The Shape of Humic Acid in Solution as Observed by Small-Angle X-Ray Scattering

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Electromagnetic scattering(e.g., light scattering, small-angle X-ray scattering, and small-angle neutron scattering)provides information on the size and shape of colloidal particles in a highly dispersed system(Feigin and Svergun 1987). The technique has been applied to various materials including biological macromolecules, synthetic polymers, amorphous solids, and other colloidal compounds(Feigin and Svergun 1987).Several attempts have also been made to evaluate the size and shape of humic acid in solution using the scattering techniques(Wershaw and Pinckney 1989;Osterberg et al.1993). Small-angle X-ray scattering(SAXS)was first applied to sodium humate by Wershaw et al.(1967)and they estimated the molecular weight, molecular volume, radius of gyration (R_g), and shape factor of the sodium humate. Thereafter, based on studies of humic substances by SAXS, R_g which can be estimated independently from the SAXS data of humic substances was obtained as the sole parameter because of the lack of information on the chemical structure of humic substances(Lindqvist 1970; Wershaw and Pinckney 1971)1973, 1977; Thurman et al.1982). In principle, however, it is possible to evaluate the shape of particles by analyzing the profile of the SAXS data, because the scattered intensity and profile reflect the number and spatial distribution of the scattering units composing colloidal particles. The excess scattered intensity is Fourier-transformed into the distance distribution function which represents the electron density distribution in a particle(Kajiwara et al.1991).The shape of humic acid can be deduced approximately from the distance distribution function without any chemical information. In addition to the distance distribution function, various methods can be used to analyze the shape of objects from the scattering profie. For example, the Kratky plot yields information on the chain stiffness of polymers or in somecases on the architecture of a molecule(Donkai et al.1985). The purpose of this study is to analyze the size and shape of humic acid by using the SAXS data from the solution.
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The Shape of Humic Acid in Solution as Observed by Small-Angle X-Ray Scattering

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