Joseph R. Feldkamp

Acid—base equilibria in clay suspension

Abstract The thermodynamics governing the equilibria of weakly basic compounds in dilute clay suspensions is developed in considerable detail. Quantities suitable for characterizing the equilibrium are shown to be: Keff, an apparent equilibrium constant for the acid—base reaction in suspension; Δ G BH + , and Δ G B , which are the changes in the partial molar Gibbs free energy for the protonated and neutral forms of the base B, respectively, which occur upon addition of clay to the system at constant T, P, and composition. The above thermodynamic treatment is then used to examine the frequently observed phenomenon that basic compounds in clay suspensions are often protonated to a much greater extent than that predicted on the basis of the pH of the bulk solution phase and the pK of the compound. It is shown that the enhanced protonation is due to the ability of the clay—water interface to significantly lower the potential of the protonated form of the molecule relative to that of the neutral form. This relative difference in stabilization of the two forms displaces the equilibrium and is shown by experiment to depend on the apparent magnitude of negative charge on the clay surface as encountered by the basic compound.

Monitoring particle interactions in aqueous suspensions by fiber optic doppler anemometry

Abstract The area under the curve (AUC) of the modified Lorentzian power spectrum as determined by fiber optic Doppler anemometry (FODA) is directly related to the number of freely diffusing particles in suspension. The initiation of particle interactions is characterized by deviation from a linear volume fraction-AUC relationship. The AUC approaches zero when an extensive particle network forms which dampens the random motion of particles. The rheological behavior supports this conclusion as a yield value was first noted when the AUC approached 0. The ability of AUC to monitor particle interactions was confirmed by a decrease in AUC when a latex dispersion was flocculated by NaCl, CaCl2, or AlCl3. Both the volume fraction at which particle interactions were initiated, φd, and the volume fraction at which particle random motion was dampened, φc, decreased when the pH of an aluminum hydroxide suspension was adjusted closer to the point of zero charge. The AUC was useful in studying the disaggregation of an aluminum hydroxide suspension by shear. Shear was found to also produce aggregation under specific conditions of surface charge and volume fraction. The reversibility of shear-induced changes in particle interactions was studied and the conditions of surface charge and volume fraction which control the reversibility of the particle structures were defined.

Effect of surface charge of carbonate-containing aluminum hydroxide on particle interactions in aqueous suspensions

Abstract Particle interactions in aqueous suspensions of amorphous carbonate-containing aluminum hydroxide were monitored by viscometry and fiber optic Doppler anemometry. An extensive, network-like particle arrangement was believed to form when the pH approached the point of zero charge. This particle network is responsible for the maximum deviation from Newtonian rheology which was observed when the pH approached the point of zero charge.

Particle Morphology of Amorphous Aluminum Hydroxycarbonate and Its Effect on Tablet Production

AbstractThe particle morphology of amorphous aluminum hydroxycarbonate was studied by disaggregation induced by phosphate adsorption and direct observation by high resolution transmission electron microscopy. The primary particles are sheet-like and have equivalent diameters of 55A or less. The primary particles are believed to correspond to planar polymers comprised of six-membered rings of aluminum ions joined by double hydroxide bridges. Secondary particles form by attractive interaction of primary particles. The size of the primary particles affects the porosity of the secondary particles. The granulating and tableting properties of two spray dried amorphous aluminum hydroxycarbonates were shown to be related to the size of the primary particles and the porosity of the secondary particles.

Use of the Tension Cell to Monitor Particle Interactions in Suspensions

AbstractThe tension cell was able to monitor the effect of adsorbing either a nonionic or ionic surface active agent on particle arrangements in kaolinite suspensions. Adsorption of polysorbate 80 affected particle interactions by steric stabilization. Adsorption of sodium dodecylbenzenosulfonate produced dispersion by a combination of electrostatic repulsion and steric stabilization. The interpretation of the effect of the adsorbed surface active agents was supported by rheological studies

Displacement of acid—base equilibria in clay suspensions as calculated from double layer theory

Abstract The effects of the clay-water interface on the acid-base equilibria of weakly basic compounds have been calculated from electrical double-layer theory. Negatively charged clays, such as montmorillonite, interact coulombically with the cationic form of the base so that the acid-base equilibrium is displaced. The degree of displacement of the reaction, as manifested in the shift of the effective p K from the solution p K value, was calculated as a function of pH (and ionic strength), clay interparticle separation distance (i.e., suspension concentration), surface charge density, and dielectric constant of the medium. The results of the calculations are compared to the experimental results obtained by allowing tetracycline, a weakly basic antibiotic, to interact with a Na-saturated montmorillonite suspension. The agreement between theory and experiment is remarkable considering the limitations of double-layer theory when used to model a montmorillonite clay suspension. The calculations show clearly that the degree of displacement of the reaction BH + = B + H + is very sensitive to the level of background salt, only mildly sensitive to the concentration of clay and surface charge density, and not at all dependent on the dielectric constant of the medium.