| Summary: | Bibliography: pages 124-129. === Electrostatic interactions between binding sites on a fulvic acid molecule are incorporated into a computer model for the simulation of fulvic acid cation equilibria. A preliminary investigation into the influence of nitrogen containing constituents on simulated metal complexation results is also reported. The metal investigated is Cu²⁺. The influence of electrostatic interactions on the complexation properties of macromolecules is evaluated by focussing on the Gibbs free energy of the complexation process. The change in Gibbs free energy, characteristic of a specific complexation reaction, is modified by the electrostatic interactions between sites. ΔG is modified by a factor dependent on the work of charging a system previously uncharged. This work is, under conditions of constant temperature and pressure, equivalent to the electrostatic contribution to ΔG. In order to evaluate the electrostatic free energy, it is necessary to evaluate the electrostatic potential on the macromolecule due to the charges present. This is accomplished using dielectric models of the macromolecules. The main points to emerge from this investigation are that the electrostatic models range from very sophisticated to very crude. Typically, the sophisticated models need detailed structural information on the macromolecule, such as atomic and charge coordinates. It is found that no such structural data are available for humic substances. This limits the choice of models to those which do not require detailed structural information. These models are however fraught with approximations, introducing uncertainty of unknown extent into the calculated results. The chosen electrostatic models are incorporated into protonation and metal complexation models of fulvic acids. Parameters which may influence simulated results to a large extent are identified. This investigation indicates that input parameters for the various modelling steps should be chosen with a great degree of circumspection. The cation complexation models are used to simulate experimental results published in the open literature. The results indicate that the incorporation of electrostatic interactions leads to some improvement between experimental and simulated results. The results further suggest that there are secondary refinements to the models which may improve the agreement between simulated and experimental results. The degree of fit between experimental and simulated results proves to be rather independent of the choice of electrostatic model. The influence of nitrogen containing ligands on the Cu²⁺ complexation capacity of fulvic acid does not turn out to be very pronounced. The assumption of neglecting these constituents as possible binding sites does not seem to be a limiting assumption. The results suggest that the predictive models developed in the course of this investigation may have some potential to simulate the cation complexation characteristics of fulvic acids. Further refinement of the models is, however, necessary.
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