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Comprehensive Study of Humic Substances-Ionic Surfactant Interaction in Aqueous Solution
- This work encompasses the several facets of humic substances-surfactant interaction in aqueous solution including the thermodynamic information, solution physico-chemistry, and conformational changes in their aggregation. The subject matter is conveniently arranged into seven chapters. The first chapter covers the brief and effective introduction of humic substances (HSs) and surfactants together with their properties and applications. The 2nd chapter deals with the amphiphilic properties of fulvic acid (FA) and humic acid (HA) evaluated by alkylpyridinium (CnPy+) binding study based on surfactant-ion- selective membrane electrode. The cooperative binding is found in CnPy+-Aso fulvic acid (AFA) system, where as the independent site binding is observed in CnPy+-Aso humic acid (AHA) system due to differences in charge density as well as hydrophobicity-hydrophilicity balance. In AFA system, the binding constants and cooperative parameters are calculated by applying Hill’s binding theory. In AHA system, the number of binding sites and binding constants are analyzed by Scatchard plot equation. Apart from electrostatic interaction, two different hydrophobic interactions are involved in HS- surfactants interaction: hydrophobic interaction among surfactants themselves so called cooperative binding (CnPy+-AFA system) and hydrophobic interaction between the hydrocarbon tail of surfactant and the backbone of HS (CnPy+-AHA system). The binding strength is increased with increasing carbon number of surfactant in both AFA and AHA systems owing to these hydrophobic interactions. In chapters 3 and 4, the thermodynamic information of C12Py+ binding with AFA and AHA are presented respectively, including the effect of pH, ionic strength, and the concentration of HSs on their binding. Thermodynamic parameters facilitate to give much deeper insight in binding mechanism. In C12Py+-AFA system, the binding strength is increased with increasing temperature. 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. Meanwhile, the temperature dependence of binding strength is not found in C12Py+-AHA system and the enthalpy of binding is slightly negative. The entropy of binding (ΔS°) in AFA and AHA systems is 95 and 61 J mol-1 K-1 respectively. In both AFA and AHA systems, the binding is obviously pH dependent and is most pronounced at pH 9.18. In AFA system, the effect of pH on the binding is investigated at two pH regions, i.e., at pH>7 and pH<7 while the ionic strength of the system is kept constant at 0.03 mol dm-3. Different binding phenomena are observed: the cooperative binding at pH>7 and non cooperative binding at pH<7. Moreover, the binding strength is decreased with increasing ionic strength due to ion screening effect in both AFA and AHA systems. The sensitivity of binding strength to electrolyte concentration is higher in AHA system than that in AFA system suggesting that the more counterions are condensed on the oppositely charged AFA chains at certain pH and ionic strength. Thus, relatively smaller extent of change in binding is observed with the additional changing of ionic strength. This observation is in consistent with the greater entropy of binding in AFA system. In chapter 4, the hydrodynamic diameters (2Rh) of C12Py+-AFA and C12Py+-AHA aggregates, investigated by using dynamic light scattering (DLS), are also included. In the absence of cationic surfactant, the hydrodynamic diameter of AHA is unattainable within the experimental condition because of their inherent polydispersity. In the presence of surfactant, however, the hydrodynamic diameter of C12Py+-AFA or C12Py+-AHA aggregates becomes measurable with high reproducibility due to the coagulation force of cationic surfactant. In both systems, the hydrodynamic diameter increases with increasing C12Py+ concentration due to the growth of C12Py+-AFA and C12Py+-AHA aggregates while maintaining a constant pH, ionic strength, and AFA/AHA concentration at 9.18 and 0.03 mol dm-3, 0.05 g dm-3, respectively. The hydrodynamic diameters of C12Py+-AFA and C12Py+-AHA aggregate increase with increasing ionic strength, which is more pronounce in AHA system. This results point up a mark for higher sensitivity of binding strength to electrolyte concentration in C12Py+-AHA system than that in C12Py+- AFA system. Chapter 5 focuses the study of the interaction between anionic surfactant, sodium dodecyl sulfate (SDS), with AHA by potentiometric titration and dynamic light scattering (DLS) methods at pH 9.18 (ionic strength 0.03 mol dm-3) and pH 3.98 (ionic strength 0.10 mol dm-3). There is no binding between SDS with AHA at pH 9.18 and ionic strength of 0.03 mol dm-3 since the strong electrostatic repulsion between these molecules outweighs any specific interaction. At pH 3.98 and high ionic strength some interaction is observed by DLS measurement since electrostatic repulsion is suppressed by counterions at this solution condition. In order to study the various aspects of HSs-ionic surfactants interaction, the effect of cationic surfactant headgroup on the binding with HSs is also reported in this chapter. The binding of dodecyltrimethylammonium (DTMA+) ions with AFA or AHA is weaker than that of C12Py+ ions, due to steric hindrance of headgroup of DTMA+ ions. On one way, the binding of C12Py+ ions with AFA or AHA is stronger than that of DTMA+ due to stronger attractive force induced by resonance effect of benzene ring carried by C12Py+ ions. From DLS measurements, it is found that the hydrodynamic diameter of DTMA+-AFA/DTMA+-AHA aggregates is smaller than that of C12Py+-AFA/C12Py+-AHA aggregates and DTMA+-AHA aggregates is smaller than DTMA+-AFA aggregates. The affinity of C12Py+ to HSs appears to vary among HSs samples of different origins since HS are continuously subject to alterations in the biosphere. In chapter 6 the binding of dodecylpyridinium (C12Py+) ions with FA and HA of different origins are examined by potentiometric titration method and the variability in binding strength is related with the structural and chemical features of analyzed HSs. On the binding with C12Py+ ions, all investigated FA of different origins (both soil and aquatic) exhibit cooperative binding behavior and all investigated HA exhibit independent sites binding behavior. However, the binding strengths are different depending on their origins. The binding affinity of C12Py+ ions is stronger with soil HA than with soil FA. In both FA and HA systems, C12Py+ binding strength is stronger with soil samples than that with aquatic samples. These results show that hydrophobicity of HSs is one of the key factors in HS- cationic surfactant binding since soil HS is more hydrophobic than aquatic one as well as HA is more hydrophobic than FA. Overall, the substantial informations are summarized in chapter 7.
Min Min Yee
- Graduate School of Science and Engineering Saga University