Chemical bonds and formation process of actinide(IV) oligomers and colloids

Tetravalent actinides show a strong tendency towards hydrolysis followed by the formation of oligomers and colloids. This formation process was investigated on Th(IV), U(IV) and Np(IV) species with EXAFS, LIBD, XPS, HEXS, UV-Vis, NMR and XRD. We investigated the involved chemical bonds, the local structure and structure development as well as the formation mechanism.
We observed that the oligomerization of tetravalent actinides was limited by the presence of chelating ligands. The presence of carboxylic acids, for example, resulted in hexanuclear complexes [1]. Such polynuclear species consist of a framework made of oxo and hydroxo bonds which is terminated by chelating carboxylic acids. In absence of such terminating ligands, the polymerization often continues until stable colloids are formed [2]. This formation process is kinetically hindered due to several reasons related with (a) the limited number of OH groups in the An(OH)n4-n precursor and (b) terminating water molecules. The polymerization comprises of olation and oxolation processes. At a later stage, the initially amorphous structure of the colloides undergoes an ordering process during which hydroxo groups are systematically replaced by oxo groups. Zeta potential measurements indicated that the colloidal particles carry positive charge at acidic pH and negative charge at alkaline pH. We found that silicate is able to stabilize such colloids at near-neutral pH through modification of the inner structure and by influencing the electrostatic repulsion caused by an enhanced negative surface charge.
UV/Vis, EXAFS spectroscopy and light scattering proved to be sensitive tools to differentiate between hydolysis species, oligomers and colloids.

[1] Takao et al. (2009) Eur. J. Inorg. Chem. 32, 4771-4775.
[2] Ikeda-Ohno et al. (2009) Inorg. Chem. 48, 7201-7210.