Particle Sizes of Standard Humic Substances Calculated as Radii of Gyration, Maximum Diameter and Hydrodynamic Radii

Average particle size and particle size distribution of humic substances under solution conditions is useful physicochemical information for investigating the role of humic substances in geochemical processes. In this study, the average particle sizes of standard humic acid (HA) and fulvic acid (FA) samples from the Japanese Humic Substance Society (JHSS) were evaluated by small angle X-ray scattering (SAXS). The scattering intensity in the observed range of scattering angles was higher in the HAs than in the FAs. The distance distribution functions, p(r), involved in the structural characteristics of the particles when dispersed in solution were obtained by a Fourier inverse transform of the scattering intensity. The radii of gyration, RG, calculated from the p(r) were 10.1, 7.3 and 1.4 nm for the Dando HA, Inogashira HA and the two FAs, respectively. Concurrently the hydrodynamic radii (RH) of these standard samples were calculated from Einstein’s equation using already-known molecular weights and intrinsic viscosity for comparison with RG values. The values of RG in the HAs were considerably larger than RH values, probably due to higher sample concentrations in the SAXS measurement. Larger ratios of RG to RH (RG/RH) in the HAs than in the FAs indicate a relatively elongate shape of the HAs as compared to the FAs. This was also confirmed by a ratio of maximum diameter of averaged particles (Dmax) and RG (Dmax/RG) obtained by the SAXS analysis. Smaller particle HAs, which were confirmed in a dilute solution system by high performance size exclusion chromatography (HPSEC), can aggregate in the more concentrated system used for the SAXS measurement. The elongate shape of the larger HA particles may result from hydrophobic interaction between smaller HA particles. In contrast, FAs with low ratios of Dmax/RG and RG/RH were dispersed as stable single small particles in the solution. Larger contents of acidic functional groups in the FA chemical structure allow single FA particles to disperse by electrostatic repulsion. The different dispersion behavior of HAs and FAs in solution can be attributed to their chemical structures.