William R. Richmond

Synthesis and characterisation of ferrihydrite/silica co-precipitates

The effect of the presence of soluble silicates on ferrihydrite precipitation and some properties of the products formed in co-precipitation of ferrihydrite and silica have been investigated. The co-precipitates were formed using a continuous crystallisation process in which a combined iron/silicon feed solution was reacted with sodium hydroxide at a constant rate, while maintaining pH at 2.65 and temperature at 85 degrees C. The products of co-precipitation and the supernatant solutions were characterised using a variety of analytical techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM) and surface charge measurements. The addition of silicates was shown to have a significant impact on the crystallinity and surface charge of the precipitates formed. For products collected after five residence times in the continuous crystalliser, co-precipitates formed from ferric sulfate solution were found to contain considerably less silica than those formed from ferric nitrate. We conclude that adsorption of silicate species on ferrihydrite surfaces speeds up the polymerisation process, and that sulfate ion competes with silicate for surface adsorption sites. Thus, the precipitation of silica proceeds much more rapidly in ferric nitrate media, than in ferric sulfate.

Cation–calixarene interactions: tetraalkylammonium cation binding by calixarene anions

Syntheses of a number of tetraalkylammonium derivatives of various calix[n]arenes (n= 4, 6, 8) have been conducted by reacting the tetraalkylammonium hydroxides with the parent calixarenes. 1 H NMR studies in acetone and dimethylsulfoxide (DMSO) solvents have shown that cation inclusion by the anionic calixarenes occurs in some cases for only the tetramethyl- and tetraethyl-ammonium cations, indicating that different forms of ion association must occur in solution. The nature of cation–anion interactions in the solid tetraethylammonium salts of the monoanions of p-tert-butylcalix[4]arene (L2) and its dihomooxa analogue (L3) has been explored by room temperature single-crystal X-ray studies of the two salts and the parent ligand L3, all recrystallised variously solvated from acetonitrile. In both solid salts, columnar arrays of alternating cations and anions are found: in the dihomooxa ligand (L3) salt, the cations are captured within the ligand cup, forming contact ion pairs, the column comprising stacks of these. By contrast, the columnar array found in the parent ligand (L2) salt consists of discrete moieties, with the anion cavity occupied by acetonitrile. The free (neutral) ligand L3, as its acetonitrile monosolvate, contains the acetonitrile within the ligand cavity also. The structure determination of the unsubstituted calix[4]arene ligand, L1, is recorded as its ⅓ acetonitrile solvate, with location of all phenolic hydrogen atoms bridging their associated oxygen atoms; the acetonitrile in this case simply occupies a lattice void, as in the isomorphous acetone solvate.

The effect of zinc sulfide on phase transformations of ferrihydrite

Abstract The addition of ZnS particles to suspensions of ferrihydrite promotes the formation of the more crystalline phases goethite [α-FeO(OH)] and hematite (α-Fe2O3) at pH ~ 2.3 and 85 °C. This previously undocumented effect appears to arise from surface-mediated reduction of Fe3+ species to Fe2+ with associated dissolution of ZnS.

Alkali-metal ion complexes of the calixarenes. Part 1. Caesium bonding in calix[4]arene systems

A crystal structure determination on the monocaesium derivative of bis(homooxa)-p-tert-butylcalix[4]arene (L2), [Cs(L2– H)(OH2)3]·xH2O (x≈ 3)[orthorhombic, Pmnb, a= 26.666(5), b= 15.928(4), c= 11.499(9)A, Z= 4, R= 0.071 for 1500 ‘observed’ reflections] showed that the caesium may be considered to be involved in polyhapto interactions with carbon atoms in two of the four phenyl rings as well as co-ordination to two phenolic oxygens, the ether oxygen and three water molecules. The nature of the polyhapto aromatic interactions seems to be similar to that of those previously observed as the exclusive mode of calixarene binding in the caesium complex of the monoanion of p-tert-butylcalix[4]arene. Both complexes can be described as cation inclusion compounds, and the retention of this solid-state structure in some solvents is associated with very low-field 133Cs nuclear magnetic resonances. On the basis of 133Cs NMR spectra, it can be concluded that a series of calix[4]arenes bridged at the upper rim by an aliphatic chain –(CH2)n– also forms inclusion complexes with Cs+ for n= 7–10, in accord with Cs+ transport studies previously reported for a similar series.

Calixarene-cupped caesium: a coordination conundrums?

Determination of the crystal structure of the mono-caesium derivative of the macrccyclic tetraphenol p-tert-butylcalix[4]arene shows the metal atom to be bound within the cup formed by the ligand; the metal is thereby in much closer proximity to the aromatic carbon atoms than to the phenolic oxygens, indicating that it is involved in polyhapto bonding to a delocalised anionic ligand.

Insights into the crystal and aggregate structure of Fe3+ oxide/silica co-precipitates

Abstract Structural characteristics of Fe3+ oxide/silica co-precipitates were investigated. The association between these materials is relevant to practically all natural aqueous systems due to the prevalence of iron and silicon in the Earth’s crust. Crystallographic information is very difficult to obtain from these precipitates due to the nanocrystalline nature of ferrihydrite and the amorphous structure of precipitated silica. Several previously undetermined key insights were gained into the structure of iron oxide/silica co-precipitates through this examination. The distribution of iron and silicon throughout co-precipitate particles is illustrated along with the influence of their association. Evidence to the governing factor behind differences in apparent crystallinity is also presented. This information culminates in the formulation of a precipitation pathway, displaying the formation of the co-precipitates.

Small molecules induce mesocrystal formation: nanoparticle aggregation directed by self-assembling calixarenes

Calixarenes have been shown to induce mesocrystal formation of barium sulfate, despite being relatively low molecular weight additives. Scanning probe microscopy has shown that a possible mechanism is the self-assembling properties of the calixarene resulting in steric stabilization of the nanoparticles, comparable to that typically requiring polymeric additives.

An electron microscopy study of β-FeOOH (akaganéite) nanorods and nanotubes

High-resolution TEM images reveal that samples of β-FeOOH (akaganeite) prepared by quenching of a condensed ferrihydrite gel contain a mixture of rod-like particles and tubes. The tubular particles are usually about 10–15 nm in diameter with a central void that is typically 1/3 of the particle diameter. Dark-field STEM images show that the tubular particles are made up of single crystals that extend across the whole tube diameter, but only rarely extend along the whole length of the tube. Both the solid rods and the tubes appear to be based on subunits of approximately 3–4 nm in diameter, and it is proposed that formation of the akaganeite particles, both tubes and rods, results from secondary nucleation of these subunits at sites on particle edges, followed by rapid linear growth along the c-direction of the akaganeite structure.

The effect of phosphonate-based growth modifiers on the morphology of hematite nanoparticles formed via acid hydrolysis of ferric chloride solutions

The effect of organic phosphonate-based additives on α-Fe2O3 (hematite) crystallization via the forced hydrolysis of ferric chloride solutions has been studied using a range of additives containing 2, 3 or 4 pendant phosphonate groups. The hydrolysis reactions were carried out at pH 1.1 with an iron concentration of 0.01 mol L−1, and with Fe ∶ additive ratios ranging from >8000 ∶ 1 down to 200 ∶ 1. In the absence of additive, the hematite particles are very uniform rhombic single crystals, with an average length of ≈100 nm. In the presence of phosphonates the particles become hexagonal, with the additives acting by inhibiting growth at (2) faces. At Fe ∶ additive ratios lower than about 200 ∶ 1, hematite formation is completely inhibited. Our results are consistent with a mechanism in which the akaganeite and hematite phases may both precipitate from the ferric chloride solution, with akaganeite apparently forming more rapidly than hematite. Inhibition of hematite formation by addition of the phosphonate additive can lead to β-FeOOH (akaganeite) being the dominant product phase, provided the concentration of Fe3+ in solution is above the equilibrium solubility of the akaganeite phase.

Alkylammonium cation interactions with calixarene anions. Part 2. Structural characterisation of a salt of 2:3 cation: calixarene stoichiometry

A room temperature single crystal X-ray structure determination on the tetramethylammonium ‘salt’ of calix[4]arene ([1.1.1.1]metacyclophane-7,14,21,28-tetraol) has revealed an unusual stoichiometry and unusual structure for this compound. Crystals of salt are tetragonal, space group P4/nnc, a= 18.230(4), c= 22.897(6)A, Z= 4 f.u.; the structure was refined to a residual of 0.087 for 1471 ‘observed’[I > 3σ(I)] reflections. The solid has the composition LH4·2[NMe4]+[LH3]–·H2O (LH4= calix[4]arene) and contains the cation, ‘C’, in two distinct environments, both involving inclusion within the calixarene units but one involving simple inclusion by a single calixarene (‘A’) and the other encapsulation by a facing pair of symmetry-related calixarene entities (‘B’). Interaggregate spacings by lattice water molecules, W, result in the ‘A’ moieties being stacked up the 4 axis ⋯ WACCAW ⋯, while the ‘B’ moieties are stacked along the 2 axes in the ab diagonal, ⋯ W′BCBW′.