Thermodynamic studies of dodecylpyridinium ion binding to fulvic acid

The binding of dodecylpyridinium ions (C12Py+) with Aso fulvic acid (AFA) has been studied from the thermodynamic point of view by using potentiometric titration method with surfactant-ion-selective membrane electrodes in aqueous solution of pH 9.18 and at the ionic strength of 0.03 mol dm−3. The cooperative binding of C12Py+ with AFA is the endothermic process driven by the positive entropy resulting possibly from the dehydration of hydrophobically hydrated water molecules around the hydrocarbon chains of the bound C12Py+ ions. The binding is obviously pH dependent and is most pronounced at pH 9.18. Different binding modes are observed at two pH regions, i.e., cooperative binding at pH > 7 and non-cooperative binding at pH < 7. The effect of ionic strength is also important in binding phenomenon. As the ionic strength decreases, the C12Py+ binding with AFA is enhanced, probably due to the lowering of screening effect of counterions. The sensitivity of binding to ionic strength is larger at high pH than that at low pH in AFA system.