Franca Jones

Proline-functionalised calix[4]arene: an anion-triggered hydrogelator

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.

Molecular modeling of water adsorption on hematite

This paper describes the results of modeling the surface hydration configurations formed when different planes of the hematite crystal were exposed to water using empirically derived potentials able to replicate the hematite, goethite and lepidocrocite structures to within 2% of their measured values. The planes chosen were the {111}, {011} and {210} planes expressed in rhombohedral coordinates. It was found that of all the surfaces studied there was a preference for hydration on the O-terminated basal {111} plane. This plane had the lowest hydrated surface energy and it was also the most stabilised by reaction with water. The Fe-terminated {111} plane was found to be unstable in the presence of excess water (67% coverage). The surface iron atoms relax away from the simulation cell to leave the O-terminated hydrated layer behind. Chemisorption may be energetically feasible at low surface coverages (<67% coverage). The {011} plane of hematite showed a preference for 100% water coverage (full coordination of the surface iron atoms). The surface energy of adsorbing water on this plane was lower than for the {210} plane particularly at high water coverages. The {210} plane was not stabilised by reaction with water at any coverage. The surfaces underwent relaxations depending on the water coverage. Large relaxations were observed at lower coverages for the {011} plane while the largest relaxations were observed at higher coverages on the {210} plane.

Controlling crystal growth with modifiers

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.

Aspartic acid as a crystal growth catalyst

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.

Investigation into the effect of phosphonate inhibitors on barium sulfate precipitation

The effect of a series of phosphonate molecules on barium sulfate precipitation was tested. While an increase in the number of phosphonate groups generally resulted in increased inhibition of barium sulfate precipitation, two notable exceptions showed that a relatively high number of phosphonate groups does not guarantee inhibition while a relatively low number of phosphonate groups does not imply no inhibition. Increasing the pH showed an increased effect of additives on barium sulfate precipitation up to pH 8. However, on increasing from pH 8 to 12, a loss of inhibition in the additives was observed which appears to be due to the barium sulfate surface changing with pH. r 2002 Elsevier Science B.V. All rights reserved.

The role of phosphonate speciation on the inhibition of barium sulfate precipitation

Abstract The inhibition of barium sulfate precipitation in the presence of phosphonate containing molecules was investigated experimentally and speciation curves were used to elucidate the interactions involved. Inhibition of precipitation was found to be pH dependent and loss of inhibition was observed at both very high and low pHs. Maximum inhibition for all the inhibitor molecules occurred at pH 8. While speciation curves showed that inhibition could be improved by the presence of 2 or more fully de-protonated phosphonate groups (for pure aminophosphonates) on the molecule at pH⩽8, at pH 12 inhibition was insensitive to the number of de-protonated phosphonate groups. It is, therefore, suggested that surface charge repulsion affects inhibition at very high pH. For molecules which are not pure aminophosphonates, stereochemistry, functional groups and the ionisation state appear to play a significant role in inhibition at 3

Infrared investigation of barite and gypsum crystallization: Evidence for an amorphous to crystalline transition

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.

IN SITU CHARACTERISATION OF CALCITE GROWTH AND INHIBITION USING ATOMIC FORCE MICROSCOPY

Real time in situ monitoring of calcite growth and inhibition on the cleavage plane is investigated using atomic force microscopy (AFM). Calcite growth and inhibition were studied using a Molecular Imaging microscope in contact mode, equipped with an in situ fluid cell. As has been reported previously, it is observed that calcite growth from aqueous solution is by motion of mono-molecular steps, and dissolution by a combination of step motion and etch pit expansion. The measured step heights were between 2.7 to 3 A. Our aim is to study the effects of a range of phosphonate-based crystal growth inhibitors of related structure, in order to provide insight into the mechanism of inhibition as a function of the additive structure. Results of this study demonstrate the effect of inhibitors on the growth steps and terraces on the surface of calcite in real time. The organic additives bind to the crystal surface, with selective binding to the step edges particularly evident in some cases. In other cases, it appears that the additive acts by binding to the terraces on the crystal surface, resulting in inhibition by coating the surface. Efforts to correlate the observations made by atomic force microscopy with bulk crystallization experiments have been made.

Advanced bile acid-based multi-compartmental microencapsulated pancreatic β-cells integrating a polyelectrolyte-bile acid formulation, for diabetes treatment

This study utilized the Seahorse Analyzer to examine the effect of the bile acid ursodeoxycholic acid (UDCA), on the morphology, swelling, stability, and size of novel microencapsulated β-cells, in real-time. UDCA was conjugated with fluorescent compounds, and its partitioning within the microcapsules was examined using confocal microscopy. UDCA produced microcapsules with good morphology, better mechanical strength (p < 0.01), and reduced swelling properties (p < 0.01), but lower cell viability (p < 0.05) and cell count per microcapsule (p < 0.01). UDCA reduced the cells’ biochemical activities, mitochondrial respiration, and energy production, post-microencapsulation. This is the first time biological functions of microencapsulated β-cells have been analyzed in real-time.

Phosphonate additives do not always inhibit crystallization

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).