Influence of pH on the surface activity of humic acid: micelle-like aggregate formation and interfacial adsorption

The influence of pH on the surface activity of humic acid (HA), was extracted from peat soil, was investigated by surface tension (γ) measurement and the increase in solubility of p-dichlorobenzene (p-DCB). The γ for an HA solution gradually decreased with increasing concentrations of HA. As the pH was decreased, the γ versus HA concentration curve shifted into a lower HA concentration region. In these curves, the beginning of the plateau was in good agreement with the aggregation point of HA as evaluated by the enhanced solubility of p-DCB. The logarithmic concentration of HA at the aggregation point, γ and the surface area occupied by an HA molecule decreased sigmoidally with decreasing pH. The aggregation point at pH 4 was 1/1.4 times that at pH 7. These results can be attributed to the acid dissociation behavior of the HA. The change in surface area occupied by an HA molecule may be due to both collaborative adsorption and a decrease in electrostatic repulsion between carboxyl groups at the air–water interface. The pC20 value, which was used as one of hydrophobic parameters, sigmoidally increased with decreasing pH, and the pC20 values at pH 4 and 5 were 1.3 times higher than those at pH 6 and 7. These results demonstrate that micelle-like aggregation and the interfacial adsorption of HA were substantially facilitated in the acidic region.