Surface-Induced Polymorphism as a Tool for Enhanced Dissolution: The Example of Phenytoin

On the crystal forms of NDI-C6: annealing and deposition procedures to access elusive polymorphs

NDI-C6 has been extensively studied for its semiconducting properties and its processability. It is known to have several polymorphs and a high thermal expansion. Here we report the full thermal characterization of NDI-C6 by combining differential scanning calorimetry, variable temperature X-ray powder diffraction, and hot stage microscopy, which revealed two different thermal behaviours depending on the annealing process. The ranking of stability was determined by the temperature and energy involved in the transitions: Form α is stable from RT up to 175 °C, Form β is metastable at all temperatures, Form γ is stable in the range 175-178 °C, and Form δ in the range 178-207 °C followed by the melt at 207 °C. We determined the crystal structure of Form γ at 54 °C from powder. The analysis of the thermal expansion principal axis shows that Form α and Form γ possess negative thermal expansion (X1) and massive positive thermal expansion (X3) which are correlated to the thermal behaviour observed. We were able to isolate pure Form α, Form β, and Form γ in thin films and we found a new metastable form, called Form ε, by spin coating deposition of a toluene solution of NDI-C6 on Si/SiO₂ substrates.

Interfacial Charge Transfer Influences Thin-Film Polymorphism

The structure and chemical composition are the key parameters influencing the properties of organic thin films deposited on inorganic substrates. Such films often display structures that substantially differ from the bulk, and the substrate has a relevant influence on their polymorphism. In this work, we illuminate the role of the substrate by studying its influence on para-benzoquinone on two different substrates, Ag(111) and graphene. We employ a combination of first-principles calculations and machine learning to identify the energetically most favorable structures on both substrates and study their electronic properties. Our results indicate that for the first layer, similar structures are favorable for both substrates. For the second layer, we find two significantly different structures. Interestingly, graphene favors the one with less, while Ag favors the one with more electronic coupling. We explain this switch in stability as an effect of the different charge transfer on the two substrates.

Engineering of a kinetically driven phase of phenoxazine by surface crystallisation

Surface crystallisation yields an unknown polymorph of the phenoxazine molecule. Tuning the crystallisation conditions causes a defined crystal growth of either the thermodynamically stable phase or the kinetic phase observed exclusively within thin films.

Role of Self-Assembled Surface Functionalization on Nucleation Kinetics and Oriented Crystallization of a Small-Molecule Drug: Batch and Thin-Film Growth of Aspirin as a Case Study

The present paper assesses the heterogeneous nucleation of a small-molecule drug and its relationship with the surface chemistry of engineered heteronucleants. The nucleation of aspirin (ASA) was tuned by different functional groups exposed by self-assembled monolayers (SAMs) immobilized on glass surfaces. Smooth topographies and defect-free surface modification allowed the deconvolution of chemical and topographical effects on nucleation. The nucleation induction time of ASA in batch crystallization was mostly enhanced by methacrylate and amino groups, whereas it was repressed by thiol groups. In this perspective, we also present a novel strategy for the evaluation of surface-drug interactions by confining drug crystallization to thin films and studying the preferential growth of crystal planes on different surfaces. Crystallization by spin coating improved the study of oriented crystallization, enabling reproducible sample preparation, minimal amounts of drug required, and short processing time. Overall, the acid surface tension of SAMs dictated the nucleation kinetics and the extent of relative growth of the ASA crystal planes. Moreover, the face-selective action of monolayers was investigated by force spectroscopy and attributed to the preferential interaction of exposed groups with the (100) crystal plane of ASA.

Searching for New Polymorphs by Epitaxial Growth

The formation of unknown polymorphs due to the crystallization at a substrate surface is frequently observed. This phenomenon is much less studied for epitaxially grown molecular crystals since the unambiguous proof of a new polymorph is a challenging task. The existence of multiple epitaxial alignments of the crystallites together with the simultaneous presence of different polymorphs does not allow simple phase identification. We present grazing incidence X-ray diffraction studies on conjugated molecules like perylenetetracarboxylic dianhydride (PTCDA), pentacene, dibenzopentacene (trans-DBPen), and dicyanovinylquater-thiophene (DCV4T-Et2) grown by physical vapor deposition on single crystalline surfaces like Ag(111), Cu(111), and graphene. A new method for indexing the observed Bragg peaks allows the determination of the crystallographic unit cells so that the type of crystallographic phase can be clearly identified. This approach even works when several polymorphs are simultaneously present within a single sample as shown for DCV4T-Et2 on Ag(111). Additionally, epitaxial relationships between the epitaxially grown crystallites and the single crystalline surfaces are determined. In a subsequent step, the experimental data are used for the crystal structure solution of an unknown polymorph, as shown for the example trans-DBPen grown on Cu(111).

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

This article aims to provide an overview of broad range of applications of synchrotron scattering methods in the investigation of nanoscale materials. These scattering techniques allow the elucidation of the structure and dynamics of nanomaterials from sub-nm to micron size scales and down to sub-millisecond time ranges both in bulk and at interfaces. A major advantage of scattering methods is that they provide the ensemble averaged information under in situ and operando conditions. As a result, they are complementary to various imaging techniques which reveal more local information. Scattering methods are particularly suitable for probing buried structures that are difficult to image. Although, many qualitative features can be directly extracted from scattering data, derivation of detailed structural and dynamical information requires quantitative modeling. The fourth-generation synchrotron sources open new possibilities for investigating these complex systems by exploiting the enhanced brightness and coherence properties of X-rays.

Surface Induced Phenytoin Polymorph. 1. Full Structure Solution by Combining Grazing Incidence X-ray Diffraction and Crystal Structure Prediction

Understanding the behavior and properties of molecules assembled in thin layers requires knowledge of their crystalline packing. The drug phenytoin (5,5-diphenylhydantoin) is one of the compounds that can be grown as a surface induced polymorph. By using grazing incidence X-ray diffraction, the monoclinic unit cell of the new form II can be determined, but, due to crystal size and the low amount of data, a full solution using conventional structure solving strategies fails. In this work, the full solution has been obtained by combining computational structure generation and experimental results. The comparison between the bulk and the new surface induced phase reveals significant packing differences of the hydrogen-bonding network, which might be the reason for the faster dissolution of form II with respect to form I. The results are very satisfactory, and the method might be adapted for other systems, where, due to the limited amount of experimental data, one must rely on additional approaches to gain access to more detailed information to understand the solid-state behavior.

Surface Induced Phenytoin Polymorph. 2. Structure Validation by Comparing Experimental and Density Functional Theory Raman Spectra

A method for structure solution in thin films that combines grazing incidence X-ray diffraction data analysis and crystal structure prediction was presented in a recent work (Braun et al. Cryst. Growth Des.2019, DOI: 10.1021/acs.cgd.9b00857). Applied to phenytoin form II, which is only detected in films, the approach gave a very reasonable, but not fully confirmed, candidate structure with Z = 4 and Z' = 2. In the present work, we demonstrate how, by calculating and measuring the crystal Raman spectrum in the low wavenumber energy region with the aim of validating the candidate structure, this can be further refined. In fact, we find it to correspond to a saddle point of the energy landscape of the system, from which a minimum of lower symmetry may be reached. With the new structure, with Z = 4 and Z' = 2, we finally obtain an excellent agreement between experimental and calculated Raman spectra.

Phase Transitions and Formation of a Monolayer-Type Structure in Thin Oligothiophene Films: Exploration with a Combined In Situ X-ray Diffraction and Electrical Measurements

A combination of in situ electrical and grazing-incidence X-ray diffraction (GIXD) is a powerful tool for studies of correlations between the microstructure and charge transport in thin organic films. The information provided by such experimental approach can help optimizing the performance of the films as active layers of organic electronic devices. In this work, such combination of techniques was used to investigate the phase transitions in vacuum-deposited thin films of a common organic semiconductor dihexyl-quarterthiophene (DH4T). A transition from the initial highly crystalline phase to a mesophase was detected upon heating, while only a partial backward transition was observed upon cooling to room temperature. In situ electrical conductivity measurements revealed the impact of both transitions on charge transport. This is partly accounted for by the fact that the initial crystalline phase is characterized by inclination of molecules in the plane perpendicular to the π-π stacking direction, whereas the mesophase is built of molecules tilted in the direction of π-π stacking. Importantly, in addition to the two phases of DH4T characteristic of the bulk, a third interfacial substrate-stabilized monolayer-type phase was observed. The existence of such interfacial structure can have important implications for the charge mobility, being especially favorable for lateral two-dimensional charge transport in the organic field-effect transistors geometry.

Drug release from thin films encapsulated by a temperature-responsive hydrogel

Control over drug delivery may be interestingly achieved by using temperature responsive encapsulants, which change their thickness and mesh size with temperature. The prototype N-isopropylacrylamide hydrogel cross-linked with di(ethylene glycol) divinyl ether p(NIPAAm-co-DEGDVE) swells at low temperature and collapses above the lower critical solution temperature (LCST), ∼29 °C in a buffer. It might be expected that drug release from such encapsulation is always favored below the LCST, due to the larger free volume present in the swollen polymer film. Recent results show contradicting behavior where some cases behave as expected and others release much less when the polymer layer is swollen. In this study, layers of the drugs phenytoin, clotrimazole and indomethacin were drop cast on glass and p(NIPAAM-co-DEGDVE) layers were then synthesized directly on top of these drug layers via initiated chemical vapor deposition (iCVD), a solvent-free and gentle polymerization technique. Dissolution experiments were then performed, in which the drug release through the hindrance of the hydrogel was measured at different pH values. The results show that not only the swelling but also the permeate (drug in this case)-polymer interaction plays an important role in the release.

Bulk and Surface-Stabilized Structures of Paracetamol Revisited by Raman Confocal Microscopy

We revisit the polymorphism of paracetamol by means of a micro-Raman technique, which has proved to be a powerful tool for structure recognition. Distinct lattice phonon spectra clearly identified the pure phases. Confocality enabled us to detect phase mixing between form II and either I or III on a micrometric scale in the same crystallite. Following the most recent findings on surface-mediated structures, we also investigated spin-coated films grown on glass, gold, and polystyrene substrates, confirming the selectivity of these surfaces for the metastable form III, which shows an unprecedented stability over a time span of several months. A mechanism of its transformation to phase II, via a partially ordered intermediate state, is suggested by polarized Raman measurements.

Improving Dissolution Properties by Polymers and Surfactants: A Case Study of Celastrol

Two polymorphs of celastrol were discovered and fully characterized by X-ray powder diffraction, thermogravimetry analysis, and differential scanning calorimetry. The single-crystal structures of form I and the isostructural solvate of form II were disclosed by single-crystal X-ray diffraction. The apparent solubility and wettability of both the crystalline forms were determined. It was found that surfactant can significantly improve the solubility of celastrol up to more than 10⁴ times. Tween 80 and sodium dodecyl sulfate largely improved the wettability of the 2 crystals. Form I shows better wettability than form II in all the buffer solutions with polymers and surfactants. Compared with form II, form I exhibits higher solubility in carboxymethylcellulose and polyvinylpyrrolidone media but much lower solubility in tween 80 and sodium dodecyl sulfate solutions. An investigation of wettability and solubility mechanisms was fully explored, and a hypothesis was proposed to understand the abnormal solubility differences.

Polymorphism of terthio-phene with surface confinement

The origin of unknown polymorphic phases within thin films is still not well understood. This work reports on crystals of the molecule terthio-phene which were grown by thermal gradient crystallization using glass-plate substrates. The crystalline domains displayed a plate-like morphology with an extended lateral size of about 100 µm, but a thickness of only a few µm. Specular X-ray diffraction patterns confirmed the presence of a new polymorph of terthio-phene. Crystal structure solution from a single crystal peeled from the film revealed a structure with an extremely large unit-cell volume containing 42 independent molecules. In contrast to the previously determined crystal structure of terthio-phene, a herringbone packing motif was observed where the terminal ends of the molecules are arranged within one plane (i.e. the molecular packing conforms to the flat substrate surface). This type of molecular packing is obtained by 180° flipped molecules combined with partially random (disordered) occupation. A densely packed interface between terthio-phene crystallites and the substrate surface is obtained, this confirms that the new packing motif has adapted to the flat substrate surface.

Substrate-Induced Phase of a Benzothiophene Derivative Detected by Mid-Infrared and Lattice Phonon Raman Spectroscopy

The presence of a substrate-induced polymorph of 2,7-dioctyloxy[1]benzothieno[3,2-b]benzothiophene is probed in microscopic crystals and in thin films. Two experimental techniques are used: lattice phonon Raman and IR spectroscopy. The bulk crystal and substrate-induced phase have an entirely different molecular packing, and therefore, their Raman spectra are characteristic fingerprints of the respective polymorphs. These spectra can be unambiguously assigned to the individual polymorphs. Drop-cast and spin-coated thin films on solid substrates are investigated in the as-prepared state and after solvent-vapor annealing. Because Raman spectroscopy is less sensitive with decreasing film thickness, IR spectroscopy is shown to be a more feasible tool for phase detection. The surface-induced phase is mainly present in the as-prepared thin films, whereas the bulk phase is present after solvent-vapor annealing. This result suggests that the surface-induced phase is a metastable polymorph.

Towards controlling the crystallisation behaviour of fenofibrate melt: triggers of crystallisation and polymorphic transformation

Fenofibrate (FEN) is a dyslipidemia treatment agent which is poorly soluble in water. FEN has tendency to form polymorphs and its crystallisation behaviour is difficult to predict. The nucleation process can be initiated by mechanical disruption such as ball milling or surface scratching which may result in different crystallisation behaviour to that observed in the unperturbed system. This study has obtained insights into the controllability of FEN crystallisation by means of regulating the exposed surface and growth temperatures during its crystallisation. The availability of an open top surface (OTS) during the crystallisation of the FEN melt resulted in a mixture containing FEN form I and IIa (I ≫ IIa) at room temperature, and in the range 40 to 70 °C. Covering the surface led to significant increases in the yield of form IIa at room temperature and at 40 and 50 °C. These temperatures also yielded the highest amount of form IIa in the OTS samples whilst crystallisation at 70 °C led to only FEN form I crystals regardless of the availability of the free surface. The metastable FEN form IIa transforms to the stable form I under the influence of a mechanical stress. Additionally, the introduction of OTS before the completion of crystallisation of form IIa led to a 'switch' of from IIa growth to form I. This study demonstrates that the polymorph selection of FEN can be obtained by the manipulation of the crystallisation conditions.

Solvent Vapor Annealing of Amorphous Carbamazepine Films for Fast Polymorph Screening and Dissolution Alteration

Solubility enhancement and thus higher bioavailability are of great importance and a constant challenge in pharmaceutical research whereby polymorph screening and selection is one of the most important tasks. A very promising approach for polymorph screening is solvent vapor annealing where a sample is exposed to an atmosphere saturated with molecules of a specific chemical/solvent. In this work, amorphous carbamazepine thin films were prepared by spin coating, and the transformation into crystalline forms under exposure to solvent vapors was investigated. Employing grazing incidence X-ray diffraction, four distinct carbamazepine polymorphs, a solvate, and hydrates could be identified, while optical microscopy showed mainly spherulitic morphologies. In vitro dissolution experiments revealed different carbamazepine release from the various thin-film samples containing distinct polymorphic compositions: heat treatment of amorphous samples at 80 °C results in an immediate release; samples exposed to EtOH vapors show a drug release about 5 times slower than this immediate one; and all the others had intermediate release profiles. Noteworthy, even the sample of slowest release has a manifold faster release compared to a standard powder sample demonstrating the capabilities of thin-film preparation for faster drug release in general. Despite the small number of samples in this screening experiment, the results clearly show how solvent vapor annealing can assist in identifying potential polymorphs and allows for estimating their impact on properties like bioavailability.

Coarse-grained modeling of crystal growth and polymorphism of a model pharmaceutical molecule

We describe a systematic coarse-graining method to study crystallization and predict possible polymorphs of small organic molecules. In this method, a coarse-grained (CG) force field is obtained by inverse-Boltzmann iteration from the radial distribution function of atomistic simulations of the known crystal. With the force field obtained by this method, we show that CG simulations of the drug phenytoin predict growth of a crystalline slab from a melt of phenytoin, allowing determination of the fastest-growing surface, as well as giving the correct lattice parameters and crystal morphology. By applying meta-dynamics to the coarse-grained model, a new crystalline form of phenytoin (monoclinic, space group P2₁) was predicted which is different from the experimentally known crystal structure (orthorhombic, space group Pna2₁). Atomistic simulations and quantum calculations then showed the polymorph to be meta-stable at ambient temperature and pressure, and thermodynamically more stable than the conventional orthorhombic crystal at high pressure. The results suggest an efficient route to study crystal growth of small organic molecules that could also be useful for identification of possible polymorphs as well.

Alteration of texture and polymorph of phenytoin within thin films and its impact on dissolution

By a change of texture and polymorph the dissolution characteristic of a drug molecule changes.