Search

The impact of macrocycles on calcium oxalate formation. Calcium oxalate is a major component of kidney stones, an ailment that affects many people globally.

Kaolinite dissolves faster at edges, and steps have a fast-dissolution direction.

Interfacial ion transfer from organic phase to aqueous phase is employed as the basis for formation of barium sulfate crystals close to the interface.

Crystallisation is performed in a non-aqueous solvent, dimethylsulfoxide (DMSO), in order to determine what the role of water is on the crystallisation pathway. In both water and DMSO environments the particles do not appear to grow by ion addition but rather appear to grow through aggregation. The main difference in water is that the aggregation processes are not random and result in particle morphologies that bear relation to the single-crystal faces, suggesting an oriented attachment mechanism. In DMSO, the aggregation processes appear less oriented and while there is some lattice registry, lattice mismatch is also observed and the aggregate shape is spherical overall. This is confirmed in the 1200 cm−1 infrared band shift to 1174 cm−1 suggesting a strained solid is formed. It is observed that the solubility of barium sulfate in DMSO is higher than in water (presumably caused by a strong Ba2+⋯OS(CH3)2 interaction), which explains the lower nucleation rates in DMSO compared to water at the same concentration. Intriguingly, there is a lower nucleation rate observed (even at a relatively high supersaturation) when Ba2+ is solvated with DMSO supporting the hypothesis that de-solvation of the cation is the rate determining step in nucleation and is of higher activation energy in DMSO than in water.

Aragonite formation and stabilization in seawater is still an area of active investigation since the thermodynamically stable product at room temperature is calcite. In this manuscript, purely inor...

International audience; Controlled formation of cocrystals is an important objective in drug development. Here, cocrystallisation of caffeine and 1-hydroxy-2-naphthoic acid was investigated at the interface between two immiscible electrolyte solutions under chemical polarisation. In this way, selective cocrystallisation was achieved, as verified by X-ray diffraction and Raman spectroscopy. Positive interfacial potentials favoured the formation of one polymorphic form of caffeine:1-hydroxy-2-naphthoic acid cocrystal. This approach to electrochemically controlled cocrystallisation opens up new possibilities for drug development. The search for innovative crystallisation methods is a subject of constant interest as most drugs appear as crystalline forms whose polymorphs may have different toxicity, hygroscopicity or physical and chemical stability. 1-3 An additional issue encountered in drug development is the low stability in humid environments

Dimension, composition and configuration are the primary aspects of semiconductor based hybrid heterostructures that govern their properties. To achieve desired functionalities within a single nanoparticle system, numerous synthetic approaches are available which may be suitable for achieving the target combination but these can be complex and tedious. In this work, a simple seeded-growth approach was employed to obtain an iron oxide–gold–cadmium sulfide (Fe3O4–Au–CdS) heterotrimer with a unique morphology manifesting magnetic, plasmonic and semiconducting properties. In a straightforward two-step synthesis process, Au–Fe3O4 dimer was injected as a seed into the Cd precursor solution for the growth of one-dimensional (1D) kinked CdS-5 nanorods on it. The synthesized Fe3O4–Au–CdS heterotrimer was tested in photocatalytic water-reduction reaction, revealing its potential as a photocatalyst for the hydrogen evolution reaction (HER), where each domain within the heterotrimer played a distinct role including light absorption, charge separation and magnetic separation. A post-synthetic modification on the semiconductor part was also performed where kinked CdS nanorod was cation exchanged with Ag2S leading to the formation of another heterotrimer of Fe3O4–Au–Ag2S while retaining the same morphology and expanding the palette of heterotrimers.

Macrocyclic compounds such as DOTP (1,4,7,10-tetraazacyclododecanetetrakis(methylenephosphonic acid)) and NOTP (1,4,7,-triazacyclononanetri(methylenephosphonic acid)) are found to inhibit precipitation of barium sulfate just as potently as their non-cyclic counterparts depending on the ionisation state of the molecule. Morphologically, DOTP has a more significant influence on the shape of barium sulfate particles formed than NOTP while turbidity results show that NOTP does not significantly affect the induction time observed, unlike DOTP, which does impact the induction time at higher concentrations. Overall, the inhibition of these macrocyclic compounds can be explained by the number of de-protonated phosphonate groups and is not significantly impacted by the presence of the ring. The presence of calcium ions during barium sulfate crystallization lowers the degree of inhibition for both NOTP and DOTP. Molecular modelling showed that an uncomplexed DOTP molecule could lattice match to the barium sulfate lattice but had fewer Ba⋯Ophos interactions than the equivalent non-macrocyclic molecule. The comparison of uncomplexed with complexed DOTP shows that the final configuration is similar for these two situations, thus in this case it is not the Ba⋯Ophos interactions but the overall replacement energy that is important.

This study presents the solubility of calcium carbonate in monoethylene glycol (MEG)/water solution at high temperatures and high MEG concentrations. The conditions simulate the water removal proce...

Three-component containing hybrid heterostructures with multiple functionalities are highly desirable but difficult to synthesize. Herein, we have demonstrated the utilization of a simple seeded-growth approach for the synthesis of a hybrid heterotrimer of iron oxide–silver sulfide–zinc sulfide (Fe3O4-Ag2S-ZnS). At first, Fe3O4-Ag was synthesized by using silver nanoparticles (Ag NPs) as a seed followed by its in situ sulfurization to produce a dimer of Fe3O4-Ag2S. This dimer was successively used as a seed under controlled experimental conditions for the synthesis of Fe3O4-Ag2S-ZnS. The availability of such trimers through this approach can shed some light on the integration of entirely different functionalities in one-particle systems.

The nature of crystallisation processes is of major interest, as they are among the most frequently occurring reactions associated with a variety of relevant processes in chemistry, biochemistry, and geochemistry. In this study, an innovative approach towards fundamentally understanding crystallisation pathways in a seemingly simple system – gypsum – has been developed via infrared spectroscopic techniques. Specifically, infrared attenuated total reflection spectroscopy (IR-ATR) was instrumental in revealing detailed information on inter- and intramolecular interactions during gypsum crystallization via subtle changes in the vibrational spectra of the involved reactants. When applying D2O as an isotope marker, it was shown that isotopically labelled water may serve as a viable spectroscopic probe during mid-infrared (3–15 μm) studies providing unique insight into the crystallization process at molecular-level detail. In addition, it was revealed that H2O and D2O give rise to distinctly different reaction kinetics during the crystallization process.

An aspartic acid functionalised azobenzene derivative is found to be a light-switchable crystal growth modifier of calcite. UV irradiation of the molecule reversibly switches it to the cis isomer, which is a significantly less effective crystal growth inhibitor than the trans isomer. Visible light, or heat switches the inhibitor back “on”. Extended irradiation degrades the inhibitor such that it is irreversibly switched “off”. It was shown that the trans isomer is preferentially absorbed on to the crystal surface, which is consistent with its greater efficacy as an inhibitor.

Four possible models for anhydrous calcium oxalate (COA) polymorphs have been investigated through ab initio quantum mechanical methods. Their structural properties, infrared and Raman spectra, and thermodynamic stability in the range of 0–800 K have been analyzed and compared. Along with the known β-COA structure, two models turn out to be possible candidates for the α- and γ-polymorphs that were observed during dehydration of weddellite (calcium oxalate dihydrate, COD) by Walter-Levy and Laniepce (C. R. Acad. Sci. Paris 1964, 259, 4685). While the calculated vibrational frequencies show that the four COA models correspond to minimum energy structures, β-COA is the thermodynamically favored phase over the range of temperatures examined in the present study. Despite the fact that computed vibrational spectra and X-ray diffraction (XRD) patterns of these polymorphs exhibit some different features, a definitive assignment of the structures based on computational results is not possible due to the lack of ac...

Barium sulfate was crystallized in a synthetic seawater mixture that was chosen to better reflect ocean conditions. The synthetic seawater contained monovalent ions, magnesium, strontium, and calcium as well as bicarbonate and boric acid. The natural pH of the synthetic seawater is 8.1, and this seawater was used to determine the impact on morphology, nucleation rates, and incorporation of foreign ions. It was found that dendritic and diamond-shaped particles are both formed. The main parameters influencing the formation of dendritic particles were the saturation index and the ion ratio, but there was also a significant synergistic effect with the other ions present. The diamond-shaped particles formed later at a lower saturation index. The nucleation rate in synthetic seawater was found to be higher than expected based on an ion ratio basis. This is most probably because the divalent ions induce a higher nucleation rate by lowering the surface free energy. Strontium was found to be the dominant ion subst...

Molecules containing tetrazole substituents have become of interest due to their being bioisosteres of carboxylic acids and like their carboxylate counterparts, tetrazolate anions have been able to affect the crystal growth of barium sulphate and calcium carbonate. In this proof of principle study, we show that this behaviour also extends to calcium oxalate and therefore opens the possibility of using tetrazole-based additives for investigating mineralization processes of human pathological relevance.

The assembly and disassembly of supramolecular gel fibres are observed in situ using variable temperature scanning probe microscopy. The results show that fibre formation can be monitored at high resolution at a surface, and the final fibre morphologies are broadly consistent with those found by ex situ analysis of the bulk gel. The impact of a gelation inhibitor upon the fibre morphology is successfully investigated, providing direct evidence for the mechanism of inhibition as a function of additive concentration.

Empirical molecular modelling was used to investigate the impact of organic additives on crystal morphology and inhibition. The replacement energy was found to correlate reasonably well with the degree of inhibition as determined from conductivity data. The replacement energy was also able to predict the barium sulfate face on which additive adsorption was most likely. While the ability of the organic functional groups to sit in the vacant sulfate lattice positions (the so-called ‘lattice matching’ criteria) appears intuitively sensible, it was found that this is not a sufficient criterion to predict real behaviour. A better criterion is the overall replacement energy as it takes into consideration the number of Ba–Oorganic interactions and whether the adsorption process overall is energetically favourable (by including the hydration energy of the ions). Thus, the replacement energy can successfully predict the effect of organic molecules on the crystal growth modification of barium sulfate.

Calix[4]arenes functionalised at the upper rim with acidic amino acid residues are found to have a significant impact on the crystal growth of model mineral systems, calcium carbonate and barium sulphate. The aspartic acid derivative is found to be most efficacious, matching or exceeding the impact of commercial phosphonate-based scale inhibitors. In some cases, the modified morphologies are found to be similar to those induced by proteins isolated from biomineralised systems.

Dipositive cation substitution in sulfate minerals has been a topic of investigation both experimentally and computationally. Here the possibility of tripositive cation substitution with accompanying vacancy formation is considered, specifically lanthanum ion substitution in barium sulfate. Experimentally, the lanthanum ions impact “bulk” crystallization in a way that is best explained by considering the solution phase ion-pair interactions. However, morphological results suggest that lanthanum ions do more than impact the free sulfate levels. Atomic force microscopy results showed that both lanthanum and calcium ions are strong inhibitors of barite growth. X-ray diffraction on bulk samples showed that lanthanum ions do indeed incorporate, as determined by lattice parameter changes. Finally, computational modeling suggests that while calcium incorporation is more energetically favorable, lanthanum ions prefer to substitute on the surface more so than calcium ions.

Calixarenes can be used as well-defined scaffolds for investigating structure–activity relationships of additives and their impact on crystallization. In this work, we present the crystal growth modification of barium sulfate by p-phosphonic acid calix[n]arenes that vary in size (n = 4, 5, 6, and 8) and thus vary in the size of the internal cavity for the same functionality in the upper rim. The tetrameric, hexameric, and octameric macrocycles induce nanoparticle formation with clear superstructure. In the case of the hexameric calix[6]arene, the initial mesocrystalline superstructure fuses over time to form almost hollow spheres, while the mesocrystals formed in the presence of the tetramer and octamer are stable over an extended period. The pentameric calix[5]arene forms more disordered aggregates of single crystals. Thermogravimetric data shows that a significant proportion of the mass of the barium sulfate-containing solid is the macrocycle, regardless of the choice of macrocycle.

The thermal annealing behaviour of an electrolyte-triggered calixarene hydrogelator is found to depend strongly on the specific metal chloride used. While the lithium chloride gel showed typical gel–sol transitions as a function of temperature, the magnesium chloride gel was found to repeatedly strengthen with heat–cool cycles. Structural investigations using small-angle neutron scattering, and scanning probe microscopy, suggest that the annealing behaviour is associated with a change in morphology of the fibrous structures supporting the gel. On prolonged standing at room temperature, the magnesium chloride gel underwent a gel–crystal transition, with the collapsing gel accompanied by the deposition of crystals of a magnesium complex of the proline-functionalised calix[4]arene gelator.

The presence of silicate during barium sulfate crystallization has different impacts depending on the pH of the solution. At pH 7 the dominance of the protonated form (H4SiO4) and possible polymerization of the silicate impacts mainly on the aggregation state and on twinning of the barium sulfate formed. At higher pH values (∼10), the silicate ion present is able to influence both morphology and partially substitute for sulfate in the lattice. Interesting fibrous particles are formed under these conditions, but this is not due to mesocrystal formation as the particles are observed to be single crystalline in nature. These fibrous sections are found to be dominant on the surface and are highly porous. These particles are different, however, to the biomorphs formed when crystallization of barium carbonate occurs in the presence of silicate. This is because the speciation of sulfate does not change over a large pH range. The impact of silicate on barium sulfate particles is similar to the impact on calcium c...

Although the effects of p-sulfonated and p-phosphonated calix[4]arene on barium sulfate morphology are similar, their ability to form mesocrystals of material are markedly different. The p-phosphonated calix[4]arene results in the formation of fibre bundles similar to those previously observed during barium sulfate crystallization in the presence of di-block copolymers. The isostructural sulfonated calix[4]arene, however, affords material consistent with the initial formation of mesocrystals which subsequently fuse. This material shows significant beam damage when viewed under a transmission electron microscope (TEM) suggesting incorporation of the sulfonated calix[4]arene, although there was no evidence of this from X-ray diffraction and atomic force microscopy (AFM) studies. We hypothesise that this calixarene is incorporated without significant change in structure of the material, and that the interaction of the macrocycle with the barium sulfate lattice is sufficiently weak that surface AFM imaging is effective in removing it from the surface.

This paper investigates crystal growth modifiers based on 1,3,5-substituted benzene derivatives. The results show that as expected, the phosphonated derivative inhibits calcite precipitation to a much greater degree than the analogous sulfonate. However, on barium sulfate, both molecules show some crystallization promotion behaviour, with the phosphonate being the more potent promoter overall. Thus, the functional group alone does not determine the impact the organic molecule will have on crystallization. This opens the way for additives that have dual purposes (inhibiting the crystallization of one phase while not impacting or promoting the crystallization of other phases).

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.

Modifying crystal growth processes using additives is a well established approach to solving problems in many processes. Nature also makes extensive use of crystalline inorganic structures modified by soluble and insoluble organic materials. This Highlight discusses some of the recent and interesting developments in this area, with an emphasis on the control of crystal growth rates, covering both inhibitors and the lesser known promoters of crystal growth. Hybrid materials resulting from the incorporation of modifiers into crystalline structures are also discussed, with an emphasis on non-classical crystallisation mechanisms, involving the oriented attachment of nanoparticles.

This paper investigates the influence of barium sulfate scaling and scaling inhibitors on the electrochemical behaviour of stainless steel. The results of a synchrotron radiation grazing incidence X-ray diffraction (SR-GIXRD) and electrochemical impedance spectroscopy (EIS) study on stainless steel shows that different scaling inhibitors interact uniquely with the substrate when barium sulfate is precipitated on the electrode surface. The main effect of the substrate in the presence of inhibitor is a tendency to form smaller barium sulfate particles. The SR-GIXRD patterns obtained in the presence of the two inhibitors were different to each other and to the control, with the carboxylate showing greater amounts of barite solids precipitated together with iron(III) and (II) sulfate, while the phosphonate showed low amounts of barite solid were precipitated. The presence of iron sulfates on the electrode surface as detected by SR-GIXRD, in the case of NTA, suggests that scaling inhibitors are not always benign, and can promote the dissolution of iron species from the substrate.

It is well understood that the solvent can affect precipitation kinetics, and all chemists are very familiar with the notion that adding an organic solvent to an aqueous solution induces the rapid precipitation of dissolved inorganic salts. The general explanation for this observation is that the solubility of the salt in the organic/water mixture is lower than in pure water and therefore precipitation occurs. Here we study the kinetics and thermodynamics of precipitation of barium sulfate from water/methanol mixtures. It is shown that addition of an organic solvent to a water solution affects not only the thermodynamics but also the kinetics of the precipitation. The kinetics of precipitation in water/methanol mixtures is faster than in pure water. This is the opposite of what would be expected from classical nucleation theory. Molecular dynamics simulations show that nucleation proceeds through the formation of amorphous aggregates that are thermodynamically more stable than the crystalline phase. The f...

Barium sulfate is a common scale in oil production installations that is treated and controlled with phosphonate inhibitors. A fundamental understanding of how these inhibitors operate, however, is only slowly emerging. In this paper, we investigate the effect on barium sulfate crystallization of two very similar phosphonate molecules that only differ in their backbone spacing, ethylenediamine-N,N,N‘,N‘-tetra(methylenephosphonic acid) (EDTMP) and hexamethylenediamine-N,N,N‘,N‘-tetra(methylenephosphonic acid) (HDTMP). It was found that the inhibitory efficacy of the organic molecules depends on their structural differences but also on the presence of other cations such as Zn2+. It appears that both stereochemical considerations and complexation strength differences between the two phosphonate additives result in different inhibitory powers. In the presence of zinc cations and EDTMP, it is found that inhibition is related to the concentration of uncomplexed (“free”) organic.

The effect of various benzoic acids on the precipitation of barite (BaSO4) and calcite (CaCO3) was investigated. The acids varied in the number of carboxylate groups, from dibenzoic acids (phthalic, isophthalic, and terephthalic) through to the hexabenzoic acid (mellitic acid). It was found that the stereochemistry of the dibenzoic acids was important, as was the pH of the solution (trimesic acid was used as a test case and showed that greatest inhibition was achieved with all carboxylate groups deprotonated). Interestingly, for both the calcite and barite systems, mellitic acid was found to be both a potent inhibitor and a significant crystal growth modifier. In the case of barite, the presence of mellitic acid produced nanoparticles that agglomerated. The nanoparticles were found to be ∼20 nm in size from X-ray diffraction (XRD) line width analysis and 20−50 nm from transmission electron microscopy (TEM). Humic acid was also tested and found to form bundled fibers of barium sulfate.

The crystallization of materials from a supersaturated solution is a fundamental chemical process. Although several very successful models that provide a qualitative understanding of the crystal growth process exist, in most cases the atomistic detail of crystal growth is not fully understood. In this work, molecular dynamics simulations of the morphologically most important surfaces of barite in contact with a supersaturated solution have been performed. The simulations show that an ordered and tightly bound layer of water molecules is present on the crystal surface. The approach of an ion to the surface requires desolvation of both the surface and the ion itself leading to an activated process that is rate limiting for two-dimensional nucleation to occur. However, desolvation on specific surfaces can be assisted by anions adsorbed on the crystal surface. This hypothesis, corroborated by crystallization and scanning electron microscopy studies, allows the rationalization of the morphology of barite crystals grown at different supersaturations.

Crystal growth of inorganic materials, calcium carbonate, barium sulfate, and calcium oxalate, was studied in the presence of calix[4]arene additives functionalized with aspartic or glutamic acids at the lower rim. The additives were synthesized by reaction of the appropriate amino acid ester with 5,11,17,23-tetra-tert-butyl-25,-26,27,28-tetrakis(chlorocarbonylmethoxy)calix[4]arene, followed by controlled hydrolysis of the ester groups. Both additives inhibited the nucleation and growth of calcium carbonate but induced different changes in calcite morphology. The aspartic acid derivative was the more potent barium sulfate inhibitor at low concentrations, with comparable performance at higher levels. Both additives induced a polycrystalline barite precipitate. The aspartic acid functionalized calixarene also stabilized calcium oxalate dihydrate at low additive concentrations, whereas the glutamic acid derivative induced morphological changes in calcium oxalate monohydrate.

Molecular modelling has been successfully used to interpret the effect of two molecules on the crystal growth of barium sulfate. The replacement energy was found to correlate with the degree of inhibition as determined from conductivity experiments. It was also able to predict the preferred barium sulfate face for additive adsorption. For EDTP (ethylenediaminetetramethylenephosphonic acid), the energetically favoured adsorption configurations were those where the phosphonate groups occupied vacant sulfate lattice sites, whilst for EDTA (ethylenediaminetetraacetic acid) it was generally not possible for all the carboxylate groups to lie within the surface and so surface barium cation to additive oxygen interactions became important. Whether in the surface or above it, the number of Ba-Omodifier interactions were important in making adsorption of the modifier energetically favourable.

A boehmite-derived gamma-alumina (γ-Al2O3) system was studied using various complementary techniques to examine surface area and pore size, the amount of hydrogen-containing species, the nature of hydrogen bonding environments, and the location of these species. Using small-angle X-ray scattering, the material examined was shown to have a significantly higher surface area than that typically expected for highly crystalline boehmite-derived γ-Al2O3. This higher surface area was associated with the presence of closed nanopores, the size of which was found to complement observations from transmission electron microscopy. More hydrogen was determined to be in the structure when measured using prompt-gamma activation analysis than indicated by loss on ignition experiments, suggesting that hydrogen-containing species other than water were also present. Neutron vibrational spectroscopy and infrared spectroscopy showed a reduction in signals associated with water and hydroxide species as the calcination temperatu...

Solution-phase and solid-state structural studies indicate that the remarkable hydrogelation properties of a proline-functionalised calix[4]arene emerge as a result of extended helical structures formed via inclusion of a proline moiety in a neighbouring calixarene cavity.

A water-soluble, chiral calix[4]arene has been found to form hydrogels when triggered by the presence of specific anions, with efficacy linked to the Hofmeister series; the gel properties are modified by the associated cations, and gelation can be reversibly switched off by increasing pH.

Infrared was used to monitor the progression of barite (BaSO4) and gypsum (CaSO4·2H2O) crystallization. It was found that barite crystallized with broad infrared peaks that narrowed over time. This was accentuated when an inhibitor was added. The infrared of gypsum in the early stages showed a phase that was clearly different to either bassinite or gypsum. It is hypothesised that in the absence of inhibitors, the rate-determining step for barite crystallization is the transformation from a disordered solid to a crystalline solid as little water was observed to be present during this process when inhibitors are not present. All of these findings are consistent with a non-classical model of crystallization where disordered solids eventually rearrange to a crystalline solid if not inhibited. It also suggests that water may be key to determining the “lifetime” of the disordered clusters.

Tetrazoles are a class of organic compound often used as carboxylic acid analogues. This analogous behaviour is shown to extend to crystallization modification, that is, tetrazoles are also able to influence crystal growth and morphology although in a different manner to their carboxylate counterparts. All the tetrazoles investigated thus far are shown to impact on barium sulfate and calcium carbonate crystallization to varying degrees. Thus, the tetrazoles represent a new class of crystal modifier.

Ion desolvation is an important kinetic step in the growth of divalent ionic crystals—a category that encompasses numerous materials relevant to biomineralization. It has recently been shown for one such divalent ionic crystal that the rate-limiting desolvation of the cation can be assisted by the anion and that this process can be surface specific. Here we show that even a simple biological molecule, such as aspartic acid, can have a measurable catalytic effect on barite crystal growth and that this effect is related to the lowering of the activation barrier for cation desolvation. We therefore suggest that growth rate enhancement on specific faces through catalysis of the cation desolvation step may be a viable mechanism for the positive control of biomineralization.

Morphology modifiers can act in many different ways on the precipitating phase. Inhibitors can alter the solution chemistry and thereby change the supersaturation, they may adsorb onto the surface and block growth and agglomeration, they may alter the critical nucleus and affect nucleation or they may do a combination of these three. Previous work showing that nitrilotriacetic acid (NTA) had a significant effect on barium sulfate precipitation has been expanded upon in order to determine the mechanism by which the observed morphology modification occurs. The absence of significant complexation to barium at the pH of the experiment suggests that NTA affects the critical nuclei of the precipitating barium sulfate. This results in the longer induction times observed using nephelometry, and adsorption onto growth features is probably the mechanism of interaction. Molecular modelling shows that NTA adsorption is favourable on terraces and that hydrogen bonding improves the ability of an additive to adsorb. A fundamental understanding of the mechanisms by which additives affect crystal growth is essential if new additives, with predictable effect, are to be designed a priori.

Barium sulfate and potassium perchlorate have the same crystal structure but differ widely in their solubility and in their ionic charge. These systems, therefore, are an interesting means to probe how growth modifiers affect precipitation and what role the charge or stereochemistry of the modifier has on precipitation. It was found that the mode of interaction of the additive is dependant on the solubility of the system. Also, additives have a far greater effect on morphology when the solubility of the salt is low.

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.

The generally accepted view that phosphonate derivatives are more potent than the analogous carboxylates as crystal growth modifiers for barium sulfate has been systematically studied by using trifunctional molecules varying from the triphosphonate through to the analogous tricarboxylate; the results suggest that predictions based on simple structural features should be made with caution.