The comprehensive models described in Chapter 9, and evaluated in Chapter 10, depend upon fitting data to equations. In Model VI, assumptions are made about molecular size and shape in the electrostatic sub-model, and in the calculation of proximity factors (defining multidentate sites). These aspects of Model VI can therefore be regarded as predictive, i.e. not dependent upon data fitting. The purpose of this chapter is to discuss how predictive modelling can be taken further, the ultimate aim being to describe cation binding from knowledge about humic molecular properties, structures and sizes, without the need to extract parameters from binding data.

Electrostatic interactions

As has already been discussed in Section 9.5, electrostatic interactions can be simulated given information or assumptions about molecular size, shape, solvent permeability and net charge. Some work has been done to explore the influence of molecular size heterogeneity, which may influence the overall binding characteristics of a humic sample. Thus, a physically more realistic model is obtained by the strategy of Bartschat et al. (1992), i.e. postulating the humic matter to consist of molecules with a range of electrostatic characteristics, dependent upon molecular size. Implementation requires knowledge or assumptions about the size distribution of the sample.

As well as size, Coulombic effects depend upon the conformations of the humic molecules, the extent to which the structures are penetrated by water, and the degree to which conformations change as a function of molecular charge and solution conditions.