Near-infrared turbidity of beta-FeOOH particle suspensions

Near-infrared transmission spectroscopy can he complicated by the light sca ttering from heterogeneous materials. For the examination of an evolving sy stem exhibiting such light scattering, transmission spectra near wavenumber v = 10(4) cm(-1) were obtained during the hydrolysis of FeCl3 solutions. A t first, the resulting turbid suspension of cigar-shaped beta-FeOOH particl es exhibits single-particle scattering, including a Rayleigh regime (attenu ation x V-4). At later times, the scattering increases strongly as the part icles aggregate, and becomes proportional to v(alpha), with alpha approxima te 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 simpl e form va with 1 less than or equal to alpha less than or equal to 4. While over limited spectral ranges a may be taken independent of v, over wide ra nges it decreases with increasing v. This behavior is consistent with the t heoretical limits of alpha = 4 at v = 0, and alpha = 0 at v = infinity. Ove rall, the results suggest that a useful form for simulating scattering back grounds in near-infrared spectroscopy is Av(alpha), with A and alpha fitted constants.