R.I. Ristic

Crystallization of paracetamol from solution in the presence and absence of impurity.

The bulk crystallization of paracetamol has been examined under controlled conditions in the presence and absence of the additive p-acetoxyacetanilide (PAA), as a function of both supersaturation and additive levels. The induction time to nucleation was found to increase with increase in PAA concentration in solution. The product micro-crystals were characterized for shape and strain/defect content using electron and optical microscopy and X-ray Laue diffraction techniques, respectively. A change in crystal habit of the pure crystals from columnar (dominant [110]) to plate-like (dominant [001]) was observed to occur with an increase in supersaturation level, whilst the addition of PAA invariably led to the development of columnar crystals with an aspect ratio that varied with impurity level and supersaturation. HPLC showed the PAA to be incorporated into the crystals with an average segregation coefficient of 14-18% depending on the supersaturation. The ready incorporation of PAA is attributed to the molecular similarity of this molecule to that of the host material. The incorporation is shown to cause a significant increase in the mosaic spread, implying the development of a significant strain/defect content in the crystals. The influence of the impurity on the time to nucleation is probably due to its effect in blocking the development of the critical nucleus. The potential implications of such variations in morphology and strain content in the design of the physical and chemical properties of the resulting particulates are discussed.

The influence of tensile strain on the growth of crystals of potash alum and sodium nitrate

Abstract Using a specially designed growth cell in which the crystal could be subjected to tensile stain during growth, studies have been made of the influence of strain on the growth rates of the habit faces of potash alum and sodium nitrate. These materials are, respectively, typical of brittle and ductile materials. In both cases, the application of strain led to a reduction in growth rate. In the case of brittle potash alum the reduction in growth rate of the {1 0 0} and {1 1 0} habit faces was directly proportional to the applied stress. On removal of the constraint, the growth rate returned to its initial value. The growth rate of the {1 1 1} faces was influenced by strain, but to a much lesser extent. In contrast, sodium nitrate showed a time dependent decrease in growth rate consistent with the gradual partial release of strain by plastic deformation. On release of the strain, the growth rate of the unconstrained crystal was higher than that observed initially. X-ray topographic analysis of the crystals showed that potash alum had undergone only limited plastic deformation during straining to produce few dislocations the orientation of which could not influence growth of the {1 0 0} and {1 1 0} faces. In contrast, sodium nitrate had undergone massive plastic deformation to form large numbers of dislocations many of which intersected the growing interface. Where this had occurred some of these dislocations converted to growth dislocations in the subsequent growth, thus contributing to the noted increase in growth rate. The results are extended to consider the equivalent behaviour of micro crystals and are discussed in the context of growth rate dispersion in industrial crystallisation.

Dissolution kinetics of paracetamol single crystals.

The dissolution anisotropy of paracetamol crystals grown in the presence and absence of the molecularly similar additive, p-acetoxyacetanilide (PAA) was studied under controlled conditions using a single crystal dissolution method in undersaturated aqueous solutions. Linear dissolution rates were determined for all the major habit faces by measuring their movement (regression) with time in a flow cell using a microscope. The rates of dissolution of particular faces of the pure material were distinctly different in crystals of different morphology grown at different supersaturations. The dissolution rates of [001] and [110] faces of crystals grown in the presence of PAA (6.02% w/w in solution) are higher than those of pure paracetamol. The results correlate with the distribution of strain in the crystal and support the concept that integral strain increases the solubility and hence the dissolution rate of the material. The mechanism of the dissolution process at the [001], [201;] and [110] faces was defined using optical microscopy and X-ray topography. At all undersaturations above 1% the dissolution studies yielded well developed, structurally oriented, etch pits on both [001] and [201;] faces while on the [110] face rough shallow etch pits were observed. On all three faces, this etch-pitting was considerably more widespread than the dislocation content of the sector and probably reflects a 2-dimensional nucleation process rather than a dislocation controlled mechanism.

The influence of mechanical stress on the growth and dissolution of crystals

Abstract A review is presented of the authors’ recent work on the influence of mechanical stress on the growth rate of materials. On the basis of the results, it is proposed that growth rate dispersion of secondary nuclei arises as a result of competition between stress reduction and dislocation enhancement of growth rates in the developing particles, with the former being the major influence at low particle sizes. The results are shown to account for the particle size and material dependence of the dispersion. Mechanistic studies suggest that the principal mechanism of stress reduction in growth rate is the influence of the applied stress on the surface free energy of the crystal. This effects both nucleation at the growth centre and the migration of growth steps across the crystal surface.

Morphological and growth rate distributions of small self-nucleated paracetamol crystals grown from pure aqueous solutions

The growth rate dispersion of small paracetamol crystals nucleated and grown from pure solution was measured over the range of supersaturation, 0%<s<20%, in the three different crystallographic directions: [0 0 1], [1 1 0] and [0 1 0]. The results were fitted to three parameter log-normal and gamma distribution functions. The average growth rates obtained from these distributions were used to predict the morphological behaviour of an assembly of small paracetamol crystals as a function of supersaturation. It was shown that a columnar shape (dominant {1 1 0}) was formed in regions of low supersaturation while a plate-like form (dominant {0 0 1}) prevailed at high supersaturations. Using optical microscopy and X-ray topography, the potential causes for both the growth rate distribution and morphological change were investigated.

The growth of large single crystals of the organic nonlinear optical material 2-(α-methylbenzylamino)-5-nitropyridine

The crystal growth behaviour of the important organic nonlinear optical material 2-(a-methylbenzylamino)-5-nitropyridine (MBANP) (space group P21) has been investigated and techniques have been developed for the preparation of large (up to 7 cm x 4 cm x 3 cm) high-quality single crystals suitable for detailed optical, dielectric, ultrasonic and piezoelectric studies and for device fabrication. Observation of the crystal growth behaviour and measurement of growth kinetics revealed a strong and anomalous dependence of growth rate on supersaturation. At relative supersaturations less than 0.08, slow growing irregular faces were formed. Once the crystal was completely bounded by such faces, little or no further growth occurred. At supersaturations below 0.24 no growth was observed in either <010> direction. Above this, rapid, but highly imperfect, growth occurred in one <010> direction while the opposite end of the crystal exhibited zero growth rate at all achievable supersaturations. At relative supersaturations greater than 0.4 considerable instability in the growth along [010] was observed. The factors potentially responsible for this behaviour are discussed. A novel technique is described for the preparation of seed crystals elongated parallel to <010>. The use of such seeds allows the growth of large crystals of MBANP at supersaturations less than 0.24, thereby avoiding the highly imperfect and unstable growth at higher supersaturations and yielding crystals of very high quality. The nature, density and distribution of growth induced defects in large crystals of MBANP have been investigated using X-ray diffraction topography. In addition to revealing growth dislocations, growth sector boundaries, inclusions and striations, these studies provide a detailed insight into the growth history of the crystals.

The influence of synchrotron radiation-induced strain on the growth and dissolution of brittle and ductile materials

Abstract Sodium chlorate (brittle) and sodium nitrate (ductile) crystals were irradiated by synchrotron radiation to produce samples in which one-half of a particular crystal was strained by radiation damage and the other not. The growth and dissolution kinetics of these samples were investigated using in situ laser interferometry and X-ray topography. An appreciable difference was observed in the growth and dissolution kinetics of the irradiated and nonirradiated halves. In the region of very low supersaturation, σ c = 0.32% for sodium chlorate and σ c = 0.065% for sodium nitrate, the irradiated portions of both types of crystals dissolved; simultaneously, the nonirradiated portions grew. Above the critical supersaturation, σ c , both halves grew. This defines a difference in solubility between the irradiated and nonirradiated material. The significant difference between the two critical supersaturations in the two materials confirms our earlier findings that a larger amount of elastic strain per unit volume can be introduced into brittle materials than into ductile ones. This, in turn, has a much stronger effect on both growth and dissolution kinetics. Irradiation is shown to yield pure point defect strain and not to introduce dislocations in the system. A possible mechanism by which strain influences the growth and dissolution kinetics is discussed.

Growth of the tetrahedral faces of sodium chlorate crystals in the presence of dithionate impurity

Abstract It has been established that trace amounts of S 2 O 2- 6 anions initiate the appearance of new faces, { 111 }, on sodium chlorate crystals grown from aqueous solution. The present study compares the growth mechanisms of the {111} habit faces and { 111 } type faces using laser interferometry techniques. The {111} faces show dislocation layer growth in which the impurity influences the growth by causing the gradual deceleration of step movement. In contrast, the { 111 } faces incorporate crystal units into an atomically rough surface. Under these circumstances the impurity on the surface creates a barrier to continuous growth. The strong impurity interaction with the { 111 } faces can be explained by the close structural similarity of the -SO 3 portion of the S 2 O 2- 6 ions to the ClO - 3 ions situated on these faces. This allows the dithionate ion to substitute for ClO - 3 , disturbing the structure and energy of the interface and inhibiting growth.

Long and short period growth rate variations in potash alum crystals

Abstract Recent studies of the growth rate of the different faces of potash alum crystals have revealed that these grow with variable growth rates. This phenomena can be considered on two time scales. In the long term “constant” growth remains for period of hours followed by growth arrests and so on. Superimposed on this are short term (minutes) fluctuations. The potential causes for these variations were discussed.

Three-parameter distribution function fit to growth rate dispersion among small crystals

This paper considers the advantages of using a three-parameter fit to both log-normal and gamma distributions rather than the two parameter fit normally used for the statistical treatment of growth rate dispersion among small crystals. It discusses the distribution functions, the procedure of searching for location parameters and the optimal number of bins for the best log-normal and gamma fits. As an example, the three-parameter fit is applied to results of the growth rate distributions of small paracetamol crystals. It is shown that the application of three-parameter distributions yields both a better fit to the experimental data than the two-parameter ones and the potential for a compound statistical treatment of growth, zero-growth and dissolution among small particles.