Study of ionic surfactants binding to humic acid and fulvic acid by potentiometric titration and dynamic light scattering

The binding of anionic surfactant, sodium dodecyl sulfate (SDS) with Aso humic acid (AHA) has been studied by potentiometric titration and dynamic light scattering (DLS) methods at two pH regions and ionic strengths that is pH 9.18 (ionic strength 0.03 mol dm−3) and pH 3.98 (ionic strength 0.10 mol dm−3). At pH 9.18 and low ionic strength no binding is observed between SDS and AHA in the investigation by both methods, whereas some interaction is observed at pH 3.98 and at high ionic strength by DLS measurement since electrostatic repulsion is suppressed by counterions at this solution condition. The binding between cationic surfactant, dodecyltrimethyl–ammonium ion (DTMA+) with Aso fulvic acid (AFA) and AHA has also been investigated by potentiometric titration and DLS methods and compared with the binding of dodecylpyridinium ion (C12Py+). The binding of DTMA+ ions with AFA or AHA is weaker than that of C12Py+ ions, presumably due to steric hindrance of headgroup of DTMA+ ions and due to higher attractive binding of C12Py+ ions induced by π–π interaction between aromatic rings. The hydrodynamic diameter of DTMA+–AFA/DTMA+–AHA aggregates is smaller than that of C12Py+–AFA/C12Py+–AHA aggregates.