Wolfgang Beckmann

Nucleation phenomena during the crystallisation and precipitation of Abecarnil

The crystallisation and precipitation process of Abecarnil is studied for a number of techniques and two different solvents, isopropyl acetate and methanol. The phenomena observed involve induction times, crystal size distribution and polymorphism. It is shown that the nucleation phenomena are mainly a function of supersaturation and only a weak function of the crystallisation technique. The particle size distribution obtained for precipitations can be correlated with the supersaturation at which the first nuclei appear. This also shows a limiting size to be attainable with very fast precipitation schemes. Abecarnil forms three different modifications, A, B, and C, with the latter form as monotropical stable form. The crystal modification obtained for the unseeded crystallisations and precipitations is mostly one of the unstable A or B modification. The exact form found depends on the primary solvent used. However, with increasing supersaturations, the B form dominates irrespective of the solvent and technique used.

Growth kinetics of the (110) face of stearic acid growing from butanone solutions - pure solutions and in the presence of an emulsifier

Abstract The growth rates of the (110) face of stearic acid growing from butanone solutions were measured in a flow system at temperatures between 13 and 25°C. Pure solutions as well as those containing 0.5 and 2% of an emulsifier (Span 60) were employed. The measurements were performed with both the B and C polymorph of stearic acid. From the determination of the activation energy for growth the growth mechanism was deduced. In pure solutions, both polymorphs grow via a spiral dislocation mechanism. By finding two distinct growth laws for the two modifications, the structure of which is very similar, a growth controlled only by surface diffusion could be ruled out. Instead, surface diffusion combined with diffusion along the steps could explain the experimental data sufficiently. For solutions containing the emulsifier, the polymorphs also follow a spiral growth mechanism. The absolute rates are decreased considerably as compared to pure solutions. The decrease in rates could be explained by an adsorption of the emulsifier at kink sites. The decrease in rates is more pronounced for the B than for the C modification, explaining the preferential precipitation of the C polymorph from solutions containing Span 60.

Calculations of the intermolecular potential and surface energies of stearic acid

Abstract Intermolecular potential calculations have been performed for the B and C polymorph of stearic acid, CH 3 (CH 2 ) 16 -COOH. The potential used was a Lennard-Jones 6–12 potential with corrections for the Coulomb forces of the charged carboxylic group. The parameters for the potential were directly taken from the literature without further adjustments. The heat of sublimation deduced from the calculations is 178 kJ/mol, which can be compared with an experimental value of 166 kJ/mol. The heat of the polymorphic transition of B into C - experimentally determined to lay between 4 and 6 kJ/mol - was calculated to be approximately 5 kJ/mol (±2 kJ/mol). The surface energies were calculated for the (100), (010), (110) and (001) faces. From a Wulff plot, the equilibrium forms were obtained. The B crystals are limited by (010) and (110) faces and the C crystals by (100) and (110) faces. The edge energies were obtained on the (100), (010), (110) and (001) faces for a variety of low indexed steps.

Growth kinetics of the (110) face of the B and C polymorphs of stearic acid growing from octanone-2 solutions

Abstract The growth rates of the B and C modifications of stearic acid growing from octanone solutions have been determined for solution having saturation temperatures of 16, 20 and 29°C. The overall growth behaviour of the two modifications deduced from the measurements differs considerably. A comparison with results for butanone as solvent shows that a mechanism of surface diffusion coupled with ledge diffusion can account for the difference.

The dissolution of stearic ACID: I. The B modification in octane

Abstract For the B modification of stearic acid, the dissolution kinetics and macroscopic etch figures were observed during the dissolution into octane at 17°C. The dissolution seems to be a quadratic function of subsaturation, the kinetics is higher than for growth. The spreading velocity of the borders of the etch pits varies linearly with subsaturation. The etch pit density of approximately 0.6 × 104 cm-2 and the slopes of the borders of the pits of 5° to 7° were found to be nearly independent of subsaturation. The results are discussed in terms of the current theories for dissolution and etching.