Poly(methylene) crystallites in humic substances detected by nuclear magnetic resonance

Crystalline domains composed of poly(methylene) chains have been detected by solid-state NMR and wide-angle X-ray scattering (WAXS) in several samples of soil organic matter, including humins, surface soil (peat), and humic acids extracted from surface soil and young coal. From the melting range of 60- >80 degrees C and H-1 spin diffusion experiments, a crystallite thickness of ca. 3 nm or 25 CH2 units is deduced. The overall fraction of (CH2)(n) carbons in these materials is up to 9%. Nearly half of this poly(methylene) is crystalline, while the rest is noncrystalline and more isotropically mobile. In humin, several crystalline and noncrystalline poly(methylene) domains form larger aggregates. The crystallites are expected to be resistant to environmental attack and thus inert in the soil and have long residence times, while the mobile amorphous regions may play a role in the sorption of nonpolar molecules in soil.