ABSTRACT
We present a comparison of experimentally and theoretically determined osmotic pressures for various colloidal dispersions. Experimental data is collected from several different silica and polystyrene dispersions. The theoretical pressure determinations are based on the primitive model combined with the cell model, and the physical quantities are calculated exactly using Monte Carlo simulations in the canonical and grand canonical ensemble. The input to the simulations in terms of colloidal particle size, surface charge density, and so forth are taken directly from experiments, and the approach does not contain any adjustable parameters. The agreement between theory and experiment is very good without any fitting parameters, showing that the simplifications behind the primitive model and the cell model are physically sound. The results reveal a surprising correspondence between the equations of state in spherical and planar geometries, indicating that the particle shape is of secondary importance in dispersions dominated by repulsive interactions. For one of the silica dispersions, we have also investigated how various monovalent counterions influence the swelling properties. Within experimental error, we are unable to detect any ion specificity, which is further support for the theoretical models used.
ABSTRACT
Adsorption on ZnO of sodium poly(acrylate) (PAA), sodium poly(styrene sulfonate) (PSS) and a monomer surfactant [hydroxyethylidene diphosphonate (HEDP)] was investigated in suspensions initially equilibrated at pH 7. Results demonstrate interplay in the adsorption mechanism between zinc complexation, salt precipitation, and ZnO dissolution. In the case of PAA, the adsorption isotherm exhibits a maximum attributed to the precipitation of zinc polyacrylate. PSS and HEDP formed high-affinity adsorption isotherms, but the plateau adsorption of HEDP was significantly lower than that of PSS. The adsorption isotherm of each additive is divided into two areas. At low additive concentration (high zinc/additive ratio), the total zinc concentration in the solution decreased and the pH increased upon addition. At a higher additive ratio, zinc concentration and pH increased with the organic concentration. The increase in pH is due to the displacement of hydroxyl ions from the surface and the increase in zinc concentration results from the dissolution of ZnO due to the complexation of zinc ions by the organics. The stability of the ZnO dispersions was investigated by measurement of the particle size distribution after addition of various amounts of polymers. The three additives stabilized the ZnO dispersions efficiently once full surface coverage was reached.
ABSTRACT
The adsorption of poly(acrylic acid) and poly(styrenesulfonic acid) (PSS) is studied on a barium sulfate powder. Comparison of the polyelectrolytes shows that the difference in binding strength corroborates the difference between the ligand strengths with barium ions. The surface excess dependence on pH correlates with the density of the ionized groups. The electrokinetic potential follows the surface coverage and the ionization ratio of the polymer up to the onset of the adsorption plateau, but continues to increase above that point. This peculiarity is explained by the release of barium ions from the adsorption layer into the solution. The phenomenon is attributed to the complexing power of unadsorbed molecules. Analysis of the displacement of small ions (Na(+), SO(2-)(4)) shows that adsorbed PSS releases sulfate ions from the surface and sodium counterions from the polymer. The displacement ratio for sulfate ions (SO(2-)(4)/PSS monomer units) remains constant over the adsorption isotherm, but that for sodium ions is constant only up to about two-thirds of the maximum coverage. From the data we deduced that about half of the monomer units of the adsorbed PSS molecules bind with surface barium ions. The other half form barium and sodium sulfonates whose ratio varies with the concentration of unadsorbed molecules. Copyright 1999 Academic Press.
ABSTRACT
Small molecules that have two carboxylic functions can adsorb from water onto calcite. The adsorption site is a -Ca+ site. The mechanism of adsorption is a complexation of the -Ca+ site by the two carboxylates, similar to the solution complexation of Ca++ ions. The complex has a ring structure where the two carboxylates are joined on one side by the -Ca+ ion and on the other by the n CH2 groups of the small molecule. Five-bond rings (n = 0) are the most stable, followed by six-bond rings (n = 1) and seven-bond rings (n = 2). Five-bond rings can also be formed with one carboxylate and one hydroxyl group (this is the case for alpha-hydroxycarboxylates) or with one enolate and one hydroxyl group (catechol). The sequence of binding strengths is enolate > carboxylate > hydroxyl; it matches the sequence of complexation efficiencies of these groups in solution and their characters as electron donors toward the metal cation. Copyright 1999 Academic Press.
