Determination of the onset of crystallization of N1-2-(thiazolyl)sulfanilamide (sulfathiazole) by UV-Vis and calorimetry using an automated reaction platform; subsequent characterization of polymorphic forms using dispersive Raman spectroscopy

Experimental Electron Density and Neutron Diffraction Studies on the Polymorphs of Sulfathiazole

High resolution X-ray diffraction data on forms I-IV of sulfathiazole and neutron diffraction data on forms II-IV have been collected at 100 K and analyzed using the Atoms in Molecules topological approach. The molecular thermal motion as judged by the anisotropic displacement parameters (adp's) is very similar in all four forms. The adp of the thiazole sulfur atom had the greatest amplitude perpendicular to the five-membered ring, and analysis of the temperature dependence of the adps indicates that this is due to genuine thermal motion rather than a concealed disorder. A minor disorder (∼1-2%) is evident for forms I and II, but a statistical analysis reveals no deleterious effect on the derived multipole populations. The topological analysis reveals an intramolecular S-O···S interaction, which is consistently present in all experimental topologies. Analysis of the gas-phase conformation of the molecule indicates two low-energy theoretical conformers, one of which possesses the same intramolecular S-O···S interaction observed in the experimental studies and the other an S-O···H-N intermolecular interaction. These two interactions appear responsible for "locking" the molecular conformation. The lattice energies of the various polymorphs computed from the experimental multipole populations are highly dependent on the exact refinement model. They are similar in magnitude to theoretically derived lattice energies, but the relatively high estimated errors mean that this method is insufficiently accurate to allow a definitive stability order for the sulfathiazole polymorphs at 0 K to be determined.

Polymorph Farming of Acetaminophen and Sulfathiazole on a Chip

PURPOSE

The aim of this paper is to understand at a given temperature (1) the role of template films, the droplet volume of a saturated sulfathiazole aqueous solution and the solvent on polymorph screening of sulfathiazole on a silicon wafer, and (2) the effect of template films on the acetaminophen crystal face at the template-crystal interface.

MATERIALS AND METHODS

Template Effect: Spun cast template films of non-annealed chitosan and annealed chitosan at 140 degrees C on silicon wafers were prepared. A 0.01-cm(3) saturated sulfathiazole aqueous solution droplets were deposited on both kinds of chitosan film. Sulfathiazole crystals were produced on those films by evaporation at 25 degrees C. Volume Effect: Different droplet volumes of a saturated sulfathiazole aqueous solution ranging from 0.01 to 0.14 to 2.7 cm(3) were deposited on non-annealed chitosan films. Sulfathiazole crystals were generated on those films by evaporation at 25 degrees C. Solvent Effect: 0.01 cm(3) saturated sulfathiazole methanol solution droplets were deposited on non-annealed chitosan films and sulfathiazole crystals were formed on those films by evaporation at 25 degrees C. The formation pathways of different sulfathiazole crystal polymorphs of the above mentioned effects were analyzed and verified by systematic studies. Template-crystal Interfacial Study: Millimeter-sized acetaminophen crystals were successfully grown on non-annealed chlorosulfonated poly(ethylene) (PE-Chl) and chitosan template films by cooling the saturated acetaminophen aqueous solution from 50 to 25 degrees C in which those template films were immersed. The bonding energies for specific carbons collected by electron spectroscopy for chemical analysis (ESCA) at the acetaminophen crystal surface, together with the molecular interactions between acetaminophen and PE-Chl and between acetaminophen and chitosan in separately prepared solid dispersion film samples detected by Fourier transformed infrared (FTIR) spectroscopy, proved to be useful for identifying the crystal face of acetaminophen essential for its specific intermolecular interactions at the template-crystal interface.

RESULTS

Thermodynamically metastable sulfathiazole Form I crystals were reproducibly obtained on the non-annealed chitosan films whereas the stable sulfathiazole Form III crystals were repeatedly formed on the annealed chitosan films. Droplet volumes and solvents were also found responsible for the polymorphic outcome of sulfathiazole in the kinetically driven area of two overlapping metastable zones from two competing polymorphs of Form I and Form III. Thermodynamically stable sulfathiazole Form III crystals were formed on the non-annealed chitosan films instead when the droplet volumes of a saturated sulfathiazole aqueous solution were increased from 0.01 to 0.14 cm(3) and 2.7 cm(3). When the solvent was changed from water to methanol, the thermodynamically stable sulfathiazole Form III crystals were again observed on the non-annealed chitosan films even from the 0.01 cm(3) saturated sulfathiazole methanol solution droplets.

CONCLUSIONS

Template surfaces were thought to provide specific functional groups to either change the energy barrier for the nuclei formation of the thermodynamically metastable Form I or alter the droplet contact angle and the droplet surface area which was related to the droplet evaporation time. The evaporation time determines the amount of time available for the polymorphic transformation from Form I to Form III. Apparently, droplet volumes could also determine the amount of time needed to reach supersaturation and the amount of time available for a polymorphic transformation from Form I to Form III. In addition, the molecular conformation and viscosity of solvents such as methanol might alter the original nucleation kinetics in water and lead to a more rapid polymorphic transformation from Form I to Form III. Template films of PE-Chl and chitosan were found to be critical for determining the face of a millimeter-sized acetaminophen crystal at the template-crystal interface. The idea of performing polymorph screening on the template film deposited on a chip has opened up a new doorway to examine the roles of: (1) various kinds of drug carrier in the form of a template film, (2) the droplet volume of a saturated solution, and (3) the type of solvent used, in polymorphic control. Growing millimeter-sized crystals directly on the chip of template has also provided a convenient technology enabling platform for examining the crystal-template interface by solid-state characterization techniques such as ESCA.

Flow-through sensor with Fourier transform Raman detection for determination of sulfonamides

A flow-through sensor system with Fourier transform (FT) Raman spectroscopy as detection technique is described. The molecular and structural information contained in Raman spectra together with the selective retention of the species of interest on the sorbent make the proposed methodology highly selective. The flow-through sensor allowed the direct quantitative determination of sulfathiazole and sulfamethoxazole in the presence of other species that are normally encountered with these analytes. The system used Sephadex QAE A-25 resin as packing material of a flow-through cell on which sulfonamides were temporarily retained. Samples were transported by a carrier solution of NaOH 10(-2) mol l(-1) (pH = 12), and 2 ml of a [NaCl (0.10 mol l(-1))/NaOH (10(-2) mol l(-1))] solution was employed as eluent. Using a sample volume of 1 ml, the analytical signal was linear in the range 0.5-7 g l(-1) and 0.5-10 g l(-1), for sulfathiazole and sulfamethoxazole, respectively. RSDs (%) lower than 4% were obtained for both analytes. The sensor was satisfactorily applied to several commercial pharmaceutical preparations for human and animals in different physical presentations, including capsules, syrup, tablets, powders, injectables and suspensions.