Colloidal Aspects of the Formation of the Interactions between Humic Substances and Surfactants

Humic substances (HS) represent fascinating heterogeneous mixture of natural molecules, which possess a high potential to interact with various substances e.g. metals, surfactants, dyes, pesticides. This beneficial property allows their further utilization as the reactive part of sorbents with possible applications in remediation processes of soils and waste waters treatment plants. Surfactants represent a group of substances, which are artificially introduced in the nature by human driven applications causing the undesirable foaming of waters and enhancing the solubility of hydrophobic organic pollutants such as highly toxic polycyclic aromatic hydrocarbons. The present work is focused on the study of colloidal aspects of interactions between selected representatives of surfactants and humic substances, as the example of a promising sorbent and flocculant. For purposes of present work Hexadecyltrimethylammonium bromide, Tetradecyltrimethylammonium bromide, and Carbethoxypendecinium bromide were used as selected cationic surfactants, Sodium dodecyl sulphate as anionic and TWEEN-20 as a non-ionic. The interactions between the individual components were studied by using a combination of dynamic light scattering (determination of the changes in particle size distributions), electrophoretic light scattering (zeta potential – stability investigation) and isothermic titration calorimetry (thermodynamic parameters of interactions) in titration mode of measurement. This set-up provided us the determination of the critical aggregation concentrations of the surfactant-humic system, which corresponded to its phase separation. Moreover, the interactions between the components were confirmed also by routine physico-chemical methods (e.g. thermogravimetry and Fourier transform infrared spectroscopy).  We believe that the outcomes of our work will help to shed a new light on the phenomenon of the formation of the interaction between surfactants and humic substances and will provide the crucial insight in the broader utilization of specific fractions humic substances as the universal sorbent for surfactants mainly from the waste waters.