In previous work, it has been shown that the crystal nucleation of salicylic acid (SA) in different solvents becomes increasingly more difficult in the order: chloroform, ethyl acetate acetonitrile, acetone, methanol, and acetic acid. In the present work, vibration spectroscopy, calorimetric measurements, and density functional theory (DFT) calculations are used to reveal the underlying molecular mechanisms. Raman and infrared spectra suggest that SA exists predominately as dimers in chloroform, but in the other five solvents there is no clear evidence of dimerization. In all solvents, the shift in the SA carbonyl peak reflecting the strength in the solvent-solute interaction is quite well correlated to the nucleation ranking. This shift is corroborated by DFT calculated energies of binding one solvent molecule to the carboxyl group of SA. An even better correlation of the influence of the solvent on the nucleation is provided by DFT calculated energy of binding the complete first solvation shell to the SA molecule. These solvation shell binding energies are corroborated by the enthalpy of solvent-solute interaction as estimated from experimentally determined enthalpy of solution and calculated enthalpy of cavity formation using the scaled particle theory. The different methods reveal a consistent picture and suggest that the stronger the solvent binds to the SA molecule in solution, the slower the nucleation becomes.

Over 2100 induction time experiments were carried out for the medium-sized, antipsychotic drug molecule, risperidone in seven different organic solvents. To reach the same induction time the required driving force increases in the order: cumene, toluene, acetone, ethyl acetate, methanol, propanol, and butanol, which reasonably well correlates to the interfacial energies as determined within classical nucleation theory. FTIR spectroscopy has been used to investigate any shifts in the spectra and to estimate the interaction of solute and solvent at the corresponding site. The solution condition has also been investigated by Density Functional Theory (DFT) calculations over (1 : 1) solvent-solute binding interactions at 8 different sites on the risperidone molecule. The DFT computational results agree with the spectroscopic data suggesting that these methods do capture the binding strength of solvent molecules to the risperidone molecule. The difficulty of nucleation correlates reasonably to the DFT computations and the spectroscopic measurements. The results of the different measurements suggest that the stronger the solvent binds to the risperidone molecule in solution, the slower the nucleation becomes.

The influence of the solvent in nucleation of tolbutamide, a medium-sized, flexible and polymorphic organic molecule, has been explored by measuring nucleation induction times, estimating solvent-solute interaction enthalpies using molecular modelling and calorimetric data, probing interactions and clustering with spectroscopy, and modelling solvent-dependence of molecular conformation in solution. The nucleation driving force required to reach the same induction time is strongly solvent-dependent, increasing in the order: acetonitrile

In the present work the induction time for nucleation of ethyl paraben (EP) and propyl paraben (PP) in ethanol, ethyl acetate and acetone has been measured at different levels of supersaturation. The induction time shows a wide variation among repeat experiments, indicative of the stochastic nature of nucleation. The solid-liquid interfacial energy and the size of the critical nucleus have been determined according to the classical nucleation theory. Combined with previous results for butyl paraben (BP), the nucleation behaviour is analysed with respect to differences in the solid phase of the three pure compounds, and with respect to differences in the solution. The results indicate that the difficulty of nucleation in ethanol and acetone increases in the order BP < PP < EP, but is approximately the same in ethyl acetate. For each of the three parabens the difficulty of nucleation increases in the order acetone < ethyl acetate < ethanol. The Gibbs energy of melting increases in the order BP < PP < EP, but the crystal structures are quite similar resulting in the basic crystal shape being very much the same. The solid-liquid interfacial energy is reasonably well correlated to the solvation energy, and even better correlated to the deformation energy, of the solute molecule within the first solvation shell as obtained by density functional theory calculations.

Controlling pharmaceutical polymorphism in crystallization processes represents a major challenge in pharmaceutical science and engineering. For instance, CO 2antisolvent crystallization typically favors the formation of metastable forms of carbamazepine (CBZ), a highly polymorphic drug, with impurities of other forms. This work demonstrates for the first time that a supercritical CO 2antisolvent crystallization process in combination with certain molecular additives allows control of the polymorphic outcome of CBZ. We show herein that in the presence of sodium stearate and Eudragit L-100, needle-shaped crystals of CBZ form II are obtained, while blocky-shaped crystals of CBZ form III are obtained in the presence of Kollidon VA64, sodium dodecyl sulfate, ethyl cellulose, and maltitol. This selectivity for pure forms in this supercritical set up contrasts to the results when the same set of additives where used in a solvent evaporation method that yielded mixtures of form I, II, and III. The type of additive used in the CO 2 -antisolvent crystallization process impacted both the product crystal polymorphic form and size. A detailed molecular-level analysis along with density functional theory calculations allowed us to give a mechanistic insight into the role of sodium stearate and Eudragit L-100 in facilitating nucleation of the metastable form II.