Crystal Growth, Structure, and Polymorphic Behavior of an Ionic Liquid: Phthalate Derivative of N-Butyl,N-methylimidazolium Hexafluorophosphate

Abstract

After the multistep synthesis of an original imidazolium hexafluorophosphate ionic liquid, [pbmim]­[PF6], two polymorphic forms were isolated from methanolic solution and characterized by XRPD, DSC, and Raman spectroscopy. Stable Form A (mp 90.3 °C) was obtained by conventional crystallization at a moderate cooling rate (<10 K/min), whereas metastable Form B (mp 83.2 °C) was spontaneously produced from a liquid–liquid demixing (oiling out) when a faster cooling rate (>10 K/min) was applied. Structural analyses carried out by using single-crystal (Form A) and powder (Form B) X-ray diffraction revealed a rotational disorder of anionic octahedrons and, more interestingly, large conformational differences between cationic moieties caused by their molecular flexibility. Crystal growth of [pbmim]­[PF6] (Form A) in methanol often leads to numerous crystal defects and revealed that most of them consist of liquid inclusions. The supersaturation ratio (β) appeared to be the predominant factor influencing the crystal growth behavior under isothermal and stagnant conditions. At low β values, a morphological transition from rod-shaped crystals to platelets was observed, presumably caused by a change in the growth mechanism of specific faces. Using high β values promotes either the formation of microscopic (<5 μm) liquid inclusions that become easily detectable upon heating or the appearance of macroscopic inclusions with an hourglass shape.

Resolution of an Atropisomeric Naphthamide by Second-Order Asymmetric Transformation: A Highly Productive Technique

“…As can be seen from Figure , the solubility variation is small from 10 to 30 °C but increases abruptly from 30 °C onward. This behavior suggests the existence of a metastable miscibility gap, at higher or lower temperatures, also called submerged liquid demixing or the metastable oiling-out phenomenon . Therefore, seeded crystallization is well suited for this compound in order to avoid liquid demixing, which may cause high supersaturation in one of the two liquids and thus uncontrolled crystallization.…”

“…As can be seen fromFigure 4, the solubility variation is small from 10-30 °C but increases abruptly from 30 °C onward. This behavior suggests the existence of a metastable miscibility gap, at higher or lower temperatures, also called submerged liquid demixing or the metastable oiling-out phenomenon 49. Therefore, seeded crystallization is well suited for this compound in order to avoid liquid demixing which may cause high supersaturation in one of the two liquids and thus uncontrolled crystallization.Racemization kinetic constants k1 obtained by polarimetry measurements48 have been used to plot an Arrhenius's graph, and the Gibbs free energy of activation (ΔG ‡) was found to be 17.In general, a torsion rotation energy barrier lower than 20-25 kcal‧mol −1 can show the interconversion within a few hours around room temperature 50.…”

Determination of the Solid–Liquid Phase Diagram of the Binary System Propionic Acid/Water

“…Since low cooling rate was applied in our case, crystallization occurred during cooling and the liquid−liquid phase separation was not observed. 24 The equilibrium is represented by the points C for solid and E for solution. 25,26 Such S-shaped systems with an undercooled metastable miscibility gap can be found in the literature, for example, the water/salicylic acid system 27 or carbon dioxide/n-hexane system.…”

Impact of a Partial Solid Solution and Water Molecules on the Formation of Fibrous Crystals and Fluid Inclusions

“…Moreover, the partial substitution of S-αMBA by NaOH in the medium increases the occurrence of fluid inclusions. Usually, the observation of reproducible fluid inclusions results from a too high crystal growth rate as soon as the crystals exceed a threshold in size [3][4][5][6][7]. However, other mechanisms have been proposed [8][9][10].…”

“…If the cooling is fast, several bubbles of gas could nucleate simultaneously, but usually after a while, only the largest one survives because of the Laplace-Young equation: ΔP = 2γ/R. In this Usually, the observation of reproducible fluid inclusions results from a too high crystal growth rate as soon as the crystals exceed a threshold in size [3][4][5][6][7]. However, other mechanisms have been proposed [8][9][10].…”