Kirill L. Shafran

The systematic study of aluminium speciation in medium concentrated aqueous solutions.

Industrial applications and environmental problems involving the aqueous chemistry of aluminium require an understanding of the speciation of this metal ion at a wide range of concentrations. The formation of polynuclear species is of special interest due to the complexity of the hydrolysis mechanisms and the diversity of the hydrolysis products. Kinetic aspects of speciation are also important considering the different stability ranges of polycationic species formed during the hydrolysis process. In the present paper we report results of systematic studies on the formation of aluminium polycations at room temperature. Automated potentiometric titrations have been used to study the hydrolysis of aluminium-ions in solutions (0.01-0.2 M) on a short time scale (2 min between titrant additions). (27)Al NMR spectroscopy and dynamic light scattering have been used for investigations on a longer time scale (24 h). The effects of alkali strength (KOH, NH(4)OH and KHCO(3)) and concentration (0.45-2.0 M), counterion identity (Cl(-), NO(3)(-), SO(4)(2-)) and ionic strength have been investigated. Optimum conditions for the generation of Al(13)-mer are proposed on short and long time scales. On a short time scale, aluminium chloride and nitrate should be used as starting materials, KOH and KHCO(3) should be used for hydrolysis and experiments conducted at low ionic strength. For solutions that have been left to age, there is a considerable hydrolysis window that can be used to generate significant quantities of the Al(13)-mer that vary little with the alkali used. Al(13)-mer species are not generated from alum as the precursor. The presence of sulphate ions alters the pathway of aluminium polymerisation to form polymeric and solid materials. On the basis of the potentiometric titration data, dynamic light scattering and (27)Al NMR measurements evidence is provided for the detrimental role of sulphate-ions in the formation of Al(13)-mer and an alternative mechanism of aluminium ion polycondensation is proposed, based on the increased stability of monomeric and oligomeric species (dimer and trimer) in the presence of sulphate-ions.

Comparative study of the influence of several silica precursors on collagen self-assembly and of collagen on ‘Si’ speciation and condensation

In order to understand the possible interactions between silicon species and collagen, the effects of sodium silicate, molecular complexes of silicon and silica nanoparticles on the collagen self-assembly process have been extensively studied at a range of concentrations from ca. 8 × 10−5 to 1 × 10−2 M. The mode of interaction between collagen and ‘silicon’ appears species dependent. Depending on its concentration, silicate solutions either promote or hinder collagen fibrillogenesis. Low concentrations of a silicate solution promote fibril formation as does the addition of a silicon catecholato complex. The presence of silica nanoparticles and concentrated silicate solutions hinders fibril formation. The data obtained suggest that there may be direct interaction between the various ‘Si’ containing species and the collagen triple helices as initially formed. The effect of collagen self-assembly on ‘silicon’ speciation/condensation has also been investigated. These studies showed that collagen modifies the equilibrium distribution of molecular silicon species in solution, but to a much lesser extent than has been observed for other proteins extracted from biosilicifying organisms.

Studies of biosilicas: structural aspects, chemical principles, model studies and the future

Silica and silicates are extensively used in industry and medicine. The materials find use in paints, foods, medicines, adhesives, detergents, chromatography materials, catalysts and photonic materials (reviewed by Iler 1979). Silica may be produced at high temperature, via aqueous processing or by largely non-aqueous routes such as the low-temperature sol-gel process (reviewed in Brinker and Scherrer 1990; Hench and West 1990).Whatever the eventual use of the silica, it is its structure that determines its properties. By structure we mean order and organization on length scales from angstroms to the size of the final object, morphology, surface area, porosity and surface functionality. The essential building block is the SiO4 tetrahedron although other structural units such as the SiO6 octahedron are also used. These units can be put together in a wide range of patterns to yield both porous and non-porous crystalline materials including silica-based zeolite materials. As well as crystalline silica and silicates, an extremely diverse range of amorphous materials exist, such as disordered precipitates, gels, glasses and shaped objects (spheres, screws,hollow tubes etc.; Stoer et al. 1968; Yang et al. 1997; Miyaki et al. 1999), produced under a wide range of synthesis conditions. The amorphous materials, in contrast to their crystalline analogues, exhibit no long-range order and are built up from SiO4 tetrahedra with variable SiiaOiaSi bond angles and SiiaO bond distances.

Interactions of aluminium hydrolytic species with biomolecules

In this contribution the formation of bioinorganic assemblies between the basic globular protein lysozyme and aqueous aluminium species including Al13-mer, Al30-mer and colloidal aluminium hydroxide have been explored and comparison made to previous interaction studies performed with bovine serum albumin (BSA). Specific charge-stabilised bioinorganic assemblies involving aluminium species and lysozyme were observed to form in contrast to the gel like structures formed on interaction of BSA with aluminium species. As demonstrated by infrared spectroscopy (structural assignment, 2D correlation spectroscopy), interactions mostly involve acidic surface groups of the proteins (Asp, Glu), with strong complexation and deprotonation in the case of BSA interacting with Al13 and Al30 and through hydrogen bonding for lysozyme interacting with the same species and aluminium hydroxide particles interacting with both biomolecules.

Super-resolution in situ ultrasonic monitoring of chemical reactions

This paper describes experiments to compare the sensitivity and robustness of ultrasound measurements with pH measurements when monitoring chemical reactions under laboratory conditions, the aim being to determine the relative suitability of the two techniques for process monitoring and control. It is shown that ultrasonic time-of-flight measurements, based on the centre of the area of an ultrasonic pulse, provides for super-resolution with respect to the sampling frequency. In comparison to reaction monitoring based on pH measurements, ultrasound was found to be superior in terms of its response time and resolution.

Resolving Complex Chemical Processes: Comparison of Monitoring by Ultrasound with Other Measurement Methods

A weak acid-strong base time-resolved titration has been monitored by ultrasonic and pH measurements. Various treatments have been applied to the acquired data, and the results obtained are presented and discussed. It has been found that the group velocity of ultrasound is the most robust parameter after enhancement of the time resolution of the ultrasonic hardware by signal processing. Application of temperature correction transformed the ultrasonic titration curves into two nearly straight lines that intersect at the equivalence point of the titration at the maximum point on the curve. The ultrasonic instrument exhibited much lower stabilisation times after titrant addition compared to the conventional pH-meter making ultrasonic spectroscopy potentially very useful for monitoring/controlling fast chemical processes including acid-base reactions