Building, 75 Montrose Street, G1 1XQ, Glasgow 3 Strathclyde Institute of Pharmacy & Biomedical Science (SIPBS), University of Strathclyde, 161

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1 THE IMPACT OF PARACETAMOL IMPURITIES ON FACE PROPERTIES: INVESTIGATING THE SURFACE OF SINGLE CRYSTALS USING TOF-SIMS. Sara Ottoboni* 1, Michael Chrubasik 1,4, Layla Mir Bruce 2, Thai Thu Hien Nguyen 2, Murray Robertson 3, Blair Johnston 1, Iain D. H. Oswald 3, Alastair Florence 1, Chris Price 1 1 EPSRC Centre for Innovative Manufacturing in Continuous Manufacturing and Crystallisation, University of Strathclyde, Technology and Innovation Centre, Level 6, CMAC, 99 George Street, G1 1RD, Glasgow, UK 2 Department of Chemical & Process Engineering, University of Strathclyde, Level 4, James Weir Building, 75 Montrose Street, G1 1XQ, Glasgow 3 Strathclyde Institute of Pharmacy & Biomedical Science (SIPBS), University of Strathclyde, 161 Cathedral Street, G4 0RE, Glasgow 4 National Physical Laboratory Scottish Hub, University of Strathclyde, Glasgow, UK Figure 1 Main faces of paracetamol form I. Paracetamol predicted morphology using the Bravais-Friedel-Donnay-Harker (BFDH) method assuming that the relative growth rates R hkl of the crystal faces is inversely proportional to the inter-planar spacing d hkl, without taken account of the chemical nature of the crystal, molecular packing arrangement or crystallising environment.

2 Table 1 Molecular packing diagrams based upon the crystallographic structures: the (001) face showing more polar surface with exposed OH and -NHCOCH 3 groups in various directions; the (20-1) face showing mainly the OH and -C=O groups exposed from the surface; the (011) face shows -NHCOCH 3 group exposed; the (-110) face showing less polar with less functional groups OH and -NHCOCH 3 exposed. The molecules can be seen to be arranged alternatively nearly parallel and perpendicular to the surface. Face (11-1) shows -NHCOCH 3 group exposed; the (200) face showing OH and -NHCOCH 3 exposed. The red shadow reprents a crystal plane. Depth of the slice 10Å, Area of the slice 20Å squared. FACET (001) main facet FUNCTIONAL GROUP EXPOSED ON EACH FACET (20 1) (0 11), (01 1) and (1 10) (-110) and (-1-10) (11-1)

3 Solubility (mg/g) Solubility (mg/g) (200) Temperature ( C) Granberg and Rasmuson-paracetamol in ethanol Prediction of 4-nitrophenol in ethanol Prediction of paracetamol in ethanol Equilibration data of paracetamol in ethanol Equilibration data of 4-nitrophenol in ethanol Temperature ( C) Prediction of 4-nitrophenol in hexane Prediction of paracetamol in hexane Equilibration data of paracetamol in hexane Equilibration data of 4-nitrophenol in hexane Figure 2 Solubility curve of paracetamol in ethanol (left hand side) and hexane (right hand side) in mg/g of solvent predicted by using COSMOTherm, evaluated from literature and measured by equilibration method (37). Enthalpy of fusion and melting temperature of paracetamol and 4-nitrophenol used in the solubility predictions were taken from the literature (31). Predicted solubility data substantially underestimated the experimentally determined values. The solubility of paracetamol in ethanol determined by equilibration shows good agreement with published data (37).

4 Table 2 Crystal lattice parameters, volume and lattice systems for PP, PN and P4%N. Single crystal X-ray diffraction data were collected for crystals of pure paracetamol (PP), pure 4-nitrophenol (PN) and those from the cooling crystallization from paracetamol with 4%mol of 4-nitrophenol (P4%N) to obtain the lattice parameters along with the face-indexing of the crystals. Sample Lattice parameters Lattice parameters α, Lattice volume Lattice system a, b, c (Å) β, γ ( ) (Å 3 ) PP ± 0.004, ± 0.007, ± PN ± , ± , ± P4%N ± 0.005, ± 0.007, ± , ± 0.004, 90 90, ± , 90 90, ± 0.007, Monoclinic P 626 Monoclinic P 780 Monoclinic P Figure 3 SC-XRD images of PP crystal (on the left) and P4%N crystal (on the right).

5 Table 3 Stereomicroscopic images of the different crystal examined in this work: paracetamol crystal (PP), 4-nitrophenol crystal (PN), cooling crystallization crystal of paracetamol in presence of 4-nitrophenol (P4%N), pure paracetamol crystal with drops of 4-nitrophenol solution (PDN) and pure paracetamol crystal with 4-nitrophenol crystal epitaxial growth (PEN). Scale bar corresponds to 10mm. Examination of large single crystals of the 5 different materials PP, PN, P4%N, PND and PEN show striking similarities from all the samples except the 4-nitrophenol (PN) which is slightly elongated and contains clear traces of coloured impurity in the lattice. Stereo microscopy of the large single crystals prepared for TOF-SIMS examination revealed different habits to the smaller single crystals used in the XRD investigation to assign faces. Both types of crystals came from the starting same solution, the small crystals used for SCXRD were harvested after a suitable growth period at 5 C at the same time the single crystals used to grow to large size were also harvested and transferred to a new supersaturated solution to grow further. As a consequence of this is has not been possible to unequivocally assign face identities to the crystals characterised by TOF-SIMS. (PP) (PN) (P4%N) (PDN) (PEN) Figure 4 OM image in DIC mode, 10X, and SEM image of paracetamol crystal (PP). SEM operating conditions: distance 7210um, emission current 58.9mA, WD 7.21mm. Paracetamol surface texture is observed in OM and SEM image, with the characteristic edge steps. These crystal surface characteristics are consistent with those reported by Thompson et al. (13) who used electron microscopy. In this case steps are higher than Thompson et al. observed due to growth at higher supersaturation (S=1.5 vs S=0.44). The crystal showed steps ranging in the height from 10 to 30 nm. Topographic analysis using AFM confirmed the microscopic characterization of the pure paracetamol and pure 4-nitrophenol crystals (Figure 9).

6 Figure 5 OM image in DIC mode, 10X, and SEM image of 4-nitrophenol crystal (PN). SEM operating conditions: distance 6220um, emission current 57.9mA, WD 6.22mm. 4-nitrophenol characteristics rounded steps are observed in OM and SEM image, shorter steps than the pure paracetamol (PP) crystal. Figure 6 OM image in DIC mode, 10X, and SEM image of cooling crystallization crystal (P4%N). SEM operating conditions: working distance 6700um, emission current 57.4mA, WD 6.7mm. Increase frequency of surface defects are apparent in OM and SEM images and are associated with the presence of the impurity 4-nitrophenol (see the arrow).

7 Figure 7 OM image in DIC mode, 10X, and SEM image of drop crystal (PDN). Red arrow highlight the drop perimeter. SEM operating conditions: working distance 6360um, emission current 58.7mA, WD 6.36mm. Needle shape 4- nitrophenol crystals are observed in the drop area both in OM and SEM images (see the arrow). The 4-nitrophenol needle like crystals were randomly distributed in the core area of the drop, while at the perimeter they were aligned in the direction of spread. Areas not covered by the spreading drops of 4-nitrophenol solution showed characteristic paracetamol surface texture. The 4-nitrophenol crystals grown by rapid solvent evaporation exhibit a needle like morphology that is quite different to the slowly grown crystal used for the SCXRD. This may be due to rapid growth by evaporation or alternatively could be due to the formation of the beta polymorph of 4-nitrophenol (41). Figure 8 OM image in DIC mode, 10X, and SEM image of epitaxial crystal (PEN). Needle shape epitaxially growth 4- nitrophenol crystals are observed (arrow). SEM operating conditions: working distance 5820um, emission current 57.7mA, WD 5.82mm. Paracetamol steps substrate texture is seen from SEM image. The presence of both large and small crystals of 4-nitrophenol on the paracetamol crystal surface are seen in the OM image. These 4-nitrophenol crystals showed the same needle like shape observed when 4-nitrophenol was deposited by evaporation of a drop of ethanol saturated solution.

8 Figure 9 AFM map of PP crystal collected in air using Scan Asyst mode, the area of detection is 10x10μm. Topography shows roughness of PP crystal and the typical crystal step growth of PP crystal with step ranging in the height from 10 to 30 nm (see white arrows). Figure 10 Two different AFM maps of P4%N crystal obtained in air with Scan Asyst mode, scan area respectively 50 and 15um. From these two AFM topographic maps rough surface with nanometric steps can be evaluated (white arrow).

9 Figure 11 AFM maps of PDN crystal obtained in air with Scan Asyst mode, scan area 50μm. The perimeter drop can be observed. On the left side of the image the surface is smooth with macro roughness characteristic of paracetamol, while on the right side the increase of micro roughness is due to the needle like 4-nitrophenol crystal randomly orientated is observe. The PDN crystals were analysed by atomic force microscopy however, the 4-nitrophenol crystals formed during rapid evaporation were not strongly bonded to the paracetamol substrate crystal surface as a consequence of this they were easily detached during the AFM analysis and became attached to the AFM tip. Therefore only boundary areas of the drop have been analysed by AFM. Figure 12 AFM maps of PEN crystal obtained in air with Scan Asyst mode, scan area respectively 50um. Nano and micro roughness can be observed. Micro roughness appears to be linked to 4-nitrophenol crystal epitaxial growth on paracetamol crystal surface, while nano roughness was probably linked the proximity of multiple 4-nitrophenol crystals. Table 4 Characteristic neutral fragment mass spectra peaks of paracetamol and 4-nitrophenol taking from NIST 08 MS Demo and AMDIS 2.6. Paracetamol Intensity Fragment 4-nitrophenol Intensity Fragment fragment mass (counts) assignment fragment mass (counts) assignment C 6 H 7 NO C 4 H 3 N Paracetamol nitrophenol

10 CHNO C 2 HN C 5 H 4 O C 5 H 3 NO 2 or C 6 H 5 O C 6 H 4 O 2 or C 5 H 6 O C 6 H 6 NO C 5 H 6 O CH 5 NO 2 or C 4 HN C 5 H 4 NO 2 or C 6 H 5 O C 6 H 6 O C 3 H 3 N C 3 H 3 N C 3 H 2 N C 2 N Paracetamol C 4 H 5 NO 3