Near-infrared turbidity of β-FeOOH particle suspensions

Near-infrared transmission spectroscopy can he complicated by the light scattering from heterogeneous materials. For the examination of an evolving system exhibiting such light scattering, transmission spectra near wavenumber v = 10(4) cm(-1) were obtained during the hydrolysis of FeCl3 solutions. At first, the resulting turbid suspension of cigar-shaped beta-FeOOH particles exhibits single-particle scattering, including a Rayleigh regime (attenuation alpha V-4). At later times, the scattering increases strongly as the particles aggregate, and becomes proportional to v(alpha), with alpha approximate to 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 v(alpha) with 1 less than or equal to alpha less than or equal to 4. While over limited spectral ranges alpha may be taken independent of v, over wide ranges it decreases with increasing v. This behavior is consistent with the theoretical limits of alpha = 4 at v = 0, and alpha = 0 at v = infinity. Overall, the results suggest that a useful form for simulating scattering backgrounds in near-infrared spectroscopy is Av(alpha), with A and alpha fitted constants.