Studies On Physicochemical Behavior Of Humic Acid At Gas-liquid Interface

There are many active substances in natural waters extensively. Most of substances have two moieties, one is hydrophobic moiety and the other is hydrophilic moiety. These amphipathic molecules reside at the gas-liquid interface. The surface-active organic substances tend to be enriched at air-liquid interface and make up of natural film. In these natural film at the air-water interface, film made up of humus substances hold leading station. Humus substances mostly are result from oxygenation and degradation of carbohydrate and albumen polysaccharide .The natural film at the air-water interface influences on a lot of dynamic and non-equilibrium processes. The exchange of organic and inorganic matter, gases, water vapor, heat and momentum may be mediated by the surface film at the air-water interface. Being the relationship between concentration of surface-active organic substances at the air-water interface and these processes, it is of fundamental importance to determine the basic chemical and biological composition of surface films and to measure their rates of formation, alteration and disappearance. Monolayer techniques used for the study of surface film at the air-water interface provide a lot of information of film molecules, including optic microscope, x-ray, wave damping, spectroscopy, surface potential measurement, surface pressure measurement. In these study methods, surface pressure measurement has great advantages, such as no pretreatment and no destructive film. It can be depicted property of surface film exactly and provides information about the molecular orientation and changes at the interface.We study physicochemical behavior of humic acid at gas-liquid interface and metal-humic interactions. From experiments we conducted, we got some important results which administer to the understanding the exchange of water and vapor in natural water system.1) Humic acid form mainly from various active chemical function group, including hydrophobic alkyl (C-H) and t hydrophilic function group (-COOH, -OH).Due to the accumulation of humic acid in natural water systems onto air-water interface, the surface film is forming. Increasing quantity of humic acid, surface pressure and area of hysteresis loop will be increased too.2) The results indicated that metal-humic interactions contained a few kinds of principle. The increased cation concentration could be causing a compression of the colloidal double layer, reducing the inter-molecular repulsion, accelerating colloidal aggregation , producing a more hydrophobic structure and increasing surface pressure. The metals could be forming chemical bonds with the functional groups in the humic acid structure, neutralising the humic charge and hence reducing the inter-molecular repulsion. They may further enhance the tendency for flocculation between humic molecules. Because metallic cation can accelerate aggregation of humic acid, surface film show agglomerate property. The diagram of surface film formed by metal-humic interactions showed that surface tension drop abruptly.In the paper, we discussed that the mechanisms were responsible for the hysteresis. It is likely that some mechanisms are responsible for the hystersis.1. Frequency of instrumental (cycle of compressible-expansible process )2. Desorption-diffusion of film3. Changing of surface structures such as micelles, phase changes in the film and collapse of the film.4. Contact angle hysteresisBased on the hypothesis who should be responsible for the hysteresis, we explain the forming of hysteresis loops by modeling.