Thermodynamic studies of dodecylpyridinium ion binding to humic acid and effect of solution parameters on their binding

Thermodynamic information of dodecylpyridinium ions (C12Py+) binding to Aso humic acid (AHA) has been investigated by using potentiometric titration method with surfactant-ion-selective membrane electrodes in aqueous solution of pH 9.18. No significant change in binding has been observed with changing the temperature in C12Py+–AHA system. The enthalpy of C12Py+ ions binding with AHA is slightly negative. The solution parameters, such as pH and ionic strength affect the binding. The binding strength is most pronounced at pH 9.18 since the functional groups of AHA are fully ionized and the degree of dissociation (α) is unity at around pH 9.18. The binding strength decreases with increasing ionic strength due to the ion-screening effect. The sensitivity of binding to electrolyte concentration is higher in AHA system than that in Aso fulvic acid (AFA) system. The hydrodynamic diameters of C12Py+–AHA and C12Py+–AFA aggregates was measured as a probe of their molecular interaction by dynamic light scattering (DLS) microscopy. In both AHA and AFA systems, the hydrodynamic diameters increase with increasing surfactant concentration. DLS measurements also give the similar results of lower sensitivity of binding to electrolyte concentration in AFA system. AHA concentration does not interfere the binding strength within the concentration range of 0.2–1.5 g dm−3.