Humic complexes of diethyl phthalate: molecular modelling of the sorption process

The sorption mechanisms and complex formation between humic acid (HA) and a successively increasing number of diethyl phthalate (DEP) molecules have b een studied theoretically using molecular mechanics, the number (n) of DEP molecules being varied from 1 to 30. The energy components of the single HA (.)nDEP complexes have been used as explanatory variables in a principal co mponent analysis for exploring the presence of similarities/dissimilarities in the energetic properties of the individual xenobiotic complexes. The so rption can be explained in terms of a two-step mechanism. Absorption takes place as long as the host humic acid structure offers (a) enough internal d ocking space and (b) favorable interactions (energy release) with the guest molecule. This takes place for up to 7 DEP molecules. Further increase in the number to 30 DEP molecules will, due to the lack of free available inte rnal voids, lead to surface controlled adsorption. The two-step sorption pr ocess apparently results in (a) a linear increase in energy gain by DEP bon ds, and similarly (b) a constant incremental rise in molecular properties o f the complexes such as volume and surface area. Three outstanding observations emerge: (1) Structural features at the atomi c level (nanochemistry), such as partial atomic charges and high aromaticit y of the humic acid, are observed to be dominating the intermolecular inter actions in the complexes at the specific sorption sites. (2) Torsional reli ef and favorable changes in bonding energy also prevail for the growing com plex. The latter indicates both the structural flexibility of the HA host a nd the stabilizing effect of DEP on the complex, by filling of the voids wi thin the HA molecule. (3) The intermolecular forces are described mainly by hydrogen bonds (electrostatic energy) and interactions between dipole-dipo le, such as carboxylic functions and uncharged moieties such as aromatic ri ngs (van der Waals energy). (C) 2001 Elsevier Science Ltd. All rights reser ved.