Humic acid, the most abundant organic matter in the soil. The basic structure of the humic acid is an aromatic ring and an alicyclic ring, and a functional group such as a carboxyl group, a hydroxyl group, a carbonyl group, a thiol group or a methoxy group is attached to the ring. so that, why the humic acid is considered to be a surfactant ? and what type of surfactant does it belong to ? Can you Enthusiastically share some references?
Sincerely thank you for your help.
Why? -----> Structure dictates function:
Humic acid has a surfactant-like structure in that it contains both hydrophobic (carboxylic and phenolic groups) and hydrophilic (alkylic and aromatic groups) domains. This makes humic acid function as surfactants (i.e. the ability to reduce the surface tension of a liquid thus improving spread, flow and wettability even through hydrophobic zones).
Type? -------> ...............tricky!!:
I would hastily want to categorize humic acid as natural surfactants but in my opinion care must be exercised. If you are using pure humic acid then I will call it NATURAL SURFACTANTS. Given that the structure of humic acids can be further modified (e.g. cross-linking) to create humic acids that is excellent for a desired purpose (e.g hydration); I am not sure if these modified structures can still be regarded to as NATURAL.
Perminova IV, Hatfield K, Hertkorn N, editors. Use of humic substances to remediate polluted environments: from theory to practice. Netherlands: Springer; 2005.
Cihlář Z, Vojtová L, Conte P, Nasir S, Kučerík J. Hydration and water holding properties of cross-linked lignite humic acids. Geoderma. 2014 Oct 1;230:151-60.
@ Wei, in addition to previous answers , you may call humic acid also as colloidal surfactant.
Humic acid (HA) is called surfactant because it lowers the surface tension of water for reasons mentioned above.
However, HA is for the rest rather different from classical surfactants. Presently we don't speak of HA molecules any more because they are considered to be aggregated particles composed of components held together by physical attration that have relatively stable properties under normal solution conditions, but the interaction between the components is dependent on the solution conditions such as pH , dissolved electrolyte ions, ionic strength, smal organic solutes, ect.
When I stated "..we do not speak about HA molecules anymore.." I should have added that when we accept the word humic acids we should speak of (nano) particles instead of molecules. Particles can also be surface active, but are less likely compared with surfacant molecules.
This point is somewhat related to the discussion indicated by Steven Larson on the name humic acids (HA), fulvic acids (FA) for a special group of natural nano-particles extracted from soils and natural waters. An important contribution to this discussion is the paper:
Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters
D. C. Olk,* P. R. Bloom, E. M. Perdue, D. M. McKnight, Y. Chen, A. Farenhorst, N. Senesi, Y.-P. Chin, P. Schmitt-Kopplin, N. Hertkorn, and M. Harir in J. Environ. Qual. 48:217–232 (2019) doi:10.2134/jeq2019.02.0041
This paper stresses that a large number of studies have shown that the extracted materials indicated as HUMIC FRACTIONS/ SUBSTANCES (HA, FA) are suitable for studying natural organic matter structure and function in field conditions. Therefore the indications HA and FA are quite useful.
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