P.L. de Bruyn

Hydrolysis-precipitation studies of iron solutions. I. Model for hydrolysis and precipitation from Fe(III) nitrate solutions

Abstract Acidified iron(III) nitrate solutions of varying concentration were titrated with base (NaOH) over a temperature range (24 to 90°C) by means of a homogeneous injection technique. Changes in the optical density of the solution were also registered continuously during the titration process. The kinetic behavior of the system was studied. Based on the findings of these experimental studies it is possible to divide the hydrolysis process into four stages: (a) hydrolysis to mono- and dimers; (b) reversible, rapid growth to small polymers; (c) formation of slowly reacting large polymers; and (d) precipitation of a solid phase. A model is proposed to explain the results.

A kinetic study of precipitation from supersaturated calcium phosphate solutions

The formation of three different crystalline calcium phosphates (DCPD, OCP, HAP) and an amorphous calcium phosphate was studied as a function of pH and supersaturation. Under the experimental conditions the formation of HAP is always found to be preceded by one or more precursors in a sequence that is in agreement with the Ostwald rule of stages. Homogeneous formation of HAP at low concentrations is never observed. A semiquantitative analysis of growth curves and of relaxation times at pH 6.7 and 26°C reveals the growth of OCP to be described best by a mononuclear growth model with fourth-order dependence on the relative supersaturation. The heterogeneous formation of HAP on OCP surfaces is noted to shift to lower α values (reaction extent) with increasing initial supersaturation. It can be described by a polynuclear growth model.

Hydrolysis-precipitation studies of aluminum (III) solutions. 2. A kinetic study and model

The hydrolysis-precipitation process of Al(OH)3 from acidified aluminum nitrate and chloride solutions was studied with a homogeneous alkali injection technique. In these studies the total Al(III) concentration (CAl) varied from 5 × 10−5 to 0.5 M, the ionic strength from about 10−4 (no added salt) to 2.8 M; and the temperature from 25 to 90°C. Continuous and discontinuous kinetic experiments with both alkali and acid suggest that different processes occur below and above an OH/Al ratio = 2.5 in solutions with cAl3+ > 5 × 10−4 M. At very low concentration the reaction proceeds solely via monomeric species. Light scattering measurements are used to determine the average degree of polymerization, Nz. It is observed that over the range 0 < OH/Al < 2.5 the average degree of polymerization changes in the order 0 < Nz < 15. The reactivity of these polynuclear complexes with acid suggests that the hydroxyl ions form bridges between the Al3+ cations up to an OH/Al ratio of 2.5. A model, which is based upon the structure of the solid, is postulated for the hydrolysis process. This model appears to be in qualitative agreement with experiment.

Hydrolysis-precipitation studies of aluminum (III) solutions. I. Titration of acidified aluminum nitrate solutions

Abstract Acidified aluminum nitrate solutions were titrated with alkali (NaOH or KOH) over a temperature range of 24°C to 90°C. A homogeneous distribution of added base was achieved by: (i) in situ decomposition of urea (90°C); and (ii) a novel method involving injection through a capillary submerged in the agitated salt solution. The experimental pH curves are characterized by two plateaus (or platforms) separated by a steep jump in pH. These characteristic features were not previously observed in continuous titration experiments. This is especially true of the second platform, which begins at an OH Al ratio larger than 2.5. Stable colloidal solutions or gels are formed on traversing this plateau. The hydrolysis-precipitation behavior was also followed by light scattering and x-ray diffraction measurements. A qualitative picture of the neutralization process is developed.

On understanding microemulsions : II. Thermodynamics of droplet-type microemulsions

A thermodynamic theory of microemulsions containing brine, oil, an ionic surfactant, and a nonionic cosurfactant is given. Conditions for phase equilibria are derived. The treatment is limited to droplettype microemulsions with emphasis on W/O + W systems. Important features are saturation adsorption of surfactant and cosurfactant, the interfacial bending stress, the standard chemical potential of the droplets in the free energy of mixing, and a quantitative treatment of the contribution of the electric double layer to the bending stress. Numerical illustrations show good agreement with experimental data on the influence of salt and cosurfactant on droplet size and interfacial tension in W/O + W equilibria.

Hydrolysis—precipitation studies of iron solutions. II. Aging studies and the model for precipitation from Fe(III) nitrate solutions

Abstract Iron(III) nitrate solutions (6.25 × 10−2 M in iron(III)) were titrated with base (NaOH). Samples of different OH/Fe ratios were taken for aging studies by means of the following techniques: pH relaxation, optical density measurements, light scattering, ultracentrifugation, and electronmicroscopy. The model introduced in a previous publication (J. Colloid Interface Sci. 56, 527 (1976)) was applied to and extended by the findings of this study. The aging process may be divided into two stages: (a) hydrolysis—polymerization leading to oxolation, nucleation, and growth to small particles (∼40 A) (b) aggregation to larger particles (200–500 A), and anisotropic flocculation to linear chains followed by slow sedimentation.

Hydrolysis-precipitation studies of aluminum(III) solutions. 3. The role of the sulfate ion

The experimental titration curve (pH vs OH/Al) for acidified Al(III) solutions in the presence of sulfate ions differs from that measured in nitrate solutions. Precipitation occurs at a much earlier stage (OH/Al ⋍ 0.4) and the characteristic second plateau disappears except at very low SO42− concentrations. Tracer experiments show most, if not all, the sulfate ions abstracted from solution during precipitation to be adsorbed. No evidence is found for the existence of basic sulfates in the precipitate. X-Ray and infrared studies combined with chemical analysis indicate the formation of a strongly hydrated hydroxide which transforms on increasing the pH to a poorly crystallized boehmite. The experimental titration curve is compared with a calculated curve based on simple thermodynamic considerations featuring a solubility product and the presence of monomeric species (AlOH2−). The role of sulfate is seen as that of a catalyst which removes the free energy barrier to the orientation and ordering of plate-like, highly charged, polynuclear complexes into growing solid particles. A comparison of the precipitation behavior in sulfate solutions with that in nitrate or chloride solutions allows one to measure the supersaturation built up in the latter systems and which eventually results in the nucleation and initial growth of the solid phase on the second plateau.

The influence of sulfate ions on the formation of iron(III) oxides

Abstract The influence of sulfate ions on the formation of an iron(III) oxide phase of colloidal size was studied by homogeneous titration of acidified iron(III) solutions with varying SO4/Fe ratios (0–2.4). During titration the optical density of the solution was continuously recorded. Solid phase analysis was performed by means of X-ray and Mossbauer spectroscopy. An analysis of the experimental results shows that the role of the sulfate ion is rather complex. The process of the nucleation and growth of the iron oxyhydroxides is described and the influence of sulfate on the various stages in that process is discussed. Certain stages are catalyzed by the sulfate ions, whereas other stages are suppressed.

Hydrolysis—precipitation studies of iron solutions : III. Application of growth models to the formation of colloidal αFeOOH from acid solutions

Abstract The development of a colloidal αFeOOH phase has been followed experimentally by measuring the dependence of the induction time on the OH Fe ratio and by recording the pH relaxation of samples of known initial OH Fe ratio. These results suggest that the growth of the solid phase is largely controlled by surface-nucleation processes. Two growth models are applied in the analysis of the kinetic data. This analysis permits an estimation of the interfacial tension of the solid to be made in acid solutions.

The influence of chloride ions on the formation of iron(III) oxyhydroxide

Abstract By homogeneous injection of alkali (NaOH) in acidified Fe(III) solutions of varying Cl Fe ratios (0–12.8) the formation of a ferrioxyhydroxide phase of colloidal proportions was studied. During the base titration experiment, compositional changes were recorded continuously through optical density measurements. The aging of samples taken at various stages ( OH Fe ratios) in the neutralization process and which eventually led to the formation of a colloidal dispersion, was followed by light scattering, ultracentrifuge and electronmicroscopic observations. An analysis of the experimental results shows that the chloride ion is an important constituent of the polynuclear particles formed during the early titration stage (pH