Manijeh M. Reyhani

Simulating micrometre-scale crystal growth from solution

Understanding crystal growth is essential for controlling the crystallization used in industrial separation and purification processes. Because solids interact through their surfaces, crystal shape can influence both chemical and physical properties. The thermodynamic morphology can readily be predicted, but most particle shapes are actually controlled by the kinetics of the atomic growth processes through which assembly occurs. Here we study the urea–solvent interface at the nanometre scale and report kinetic Monte Carlo simulations of the micrometre-scale three-dimensional growth of urea crystals. These simulations accurately reproduce experimentally observed crystal growth. Unlike previous models of crystal growth, no assumption is made that the morphology can be constructed from the results for independently growing surfaces or from an a priori specification of surface defect concentration. This approach offers insights into the role of the solvent, the degree of supersaturation, and the contribution that extended defects (such as screw dislocations) make to crystal growth. It also connects observations made at the nanometre scale, through in situ atomic force microscopy, with those made at the macroscopic level. If extended to include additives, the technique could lead to the computer-aided design of crystals.

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

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.

Atomic force microscopy study of the growth mechanism of gibbsite crystals

A combination of atomic force microscopy (AFM) and scanning electron microscopy (SEM) has been used to investigate the crystal growth mechanism of gibbsite (aluminium trihydroxide) in pure solutions. Under the conditions studied, the growth on the basal face of synthetic gibbsite prepared from sodium aluminate solution proceeds by a continuous birth and spread mechanism. The nuclei formed on the surface of the basal face of gibbsite grow both laterally and vertically, with lateral growth being much faster than vertical growth. Moreover, a remarkable cyclical, smooth→rough→smooth→rough process has been directly observed. The effect of alkali ions on the crystal growth of gibbsite has been investigated. Curved features and a growth hillock were observed for the first time on the basal face of gibbsite prepared from potassium aluminate solution; whereas, steps terminating within the plane and hexagonal shaped features tilted at an angle and related to the symmetry of the face were imaged on the basal faces of gibbsite prepared from sodium aluminate solution. The results suggest faster growth on the basal face of gibbsite prepared from potassium aluminate compared to that prepared from sodium aluminate, leading to the observed elongated morphology.

Anomalous behaviour within a systematic series of barium sulfate growth modifiers

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.

A comparison of secondary nuclei produced by contact of different growth faces of potash alum crystals under supersaturated solutions

Secondary nuclei of potash alum crystals may easily be produced by gentle crystal contact. In this investigation, crystal faces of the {100}, {110} and {111} families were identified in a parent crystal, and gentle contact between these and a solid surface in a slightly supersaturated solution of potash alum produced many secondary nuclei of the same orientation. Breeding of the large number of particles produced by contact between a parent crystal and a glass surface under supersaturated aqueous solution was directly observed by optical microscopy with an in situ, thermostatted cell. A strong correlation was found between the symmetry of the nuclei produced and that of the parent crystal face. Ex situ scanning (SEM) and transmission electron microscopic (TEM) measurements were also carried out to study this type of secondary nuclei, produced from a known surface geometry. In these cases, many small nuclei in the size range of 50 nm to 1 μm were produced and studied. The larger crystals displayed morphologies commensurate with that of the parent face; the very small nuclei, whilst frequently showing very poorly ordered boundaries, nonetheless were highly ordered internally, as shown by electron diffraction, the symmetry observed reflecting that of the parent face.

Linking Additive Structures to Nanoparticle Properties

The effects of a series of polyphosphonate and poly-carboxylate additives have been investigated in the crystallization of various inorganic salts. Systematic variation of the additive structure has been used to provide insight into the dominant factors in additive-crystal interactions. The results obtained for barium sulfate and hematite (α-Fe2O3) show that the morphological effects do not necessarily follow the trend one might expect on the basis of the structural features of the additives. Molecular modeling, coupled with in-situ AFM imaging is being used to develop an approach that will allow more informed systematic design of crystal growth modifiers.