Surface Active Properties of Marine Humic Acids

Abstract

The surface tension of various humic acid solutions was measured. The marine humic acids extracted with 0.5N NaOH from the marine sediments sampled from Suruga Bay and Sagami Bay show high surface activity compared to the commercially available terrestrial humic acids. From the relation between surface tension and marine humic acid concentration, the surface tension decreases linearly with the logarithmic concentration and becomes constant value of about 40 dyn/cm at the concentration of 0.1%. This bending point at 0.1% is removed to the lower concentration by addition of salts like an ionic surfactant. The presence of a bending point and salt effect to it may suggest the association of marine humic acids.The marine humic acids were fractionated by ultrafiltration and the surface tension of each fraction was measured. The results show that the higher molecular weight fractions, which occupy 75% of the sample, exhibit high surface activity : bending points are also observed for their surface tension-concentration curves at 0.1%.The high performance liquid chromatographic analysis of humic acids by use of three kinds of gel indicates that the humic acid is a mixture of compounds molecular weight of which are widely ranged. The chromatographic results of molecular size distribution agreed on the whole with those of fractionation by ultrafiltration. From chromatograms of marine humic acids, the higher molecular weight fraction seems to be less hydrophilic and this weak hydrophilic character may cause the surface activity of them.