The silicotungstate anion-- [ SiW 1204OJ ‘4, referred to in this document as “STA”-- has a diameter of 1 nm and is used as a model particle to investigate the role of short range forces in controlling the colloidal stability of metal oxide nuclei formed in the early stages of precipitation reactions.
1) The role of counter ions has been studied by investigated the interactions of STA in the presence of H+, Li+, and Na+.
2) By measuring the light scattered from dilute STA suspensions, the second virial coefficient was determined in ranges of 0.3-5 M HCl, LiCl and N&l solutions where it was found to be a decreasing function of ionic strength. Only small differences appear with changes in cation type.
3) Osmotic stress techniques indicate that the STA anions crystallize from solution at osmotic pressures that are weak functions of cation type. However, the degree of STA hydration is very sensitive to the cation. Osmotic pressures at crystallization lie between (2.5-3.5)x107 Pa. The largest hydrate of H4STA is 31 while for Li4STA and Na4STA, the largest hydrates are 25-26 and 13 waters per STA molecule respectively. With increasing osmotic pressure, the crystals dehydrate in a series of steps with the osmotic pressures required to induce a step change in hydration varying with temperature and cation type. To a first approximation, the pressures required to induce the first dehydration step vary as. Na>Li>=H.
4) By comparing the volume fractions at crystallization, the osmotic pressures, and the second virial coefficients of STA particles with the values expected for hard spheres, we conclude that at crystallization, the particles have hard cores with diameters of 1.1-1.24 nm and feel an attraction of depth (0.3-0.8) kT,
5) Our analysis indicates that the interactions of nanometer sized particles are sensitive to electrostatic, van der Waals, and hydration interactions. To a first approximation, hydration interactions screen van der Waals attractions and block aggregation into the primary minimum, However, the hydration interactions may also provide an attractive minimum in pair potential energy which facilitates aggregation of a reversible type. The hydration interactions are controlled by a competition for water by various species in solution. As a result, the degree of aggregation will be controlled by water activity. This conclusion implies that in precipitation reactions, the activity of water (or its chemical potential) will play a significant role in determining the colloidal stability of growing solid particles.
6) Working in collaboration with Dr. F. van Swol, we are investigating the role of solvent chemical potential in controlling the state of aggregation of colloidal particles. The calculations of Frink and van Swol [4] and Kokkoli and van Swol demonstrate that these interactions can be understood in terms of the affinity of the solvent for the solid and the stacking of solvent molecules between the surfaces. As the chemical potential of the solvent is raised, solvent will begin to partition to wetting surfaces and thus force the surfaces apart. When submerged in pure solvent, the only way to increase the chemical potential of the solvent further is to apply a hydrostatic pressure to the liquid. With increasing hydrostatic pressure the clay swelling continues. While the current models deal only with nonionic solvents, extensions to ion containing solvents and charged surfaces are in progress which demonstrate that solvation interactions continue to play a major role when the surfaces are held at separations on the order of a few solvent molecules.
These models indicate that solvent activity plays an important role in determining the state of particle aggregation. With this concept in mind, the experimental portion of this contract is aimed at developing methods of characterizing the nature of solvation interactions and using solvent chemical potential to manipulate the state of aggregation of particles which model primary particles produced in homogeneous precipitation reactions. For this purpose we have chosen to work with silicotungstate anions [SiW 120a]-4 which are spherical and have a crystallographic diameter of 1 nm. These anions are highly soluble, and when they crystallize, the solids have many waters of hydration.
The chemical potential of the continuous phase is manipulated by two methods in these studies. In the first, the osmotic stress technique of Parsegian and coworkers is used [5]. Here STA solutions are equilibrated with a vapor of known humidity (water activity) and the degree of hydration is measured. In these experiments, the chemical potential of the continuous phase is set by the relative humidity of the water above the solution. The water in the solution equilibrates by increasing or decreasing the concentration of the STA particles and their counterions. At equilibrium, the osmotic pressure of the STA liquid or solid is determined from n: = -RT ln(p/po)/v where RT is the product of the ideal gas law constant (R) and absolute temperature (T), p is the vapor pressure of water above the STA, po is the saturated vapor pressure of pure water, and v is the molar volume of water (with p, po and v all at temperature T). The osmotic pressure (7~) is the pressure that would have to be applied to the STA liquid or solid to maintain the solid volume fraction if it were exposed to pure water.
The second method of controlling the activity of the water and the eIectrostatic screening ability of the continuous phase is by varying the added electrolyte concentration. Currently we have focused on using electrolytes to alter both the ionic strength and solvent activity. The intensities of the light scattered from dilute solutions of STA at fixed electrolyte concentration are used to determine the second virial coefficient in a concentration expansion of the suspension osmotic pressure. In the limit of small particles, the intensity of the light scattered form the suspension is independent of angle and can be written as:
-----------------------, dn/dc = refractive index increment, ho = incident wavelength in vacuum, no = solvent refractive index at h,, N, = Avogadro’s number 3 (6.02 x 1023), c = mass concentration, RQ = Rayleigh scattering intensity at 8,9 = angle, M, = weight-average molecular weight, and A, = second virial coefficient. In these experiments, as the ionic strength of the supporting electrolyte is increased, the electrostatic repulsions between the STA anions are screened. In addition, the activity of the continuous phase is reduced.
In attempts to investigate the role of the counterion in determining the magnitudes of hydration forces, the dehydration of STA crystals and the second virial coefficients of dilute STA suspensions have been measured with counterions of H+, Li’, and Na+. For this purpose, H&TA, LQSTA and Na&TA materials were synthesized and their composition confirmed with NMR and elemental analysis.