Electrical charge acquired by colloids or solid surfaces in aqueous systems plays a significant role in determining their properties. Homo- and heterocoagulation and flocculation, nonspecific ion adsorption and ion exchange, electrode process rates, and the configuration of organic macromolecules are all affected to some extent by surface charge or potential. Over the past decade, several lines of research and model development involving solids or surfaces as different as metallic mercury, virtually insoluble oxides, silver halides, proteins, polystyrene latexes, ionizable monolayers, and clays have converged. For colloids and hydrosols that contain acidic, basic, or amphoteric functional groups accessible at their surfaces, it is now possible to present a single, conceptually simple and easily visualized model capable of predictively modeling the development of surface charge and potential, the zeta potential of the solid, the conductivity of the sol, electrolyte adsorption densities, and related properties. It is our purpose to review the origins of this model, the methods of characterizing solids and colloids for its use, and the results obtained when it is applied to inorganic oxides, polystyrene latexes, and clays. We are convinced that the fundamental concepts embodied in the model provide a more realistic understanding and representation of ionic processes at surfaces than any single previous model, though it is a direct outgrowth of several earlier approaches. We hope that this review will stimulate a more unified view of organic and inorganic colloids and that it will trigger systematic study of the ionization and complexation reactions at surfaces and of the appropriate intrinsic equilibrium constants for surface functional groups.

Characterization of Aqueous Colloids by Their Electrical Double-Layer and Intrinsic Surface Chemical Properties

Formation and stability of schwertmannite in acidic mining lakes

The solubility of iron hydroxide in sodium chloride solutions

Arsenate and chromate incorporation in schwertmannite

Magnetite nanoparticles for biomedical applications are typically prepared using the coprecipitation technique, which is the most convenient method. However, the reaction pathways leading to the production of the magnetite phase in the coprecipitation reaction are not fully understood, despite the fact that the reaction path may be of significant importance in controlling the crystal structure, morphology, and particle size of the magnetite nanoparticles. In the present study, we identified the reaction pathways in the coprecipitation of magnetite; when base was slowly added to an iron chloride solution, akaganeite nucleated and transformed through goethite to magnetite. At high addition rates, an additional pathway in which ferrous hydroxide nucleated and transformed through lepidocrocite to magnetite competed with the former pathway. This difference was due to the pH inhomogeneity in the reaction medium that was present before homogeneous mixing. In most coprecipitation reactions, these magnetite format...

Formation Pathways of Magnetite Nanoparticles by Coprecipitation Method

0.5 molal iron(III) chloride solutions were hydrolysed at room temperature by base additions in the range OH/Fe mole ratio 0–2.75. After an ageing period the hydrolysed solutions were used to produce amorphous hydroxide gels from which crystalline products were grown at 65°C, at low pH or high pH. Examination of crystal composition and morphology and comparison with similarly treated nitrate solutions showed that the nucleation of hematite and goethite is inhibited in chloride containing solutions, which allow growth of small rod shaped β-FeOOH to predominate or occur exclusively in gels at pH 1–2. The addition of seed crystals of hematite and goethite allows competitive growth of all three minerals. The transformations β-FeOOH → α-Fe2O3 and β-FeOOH → α-FeOOH at pH 1–2 proceed by dissolution and reprecipitation and are promoted by adding seed crystals.

R. J. Atkinson

Papers

Crystal Nucleation and Growth in Hydrolysing Iron(III) Chloride Solutions