THE GROWTH OF FRACTAL HUMIC ACIDS - CLUSTER CORRELATION AND GEL FORMATION

The growth of humic acids, prepared by a gentle method from two differ ent kinds of soils (I and II), has been studied using small-angle neut ron scattering at an acidity corresponding to pH 5.0 and 0.10 M ionic strength (NaCl). Humic acids aggregate either to large clusters with a fractal dimension of 2.3(5) and an average diameter of 1720 Angstrom (I) or to clusters with an average diameter of 700 Angstrom (II). Afte r storage for 2 days at 4 degrees C, the latter aggregates (II) formed a gel. In a step toward gelation, we observed cluster-cluster interac tion from the neutron-scattering data in the form of a correlation pea k. These differences in size can be explained by assuming that the sma ller particles (II) are trapped into a nonequilibrium state characteri zed by the temperature-solvent condition. The importance of a humic ac id gel network for the conservation of water and nutrients in the envi ronment is discussed.