could yield birefringence- and weight-average quantities, respectively. Observed τ   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 〈τ〉w〈τ〉w was calculated with Perrin's expressions theoretically from the length, width, and volume of β  -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 δ/dδ/d value was always positive. The infinitely high-field (δ/d)∞(δ/d)∞ values and the reduced optical anisotropy factor Δg/nΔg/n were evaluated by fitting to theoretical orientation functions. The intrinsic birefringence (n3−n1n3−n1) could be estimated with the mean refractive index npnp reported in the literature. For the spindle-shaped particle with an axial ratio of ca. 4, the sign of Δg/nΔg/n is always positive, whereas the quantity (n3−n1n3−n1) was either negative (np>2.35np>2.35) or positive (np<2.05np<2.05) in sign or nearly zero (ca. np=2.26np=2.26), depending critically on the npnp values.

Graphical abstractThe light extinction at 633 nm of β-FeOOH suspension per unit absorption cell length against particle concentration in the absence of applied electric field. The apparent Lambert–Beer law holds over the entire concentration region. The concentration of NaCl in the suspension is kept in the 0.03–0.1 mM range. The solid line represents a linear fit through the origin of measured points (squares).Figure optionsDownload full-size imageDownload as PowerPoint slide