Joop H. ter Horst

Self-Association during Heterogeneous Nucleation onto Well- Defined Templates

We investigated the interplay between self-associates in solution and surface templating by studying the crystallization behavior of isonicotinamide (INA) and 2,6-dihydroxybenzoic acid (DHB) in the presence of self-assembled monolayers (SAM). The end group of the SAM as well as the hydrogen-bonding capabilities of the solvent and self-association of INA and DHB were found to be important in polymorph crystallization on SAMs. In the case of INA in ethanol, both chain and dimer self-associates are present in the solution. In the absence of SAMs the polymorph form II (dimer structure) is the crystallization outcome. In ethanol the 4-mercaptopyridine and 4-mercaptobenzoic acid SAMs organize INA chain associates at the template surface and enable the crystallization of form I while the 16-mercaptohexadecanoic acid SAM results in the crystallization of form II. Raman spectroscopy suggests that molecular interactions between INA and the SAM are responsible for the formation of specific polymorphs. XRPD results in the identification of the orientation of the crystal on the surface that further verified the results obtained by Raman spectroscopy. In nitrobenzene and nitromethane INA associates in solution only as chains and crystallization results in the formation of form IV and form I, respectively (both chain forms). The crystals formed in the bulk solution and on SAMs were the same, which seems to indicate that the self-association in nitrobenzene and nitromethane is not influenced by the presence of templates. In the case of DHB in toluene and chloroform, all three SAMs nucleated only one type of polymorph (stable form 2). In the case of toluene the polymorphic outcome was stable form 2 instead of metastable form 1, which is favored in toluene in the absence of the SAMs. Again, Raman spectroscopy and XRPD suggest that DHB-SAM molecular interactions may be responsible for the formation of form 2.

Fundamentals of industrial crystallization

Industrial crystallization processes aim at the large-scale production of crystalline products through the formation of involving a suspension of growing particles in a solution. The product quality is defined by the kind of crystalline phase produced, the crystal size distribution, the crystal morphology, and the product purity. These product quality aspects are determined by the subprocesses of crystallization of which crystal nucleation and growth are usually of main importance. The driving force for these subprocesses is usually established either by evaporating solvent to increase the concentration or cooling the solution to decrease the solubility. The recent pharmaceutical research interest in continuous crystallization processes is fortified by claims of improved product quality, efficient use of materials and energy resources, and waste stream reductions.

Prenucleation self-assembly and chiral discrimination mechanisms during solution crystallisation of racemic diprophylline

The crystallisation behaviour of (RS)-diprophylline (DPL) in two different solvents is investigated to assess the incidence of solvated pre-associations on nucleation, crystal growth and chiral discrimination. In the solvated state, Raman spectroscopy shows that dimeric associations similar to those depicted in the crystalline solid solution (ssRII) predominate in isopropanol (IPA), which may account for the systematic spontaneous nucleation of this crystal form from this solvent. By contrast, spontaneous nucleation in DMF yields the stable racemic compound RI, consistently with the distinct features of the Raman spectrum collected in this solvent. A crystal growth study of ssRII in IPA reveals that the crystal habitus is impacted by the solution enantiomeric excess; this is explained by increased competition between homo- and heterochiral pre-associations. This is supported by a molecular modelling study on the enantiomeric selectivity of the DPL crystal lattices. The combination of assessment methods on solution chemistry, nucleation and chiral discrimination provides methodological tools from which the occurrence of solid solutions can be rationalised.

Solvent and additive interactions as determinants in the nucleation pathway: general discussion

Sarah Price opened a general discussion of the paper by Sven Schroeder: I have been generating the thermodynamically plausible crystal structures of organic molecules for many years, and back in 2004 we did a crystal structure prediction (CSP) study on imidazole1 and found that it was relatively straightforward. Following your paper, we have reclassified the low energy structures according to the tilt within the hydrogen-bonded chain and the relative direction of the chains. Although the observed structure was the global minimum, two other structures with a displacement of otherwise identical layers are very close in energy. Do you think that if imidazole had crystallised in one of these alternative structures it would be distinguishable by NEXAFS? This would be a very sensitive test of whether NEXAFS combined with CSP could be used in characterising crystal structures.

Solid Separation from a Mixed Suspension through Electric-Field-Enhanced Crystallization

When applied to a pure component suspension in an apolar solvent, a strong inhomogeneous electric field induces particle movement, and the particles are collected at the surface of one of the two electrodes. This new phenomenon was used to separately isolate two organic crystalline compounds, phenazine and caffeine, from their suspension in 1,4-dioxane. First, crystals of both compounds were collected at different electrodes under the influence of an electric field. Subsequent cooling crystallization enabled the immobilization and growth of the particles on the electrodes, which were separately collected after the experiment with purities greater than 91u2009%. This method can be further developed into a technique for crystal separation and recovery in complex multicomponent suspensions of industrial processes.

Crystal Nucleation of Small Organic Molecules

The crystal nucleation of small organic molecules from solution is reviewed from a theoretical viewpoint of the classical nucleation theory. In experiments heterogeneous nucleation is occurring which, due to the lack of information on the foreign particles onto which nucleation takes place, hampers the molecular interpretation of nucleation rate data. Due to the various nucleation rate measurement methods, analytical techniques, and simulation methods now available, crystal nucleation research will show substantial progress in the coming years.

Solution co-crystallisation and its applications

Even though the term co-crystal remains topic of debate, general consensus evolves towards defining co-crystals as crystalline compounds constructed of 2 or more neutral components which are, in their pure form, solid at room temperature and under atmospheric pressure. In most cases this definition clearly distinguishes co-crystals from salts or solvates. For solvates one of the components in its pure form, the solvent, is a liquid. A co-crystal is different from a salt since it is constructed from two distinct neutral components without any charge transfer taking place between components. Co-crystals can therefore be formed of components that lack ionizable functional groups.

Time and space resolved methods: General discussion

Jim De Yoreo presented some slides on in situ AFM, TEM, dynamic force spectroscopy (DFS) and optical spectroscopy investigations of nucleation in the calcium carbonate system: The free energy barrier to homogeneous nucleation of calcite calculated within the framework of classical nucleation theory (CNT) is prohibitive, even at concentrations exceeding the solubility limits of the amorphous phases. Consistent with this analysis, during nucleation in pure solutions, in our in situ TEM experiments we observed direct formation of all phases, including amorphous calcium carbonate (ACC), as well as the three predominant crystalline phases: calcite, vaterite, and aragonite, even under conditions in which ACC readily forms. In addition to direct formation pathways, we observed indirect pathways in which ACC transforms to aragonite and vaterite through nucleation within or on the precursors, rather than via dissolution and reprecipitation. We also observed aragonate transformation to calcite, but never recorded an instance in which ACC transforms into calcite, except via dissolution–reprecipitation reactions.