In this IFPRI funded project we investigate the role of short range forces in controlling the morphology of sub-micron precipitates. Our progress can be summarized as:
1) Silicotungstic acid (STA) has been chosen as a model colloidal particle to mimic the properties of primary particles (nuclei) in precipitation reactions involving metal oxides.
2) STA forms crystal hydrates where the number of waters of hydration per STA molecule varies with the relative humidity of the vapor with which it is in equilibrium.
3) The waters of hydration of STA crystals are lost in discrete steps in solid/solid phase transitions involving changes in the crystal lattice.
4) The locations of the phase transitions can be related to the osmotic pressure which would have to be applied to hold the crystal at a particular solid’s volume fraction if it were equilibrated with pure water. These osmotic pressures are large (the highest hydrate is not formed until an osmotic pressure of approximately 300 atmospheres is applied at 25°C).
5) These studies demonstrate that large driving forces must be applied to squeeze solvent from between small metal oxide particles with a strong affinity for the continuous phase. Resulting from the small particle size and the particle/continuous phase affinity is a strong, short range repulsion which will completely dominate over electrostatic and Van der Waals forces.
6) Future studies will involve developing measures of the spatial dependence of the solvation interaction energy and the effects of varying continuous phase/particle affinity.
7) Our findings suggest that for precipitation reactions producing particles with a strong affinity for the continuous phase, classical nucleation events may never occur. The implications of this concept require better understanding of the role or solvent/particle interactions in controlling particle interaction potentials and molecular attachment rates to growing particles.