Structured ternary fluids as nanocrystal incubators for enhanced crystallization control

Crystallization in structured ternary fluids can proceed via higher nucleation rate and slower crystal growth pathways that are impossible to access in normal unstructured solutions. Hence, structured ternary fluids can act as nanocrystal incubators.

Synthesis of γ Phase and Amorphous Solid Dispersion of Glycine from α-Glycine During the Solvent-Free Ball Milling Process

Nano α-glycine crystals, γ-glycine crystals, and amorphous solid dispersion (ASD) of glycine were prepared through solvent-free ball milling of commercial α-glycine. The solid-state polymorph conversion of glycine from α to γ was completely realized by ball milling with 0.2 wt.% NaCl for 1 h or by ball milling with 0.02 wt.% NaCl for 1 h with subsequent storage for one week. The ASD of glycine was prepared by ball milling α-glycine with an equal amount of CaCl₂ for 1 h. We studied the effect of inorganic salt types and their concentrations on the extent of polymorph conversion and amorphization of glycine in our experiments. This solvent-free ball milling method could be used for the synthesis of polymorphs and amorphous phase of drugs and other organic materials.

Understanding the Salt-Dependent Outcome of Glycine Polymorphic Nucleation

The salt-dependent polymorphs of glycine crystals formed from bulk solutions have been a longstanding riddle. In this study, in order to shed fresh light, we studied the effects of seven common salts on primary nucleation of the metastable α-glycine and the stable γ-glycine. Our nucleation experiments and in-depth data analyses enabled us to reveal that (NH₄)₂SO₄, NaCl and KNO₃, in general, promote γ-glycine primary nucleation very significantly while simultaneously inhibiting α-glycine primary nucleation, thereby explaining why these three salts induce γ-glycine readily. In comparison, Ca(NO₃)₂ and MgSO₄ also promote γ-glycine and inhibit α-glycine primary nucleation but not sufficiently to induce γ-glycine. More interestingly, Na₂SO₄ and K₂SO₄ promote not only γ-glycine but also α-glycine primary nucleation, which is unexpected and presents a rare case where a single additive promotes the nucleation of both polymorphs. As a result, the promoting effects of Na₂SO₄ and K₂SO₄ on γ-glycine do not enable γ-glycine nucleation to be more competitive than α-glycine nucleation, with γ-glycine failing to appear. These observations help us to better understand salt-governed glycine polymorphic selectivity.

Amino acids nanocrystals for piezoelectric detection of ultra-low mechanical pressure

Developing biocompatible nano-materials with the ability to detect ultra-low mechanical pressure is promising for biomedical sensors. This paper reports the detection of pressure as low as 1 Pa in the environmental pressure of 1 atm (10^-3% pressure change) by nanocrystals of amino acids glycine and alanine through the piezoelectric effect. Piezoelectricity enables detection of pressure by a change of dielectric polarization when the material is subjected to external pressure. This work exploits the non-centro-symmetric structure of some amino acids and their weak hydrogen bonds to develop sensitive mechanical pressure sensors. The β-glycine and l-alanine nanocrystals were grown from aqueous solution inside porous alumina substrate. The nanocrystals exhibit pronounced preferred crystallographic orientation. The sensitive piezoelectric response to ultra-low mechanical pressure is discussed based on atomic and crystal symmetry considerations.

Polymorph evolution during crystal growth studied by 3D electron diffraction

3D electron diffraction (3DED) has been used to follow polymorph evolution in the crystallization of glycine from aqueous solution. The three polymorphs of glycine which exist under ambient conditions follow the stability order β < α < γ. The least stable β polymorph forms within the first 3 min, but this begins to yield the α-form after only 1 min more. Both structures could be determined from continuous rotation electron diffraction data collected in less than 20 s on crystals of thickness ∼100 nm. Even though the γ-form is thermodynamically the most stable polymorph, kinetics favour the α-form, which dominates after prolonged standing. In the same sample, some β and one crystallite of the γ polymorph were also observed.

Growth of β-glycine crystals promoted by standing surface acoustic waves (SSAWs)

Glycine has three polymorphic forms, the α-, β- and γ-phase, where the β-phase is less stable than the other two, but it is the most useful one.

`NMR Crystallization': in-situ NMR techniques for time-resolved monitoring of crystallization processes

Solid-state NMR spectroscopy is a well-established and versatile technique for studying the structural and dynamic properties of solids, and there is considerable potential to exploit the power and versatility of solid-state NMR for in-situ studies of chemical processes. However, a number of technical challenges are associated with adapting this technique for in-situ studies, depending on the process of interest. Recently, an in-situ solid-state NMR strategy for monitoring the evolution of crystallization processes has been developed and has proven to be a promising approach for identifying the sequence of distinct solid forms present as a function of time during crystallization from solution, and for the discovery of new polymorphs. The latest development of this technique, called `CLASSIC' NMR, allows the simultaneous measurement of both liquid-state and solid-state NMR spectra as a function of time, thus yielding complementary information on the evolution of both the liquid phase and the solid phase during crystallization from solution. This article gives an overview of the range of NMR strategies that are currently available for in-situ studies of crystallization processes, with examples of applications that highlight the potential of these strategies to deepen our understanding of crystallization phenomena.

Supersaturation dependence of glycine polymorphism using laser-induced nucleation, sonocrystallization and nucleation by mechanical shock

Nucleation of glycine by laser, ultrasound and mechanical shock exhibits a transition from the alpha to the gamma polymorph with increasing supersaturation.

Salt-dependent growth kinetics in glycine polymorphic crystallization

Divalent cation salts inhibit the growth of α-glycine far less severely than that of γ-glycine, causing α-glycine to have a greater competitive advantage over γ-glycine, thus further reinforcing the preferential formation of α-glycine on a relative basis.

Polymorph crystal packing effects on charge transfer emission in the solid state

Condensation of 1,8-naphthalic anhydride with N,N-(dimethylamino)aniline produced the donor-acceptor compound DMIM, which crystallised from a chloroform-diethyl ether mixture to afford two different coloured crystal polymorphs. Crystals for one polymorph are small and green, whereas the other crystals are orange and needle-like. X-ray crystal structures for both polymorphs were determined. The donor N,N-dimethylaniline and acceptor naphthalimide groups are twisted with respect to each other; the degree of twist is marginally different for the two structures. The orange crystal polymorph crystallises in the monoclinic space group C2/c and contains two slightly different molecular conformers in the unit cell (calculated density is 1.410 g cm-3). The green crystal polymorph crystallises in the triclinic space group P1 and contains only one type of molecule in the unit cell (calculated density is 1.401 g cm-3). The crystal packing motifs for the two polymorphs are subtly different, explaining the small variance in the observed densities. Very weak room temperature emission was observed for DMIM in a CHCl3 solution, but crystals deposited on a glass slide glowed when irradiated at 488 nm using a fluorescence microscope. Disparate solid-state emission spectra and lifetimes for the two polymorphic crystal forms are observed for the dyad. The emission is assigned to charge recombination fluorescence from a charge transfer state.

New in situ solid-state NMR techniques for probing the evolution of crystallization processes: pre-nucleation, nucleation and growth

The application of in situ techniques for investigating crystallization processes promises to yield significant new insights into fundamental aspects of crystallization science. With this motivation, we recently developed a new in situ solid-state NMR technique that exploits the ability of NMR to selectively detect the solid phase in heterogeneous solid–liquid systems (of the type that exist during crystallization from solution), with the liquid phase “invisible” to the measurement. As a consequence, the technique allows the first solid particles produced during crystallization to be observed and identified, and allows the evolution of different solid phases (e.g., polymorphs) present during the crystallization process to be monitored as a function of time. This in situ solid-state NMR strategy has been demonstrated to be a powerful approach for establishing the sequence of solid phases produced during crystallization and for the discovery of new polymorphs. The most recent advance of the in situ NMR methodology has been the development of a strategy (named “CLASSIC NMR”) that allows both solid-state NMR and liquid-state NMR spectra to be measured (essentially simultaneously) during the crystallization process, yielding information on the complementary changes that occur in both the solid and liquid phases as a function of time. In this article, we present new results that highlight the application of our in situ NMR techniques to successfully unravel different aspects of crystallization processes, focusing on: (i) the application of a CLASSIC NMR approach to monitor competitive inclusion processes in solid urea inclusion compounds, (ii) exploiting liquid-state NMR to gain insights into co-crystal formation between benzoic acid and pentafluorobenzoic acid, and (iii) applications of in situ solid-state NMR for the discovery of new solid forms of trimethylphosphine oxide and l-phenylalanine. Finally, the article discusses a number of important fundamental issues relating to practical aspects, the interpretation of results and the future scope of these techniques, including: (i) an assessment of the smallest size of solid particle that can be detected in in situ solid-state NMR studies of crystallization, (ii) an appraisal of whether the rapid sample spinning required by the NMR measurement technique may actually influence or perturb the crystallization behaviour, and (iii) a discussion of factors that influence the sensitivity and time-resolution of in situ solid-state NMR experiments.

Salting out the polar polymorph: analysis by alchemical solvent transformation

We computationally examine how adding NaCl to an aqueous solution with α- and γ-glycine nuclei alters the structure and interfacial energy of the nuclei. The polar γ-glycine nucleus in pure aqueous solution develops a melted layer of amorphous glycine around the nucleus. When NaCl is added, a double layer is formed that stabilizes the polar glycine polymorph and eliminates the surface melted layer. In contrast, the non-polar α-glycine nucleus is largely unaffected by the addition of NaCl. To quantify the stabilizing effect of NaCl on γ-glycine nuclei, we alchemically transform the aqueous glycine solution into a brine solution of glycine. The alchemical transformation is performed both with and without a nucleus in solution and for nuclei of α-glycine and γ-glycine polymorphs. The calculations show that adding 80 mg/ml NaCl reduces the interfacial free energy of a γ-glycine nucleus by 7.7 mJ/m(2) and increases the interfacial free energy of an α-glycine nucleus by 3.1 mJ/m(2). Both results are consistent with experimental reports on nucleation rates which suggest: J(α, brine) < J(γ, brine) < J(α, water). For γ-glycine nuclei, Debye-Hückel theory qualitatively, but not quantitatively, captures the effect of salt addition. Only the alchemical solvent transformation approach can predict the results for both polar and non-polar polymorphs. The results suggest a general "salting out" strategy for obtaining polar polymorphs and also a general approach to computationally estimate the effects of solvent additives on interfacial free energies for nucleation.