FRACTAL DIMENSION OF HUMIC ACIDS - A SMALL-ANGLE NEUTRON-SCATTERING STUDY

Small-angle neutron scattering experiments have been made on solutions of humic acid aggregates with an acidity corresponding to pH 5.0 and at 0.1 M ionic strength. We observe power-law decay of the intensity over one decade of the scattering vector, Q, indicating that the aggregates are fractal. We explain the normalized intensity in the entire Q-range by assuming that the humic acid particles can be described by building units of a radial size, less-than-or-equal-to 25 angstrom, aggregated into clusters with an average radius of 400 500 angstrom. For humic acids obtained from two different sources, we determine the fractal dimension, D = 2.3 +/- 0.1. For small values of Q, the measured data of one of the samples extend into the Guinier range giving an average radius of gyration of 320 +/- 20 angstrom.