Near-Infrared Turbidity of β-FeOOH Particle Suspensions

Abstract

Near-infrared transmission spectroscopy can be complicated by the light scattering from heterogeneous materials. For the examination of an evolving system exhibiting such light scattering, transmission spectra near wavenumber v = 10⁴ cm⁻¹ were obtained during the hydrolysis of FeCl₃ solutions. At first, the resulting turbid suspension of cigar-shaped β-FeOOH particles exhibits single-particle scattering, including a Rayleigh regime (attenuation ∞ v⁴). At later times, the scattering increases strongly as the particles aggregate, and becomes proportional to vα, with α ≈ 2, consistent with scattering models that interpret the structure of aggregates in terms of a fractal dimension d_f roughly equal to 2. In all cases investigated, the attenuation due to scattering is spectrally smooth and increases monotonically with wavenumber. It can be written in the simple form vα with 1 ≤ α ≤ 4. While over limited spectral ranges α may be taken independent of v, over wide ranges it decreases with increasing v. This behavior is consistent with the theoretical limits of α = 4 at v = 0, and α = 0 at v = ∞. Overall, the results suggest that a useful form for simulating scattering backgrounds in near-infrared spectroscopy is Avα, with A and α fitted constants.