The role of short range interparticle forces in controlling the size distribution of submicron particles precipitated from solution has been investigated. The central hypothesis explored is that primary particles formed early in precipitation reactions are subject to short range forces that can be repulsive or attractive depending of the solvent chemical potential and the particle separation.
1) A model system was used to explore the role of short range forces in controlling the colloidal properties of sub-10 nm metal oxide particles. Criteria sought in looking for the particle were i_ that it be on the order of the size of primary particles formed in precipitation reactions, ii) be readily available for investigation and iii) have a metal oxide composition. The particle chosen was the silicotungstate anion, SiW12O40,(STA) which is a sphere carrying four negative charges with a diameter of 1.1-1.2 nm. STA is readily soluble in water and is commercially available.
2) The solubility of the acid, lithium and sodium forms of STA was investigated as a function of supporting electrolyte concentration of HCl, LiCl and NaCl, respectively. Second virial coefficients of STA suspensions were measured by static light scattering.
3) The second virial coefficients were converted into an effective temperature, 7,by assuming the particles interact with an attractive pair potential with an extent which is a small fraction of the particle diameter. As T decreases, the strength of the interparticle attraction increases. A comparison was made between predicted and measured phase behavior where *c is plotted as a function particle concentration at the solubility limit. An excellent comparison was found suggesting the adhesive hard sphere model provides an adequate description of STA suspension thermodynamic properties.
4) These results demonstrate that as the supporting electrolyte concentration is increased, interparticle attractions increase. Detailed calculations suggest the attraction is stronger than can be reasonably attributed to van der Waals attractions. The conclusion is drawn that the salting out behavior seen in STA suspensions has an origin in the relative affinity of the solvent for the STA particles and the supporting electrolyte. We hypothesize that as the electrolyte concentration is increased, the water would rather hydrate the supporting ions than the STA particles resulting in a net interparticle attraction. This study clearly shows that the pair potential can be modulated by influencing the chemical potential of the solvent. In addition, these studies indicate that small particles feel weak attractions which will grow in magnitude as the solvent chemical potential is reduced. Note however, that this attraction does not bring particles into contact. Particles remain hydrated in the aggregated state.
5) The interactions of the STA particles were also investigated using osmotic techniques. Here STA crystals were equilibrated with nitrogen streams with different relative humidities of water. The dehydration properties of the STA crystals are very sensitive to the counterion. These studies indicate that the affinity of STA/counterion particlies for water is high and that complete dehydration of STA does not occur at 25 C until the relative humidity is less that 0.05 STA crystals dehydrate in steps indicating that the pair potential is oscillatory in nature. If the relative humidity is converted to an osmotic pressure, 'IC (= -kT/vln(RH), where n is the osmotic pressure, v is the molecular volume of the solvent and RH is the relative humidity), one finds that at crystallization, the suspensions must be compressed to a pressure of near 300 atm if they are kept at a constant volume and exposed to pure water. Osmotic pressures of near 1000 atm are required to completely dehydrate the particles.
6) The dehydration experiments indicate that while STA crystals are heavily hydrated, the particle interactions are sensitive to counterion. From this result we conclude that oscillatory interactions arise from counterion hydration rather than particle hydration. Never-the-less, both the dilute solubility experiments and the crystal dehydration experiments indicate that the degree of aggregation (or the separation distance of the particles) can be controlled by alterations in the solvent chemical potential.
7) We conclude that in precipitation reactions, clusters or primary particles grow by molecular addition but do not aggregate because: i) van der Waals forces between small particles are weak, and ii) the hydrated state of the particle surface screens the van der Waals attractions. As the particles grow,the extent and strength of attractive forces increase and aggregation may occur. If the particles remain reactive, such flocculation can result in irreversible agglomerates. In addition, if over the course of the reaction, the solvent chemical potential is decreased, our results suggest that attractions will increase and may lead to aggregation and thus producing a broad particle size distribution.
8) Means of controlling the state of aggregation of sub 50 nm particles suggested by the preceding include control of the solvent chemical potential, and/or the adsorption of small stabilizing agents (such as citrate used in the control of particle size in the precipitation of gold from the reduction of auric acid).