Evaluation of amphiphilic properties of fulvic acid and humic acid by alkylpyridinium binding study

Amphiphilic properties of Aso fulvic acid (AFA) and Aso humic acid (AHA) are evaluated through the study on the binding of N-alkylpyridinium bromide (CnPy+Br−, n = 12, 14 and 16), using a potentiometric titration method with surfactant-ion-selective membrane electrodes in aqueous solution of pH 9.18 and ionic strength of 0.03 M at 25 °C. Different binding behaviors are observed between AFA and AHA due to the differences in the density of carboxylate groups as well as hydrophobicity–hydrophilicity balance. Independent sites binding behavior is observed in the CnPy+–AHA system, however, cooperative binding is observed in CnPy+–AFA system. The binding of CnPy+ to AHA is stronger than that to AFA, reflecting the importance of hydrophobic interaction between surfactant molecules and the backbone of AHA molecules. The chain length dependence of the free energy of binding per CH2 group amounts to about 3.1 kJ/mol in CnPy+–AFA system, which is comparable to CnPy+ binding to dextran sulfate and that for CnPy+–AHA system is 2.3 kJ/mol.