J. Phys. Chem. C 2008, 112, 15771–15776
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Probing Crystal Nucleation from Fluid Phases: The Nucleation of para-Azoxyanisole from Its Nematic Liquid Crystalline State Sophie Janbon, Roger J. Davey,* and Geoffrey Dent The Molecular Materials Centre, School of Chemical Engineering and Analytical Science, UniVersity of Manchester, P.O. Box 88, Manchester, M60 1QD, U.K. ReceiVed: February 26, 2008; ReVised Manuscript ReceiVed: July 11, 2008
Detailed understanding of structural evolution during the process of crystal nucleation is limited. In this contribution nucleation from the single component nematic phase of para-azoxyanisole is studied with particular emphasis on the structural relationships between liquid crystalline and crystalline phases in this polymorphic system. The data are interpreted and discussed in terms of their relevance to processes occurring within a nucleating cluster. It is clear that while positional order in the supersaturated phase does dictate the structural outcome of crystallization, an activation barrier still exists limiting the overall kinetics of the nucleation process. Introduction Relatively little is known of the molecular processes that surround the nucleation event in crystallization from solutions and melts. Viewed from the supramolecular standpoint, it is evident that there can be a correspondence between the selfassembled species in the liquid phase and their counterparts in the solid state. Thus, for example the polymorphic behavior of inosine,1 tetrolic acid2 and sulfamerizine3 has been rationalized from NMR and FTIR studies on concentrated solutions which reveal solution phase dimers capable of transferring structural information from solution to molecular clusters. In other systems, however, such as inosine dihydrate and (R,S) mandelic acid1,4 such correspondence is apparently lacking. On the other hand, from experiments with alkane multilayers at the air-water interface it is also known that even bilayers can adopt the packing of a mature crystal.5 Oxtoby6 has recently posed a straightforward question: ‘In crystallization from solution, when particles join a growing cluster, are they ordered in periodic structures that resemble the eventual crystalline solid?’. In considering the answer to this question it is clear7,8 that, in the context of order parameters, both solute concentration (or density) and the (crystalline) structure must change during the creation of a nascent crystal from a supersaturated fluid phase. In classical nucleation theory both are assumed to change simultaneously yielding critical clusters which have the same packing as the mature crystal. Indeed this assumption has proved successful in the design of auxiliary molecules to inhibit or catalyze a nucleation event.5 However, recent simulations8,9 and experimental results10-12 indicate that there are cases where the density change may precede the appearance of crystalline order to give amorphous clusters which evolve into crystalline entities with time and increasing size. The development of strong, periodic intermolecular interactions, densification and the concomitant elimination of solvent would surely all play roles in this process. Such a reality would, of course, not be inconsistent with any of the foregoing experimental knowledge but in truth we simply do not know whether clusters are * To whom correspondence should be addressed. E-mail: roger.davey@ manchester.ac.uk.
Figure 1. Molecular structure of PAA.
disordered or ordered; whether they contain solvent or how structure evolves within them. More detailed investigations of the nucleation process are surely needed and yet experimentally this is problematic due to rapid kinetics and the relatively small phase volumes involved. However, insight may be gained in other ways. One central question in this overall process concerns the evolution of the order parametersdo orientational and positional order precede the appearance of the macroscopic crystal structure in such clusters? One means of probing this question, which we explore in this present contribution, is to use thermotropic liquid crystalline materials as models since such systems exhibit liquid states which are anisotropic and retain varying degrees of orientational and positional order over micron length scales. It is apparent that in such systems the process of cooling crystallization will then proceed from an isotropic liquid via a supersaturated anisotropic liquid crystalline phase before adopting a crystalline form. By studying these temperature induced transitions and in particular by assessing the structural relationships between liquid crystal and crystalline phases we hoped to shed further light on the ways in which structure may evolve during the nucleation process. We have chosen p-azoxyanisole (PAA, see Figure 1)) as a material for study. PAA was examined in 1933 by Bernal and Crowfoot13 who reported the existence of a nematic liquid crystalline phase and two crystalline polymorphs. Optical examination of the phases led them to the conclusion that the nematic phase was structurally related to the metastable rather than the stable crystalline form. The detailed crystal structure of the stable form (III) was determined by Krigbaum, Chatani and Barber in 1970,14 Carliste and Smith in 197115 and Chebli and Brisse in 1995.16 The crystal structure of the metastable polymorph (I) has only been reported recently17 and this is an important factor in enabling further understanding of nucleation in this system.18 More extensive thermal studies19,20 of this
10.1021/jp801676w CCC: $40.75 2008 American Chemical Society Published on Web 09/12/2008
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Figure 2. Optical micrographs showing the sequence of phase evolution during melting (a, b, c) and crystallization (d, e).
material have revealed the existence of two additional, so far uncharacterised, polymorphs. Delord and Malet,21 proposed a model of the nematic phase derived from X-ray scattering data in which molecules are essentially aligned with their molecular axes parallel to the director and packed in a hexagonal array. Experimental Section PAA was purchased from Sigma-Aldrich. Purified Form III was obtained by evaporative recrystallization, under ambient conditions, from a 20 g/L acetone solution. Flat yellow, millimeter sized parallelepiped crystals grew over a period of a few days and were used for all subsequent studies. For optical microscopy a Zeiss Axioplan 2 polarizing microscope was used, fitted with a Linkam hot stage, in order to image directly the different phases as they disappeared/appeared on melting and during crystallization. Heating and cooling rates were 5 and 0.01 °C/min, respectively. DSC data were recorded using a Mettler DSC 30 Low Temperature Cell controlled using DSC 30-glass Mettler software with heating and cooling rates of 5 °C/min. Infrared spectra were recorded using an Avatar FT-IR spectrometer with a Golden Gate diamond ATR crystal in conjunction with Nicolet Omnic software. Raman spectra were recorded using a Jobin-Yvon Horiba Labram 300 Raman microscope powered by an 11mW, NeHe laser and using a 10× objective. Powder X-ray diffraction experiments were performed at the STFC Daresbury SRS, UK, on stations 9.1 (wavelength 1 Å) and 14.1 (wavelength 1.488 Å). Powdered samples were loaded into 0.7 or 1 mm glass capillaries which were then flame-sealed. On station 9.1, the powder diffraction data of Form III was recorded at room temperature from 1° to 20° (2θ) in 0.01° steps. The sample was then heated to a temperature above the Form III-nematic phase transition (125-130 °C). Having established the existence of the nematic phase at 130 °C scans were recorded every 5 °C on cooling to the crystallization point (80-85 °C). On station 14.1, WAXS patterns were recorded for 60 s while heating from 100 to 145 °C and cooling from 145 to 30 °C at rate of 1 °C/min in temperature steps of ∼5 °C. Results and Discussions Phase Appearance. The optical microscopy results are summarized in Figure 2a-d. On heating, crystals of Form III (Figure 2a) transformed to the nematic phase (melting) at 118 °C (Figure 2b) and formed the isotropic melt (clearing point)
at 138 °C (Figure 2c). On cooling, the nematic phase reappeared at 137 °C and the first crystals nucleated at around 90 °C. This crystallization phenomenon was complete in less than a second and the entire sample had a dendritic polycrystalline appearance (Figure 2d). Its birefringence changed at the lower temperature of 60 °C (Figure 2e) presumably as a result of a solid state phase transformation.17,20 No further changes were observed. The DSC data (not shown), were entirely consistent with these microscopic observations and the previous reports of Crowfoot and Bernal13 and Robinder and Poirier.19 The first phase to crystallize from the nematic at around 75 °C (Form I) transformed on heating to the nematic at 103.3 °C ((0.5 °C) with a heat of transition ∆H(1) ) 24.9 kJ/mol. The phase present at room temperature (Form III) melted to the nematic at 117 °C ((0.5 °C) with ∆H(3) ) 31.9 kJ/mol. The transition from nematic to isotropic melt occurred at 135 °C with ∆Hclearing ) 0.7 kJ/mol. The identity of the phases as observed by optical microscopy and DSC were confirmed by in situ pXRD as seen in Figure 3. Since both crystal-nematic transitions are endothermic, reversible and have well defined transition temperatures it is concluded that these phases are related enantiotropically. The transition between the polymorphic forms however is exothermic (∆HIfIII ) -3.1 kJ/mol) with no evidence of reversibility and appears therefore to suggest a monotropic relationship. (It is noted that the equality ∆HIfIII + ∆H(1) ) ∆H(3) does not appear to be true for these data. This may result from the impact of both the heat capacity on the crystal to nematic transitions and the difficulty of measuring the value of ∆HIfIII in a region where Form II appeared.) The kinetically favored, Form I apparently nucleates directly from the nematic phase (at 85 °C ((10 °C)) and subsequently transforms to the final, stable Form III. A schematic energy-temperature diagram summarizing the relationship between the isotropic melt, the nematic phase and Forms I and III is seen in Figure 4. Development of Long Range Order. The X-ray diffraction results shown in Figures 3 and 5 are used here to explore the development of crystalline order in the undercooled nematic phase. The nearest neighbor maxima in the melt and nematic phases were evaluated ((0.01) by fitting a Lorentzian profile to the data. The X-ray diffraction pattern of the isotropic melt at 145 °C (Figure 3a) has two maxima at 4.78 and 12.5 Å. On cooling into the nematic region at 125 °C, (Figure 3b), the first
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Figure 3. X-ray diffraction patterns of the isotropic melt (a), the nematic phase (b, c), Form I (d) and Form III (e) of PAA.
Figure 6. Coordination shells of a central molecule in the nematic phase at 100 °C taken from the hexagonal model from Delord and Malet. Figure 4. Schematic free energy/enthalpy-temperature diagram, showing the thermodynamic relationships between the PAA phases.
Figure 5. Comparison of the simulated diffraction patterns of Forms I and III of PAA with that of the experimental nematic phase (80 °C).
of these peaks shifts to the lower value of 4.60 Å while the second remains unchanged. This latter peak is the broader and weaker of the two in both phases. On cooling the nematic phase to 80 °C (Figure 3c) three maxima at 4.49, ∼7.1 and ∼12.5 Å become evident. These are related to the radial distribution functions defining the orientational and liquid order with the peak positions giving the distances between molecules (the azoxy groups) and their
breadth the numbers of nearest neighbors. Precise analysis of these data has not been carried out in this study since they appear to be in good agreement with the previous detailed work of Delord and Malet21 who used X-ray scattering to calculate the atomic distribution functions leading to the model summarized in Figure 6. The d-spacings at 4.49 and 7.1Å (cf. 4.6 and 7.9 Delord and Malet21 at 100 °C) are thus related to the intermolecular distances in the plane perpendicular to the director (xy) and define the first and second coordination shells respectively. Given the approximate molecular length of 13.3 Å, the 12.5 Å peak is presumed to be the distance between molecules along the z-direction and suggests some small overlap of adjacent molecules along the director. Comparing this model with equivalent crystal structure projections viewed down the molecular axis in Forms I and III (Figure 7) clearly shows how the hexagonal array in the nematic phase is perpetuated in Form I, (with comparable dimensions of 4.4 and 8.4 Å) but disappears in Form III. Comparison of the simulated diffraction patterns of the polymorphs with the experimental nematic phase scattering in Figure 5 is consistent with this hypothesis. While the positions of the diffraction peaks are comparable for the different phases it maybe surmised from their intensities that the nematic phase is more closely related to Form I than to Form III. This is particularly noticeable in the 10.0 Å peak in the powder pattern of Form I which is significantly more intense than in Form III, due to the position of the azoxy groups which lie in the (001)
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Figure 7. Crystal structures projections down the molecular axis [301] for Forms I and III of PAA.
Figure 8. Evolution of d-spacing, density and order parameter, with temperature during crystallization of PAA.
plane. This diffraction peak seems to correlate with the 12.5 Å peak in the pattern of the nematic phase both in terms of position and intensity, and corresponds to the azo separation of ∼12.5 Å along the director. The equivalent azo separation in Form III lies at the smaller vaule, ∼9 Å. The 7 and 4 Å peaks in the nematic are related to the xy packing of Form I but the added complexity of packing in these planes leads to greater equivalence of intensities between the polymorphs so that it is not possible to make an unequivocal link. Overall, however it is concluded that ordering in the nematic phase leads to the Ostwald’s Rule behavior in this system with the preferred nucleation of the kinetic Form I rather than the direct nucleation of the stable Form III. This crystallization process is further investigated in Figure 8 which follows the simultaneous evolution of the nearest neighbor intermolecular separation (d spacing), the density (reported previously by Jaeger22) and the orientational order parameter (related to the average difference between the molecular axis and the director, reported previously23,24) on cooling a sample from the isotropic melt through the nematic region until crystallization of Form I occurs. The measured d spacing falls monotonically from 4.78 to 4.63 Å as the temperature falls between 145 and 130 °C and the system enters the nematic region. From 127 to 80 °C the d-spacing decreases linearly to 4.49 Å prior to the crystallization of Form I. We can infer from these results that the closer packing in the nematic phase on cooling, is associated with the molecular alignment along the director, as reflected in the evolution of the order parameter (S) from 0.4 to 0.7. Given the totality of these data it would appear that with falling temperature densification and
ordering occur simultaneously within the stability range of the nematic phase. The overall result is that the isotropic melt gives way to a nematic state with its positional order and then to a crystal form that utilizes the nematic alignment in an efficient way. Molecular Self Association. The changes in chemical environment of PAA which accompany the structural evolution were revealed in FT-IR and Raman spectra. The 1600 cm-1 region in the IR corresponds to the CdC aromatic stretching mode, and its position (spectra not shown) underwent subtle but informative changes. For example, in the Form III spectrum it appeared at 1591 cm-1 while on heating to the nematic phase at 125 °C and subsequent cooling to Form I, (90-75 °C) it shifted to the higher wavenumber of 1595 cm-1. Further cooling, below 60 °C returned the band to 1591 cm-1, its starting position in Form III. These shifts are consistent with the known molecular packing in the crystal structures of Forms I and III16,17 in which the aromatic rings in Form III interact as dimers via π-π stacking (Figure 9b) while in Form I this interaction is weakened (Figure 9a) by only partial overlap of the π systems. The equivalence between the nematic and Form I spectra suggest that the two phases have similar, weak π-π interactions and hence share a common mode of self-association. This equivalence is further evidenced in the Raman spectra, Figure 10 which show the 1430-1630 cm-1 region, relating to the C(sp3)-H deformation in the methoxy groups and the aromatic CdC stretching respectively. Three methyl -C-H stretches are clearly visible in the Form III spectrum between 1450 and 1470 cm-1 but these merge into a shoulder on heating to Form I yielding an absorption peak which closely matches the nematic.
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Figure 9. π-π interactions involved in the polymorphs of PAA; (a) in Form I (in which the oxygen is disordered across two sites17 and (b) in form III.
Figure 10. Raman spectra of the polymorphic and nematic phases of PAA between 1430 and 1630 cm-1.
The aromatic CdC stretch is a clear doublet in the Raman of Form III. This resolution was not seen in the IR (discussed above) but arises from the slightly different stacking environments of the two phenyl rings of the PAA molecule in the Form III packing. In Form I this distinction is lost as the local environment of the two rings become equivalent. As a consequence the doublet reduces to a single peak with a shoulder, a situation mirrored by the nematic phase. Overall these data compliment the foregoing structural studies, suggesting that the packing similarities between Form I and the nematic are underpinned by similar modes of molecular self-association involving weak π-π interactions. Nucleation Kinetics. Given that the clearing points of the nematic-isotropic transition are 103 and 117 °C for Forms I and III respectively this must mean that at temperatures below 117 °C this system is supersaturated with respect to Form III and below 103 °C with respect to both Forms I and III. This is also evident from the energy-temperature diagram in Figure 4, with these regions of stability and metastability of the nematic phase being shown in Figure 8. Surprisingly, despite the existence of this undercooled nematic phase, which is predisposed to transfer structural information to Form I, all attempts to observe nucleation at temperatures from 117 to about 80 °C failed. At 95 °C (supersaturation σI ) 0.18 and σIII ) 1.91 of Forms I and III, respectively), for example, no crystallization was observed over a 2 month period. Only beyond 80 °C (supersaturation σI ) 0.52) was crystallization instantaneous. This behavior precluded the measurement of induction times for the nematic to Form I nucleation and hence prevented us confronting these data with nucleation theory and, for example,
obtaining a value for the interfacial tension. However it did indicate that despite a clear structural pathway for the crystallization process the nucleation step is still activated, requiring a critical driving force to overcome the penalty of creating surface. When this step was removed by seeding the nematic phase with Form III, within the metastable zone, then nucleation was always facile. Conclusions: Wider Implication for Crystal Nucleation The overall transition from isotropic melt to the formation of the first PAA crystals can now be considered. The process of densification and the concomitant evolution of structure are dictated by the balance between the available thermal energy, the rod-like shape of the molecule and the π-π interactions between the aromatic rings of nearest neighbors. In the nematic phase this leads to orientational order with a hexagonal arrangement in the plane perpendicular to the director as reported by Delord and Mallet.21 There is little agreement on the conformation of the molecule in the nematic phasesthe methoxy group will be spinning, NMR suggests that the oxygen is localized on one of the nitrogen atoms and the phenyl rings are not coplanar. Molecules are held as weak dimers through partial overlap of aromatic rings, a feature which together with the hexagonal arrangement dictates that crystal nucleation will result in the metastable Form I. At the same time, of course, the conformation becomes more restricted upon nucleation, with the methoxy groups cis to one another and the phenyl rings now coplanar. To what extent this change contributes to the activation barrier for nucleation is not known. The transforma-
15776 J. Phys. Chem. C, Vol. 112, No. 40, 2008 tion to the stable Form III is then accompanied by more effective overlap of aromatic rings and a significant modification in the packing.17 In the context of a critical sized cluster within a supersaturated fluid it may be inferred that, if such a cluster existed initially in a disordered state, the process of densification may result in structural rearrangement allowing information to be transferred to the crystalline state. PAA melts may provide a useful model for the behavior of clusters in a supersaturated solution particularly since we observed crystallization of PAA from DMSO solutions to follow the same polymorph sequence as from the nematic phase discussed here. Solvation of PAA evidently mirrors the effect of temperature presumably by inhibiting the formation of Form III type dimers. However, it is clear from this example that the existence of positional order alone is not sufficient to remove the barrier to nucleation associated with creating an interface between ordered and crystalline regions of the supercooled material. Hence the idea that an amorphous cluster might undergo a concerted, activation-free transformation into an ordered crystalline nucleus is not supported. Preorganization may direct the structural outcome but it provides little, if any, catalysis of the nucleation processsa significant activated barrier still exists. Acknowledgment. S.J. is grateful for a Marie Curie Fellowship, and the authors thank Profs L. Leiserowitz and S. Black for helpful discussions. References and Notes (1) Chiarella, R. A.; Gillon, A. M.; Burton, R. C.; Davey, R. J.; Sadiq, G.; Auffret, A.; Cioffi, M.; Hunter, C. A. The nucleation of inosinethe impact of solution chemistry on the appearance of polymorphic and hydrated crystal forms. Faraday Discuss. 2007, 136, 179–193. (2) Parveen, S.; Davey, R. J.; Dent, G.; Pritchard, R. G. Linking solution chemistry to crystal nucleation: the case of tetrolic acid. Chem. Commun. 2005, 1531, 1533. (3) Spitaleri, A.; Hunter, C. A.; McCabe, J. F.; Packer, M. J.; Cockroft, S. L. Cryst. Eng. Commun. 2004, 6, 489-493. (4) Davey, R. J.; Mughal, R. G.; Parveen, S. Concerning the relationship between structural and growth synthons in crystal nucleation: solution and crystal chemistry of carboxylic acids as revealed through IR spectroscopy. Cryst. Growth Des. 2006, 6, 1788–1796.
Janbon et al. (5) Weissbuch, I.; Leiserowitz, L.; Lahav, M. Towards stereochemical control monitoring and understanding of crystal nucleation. Cryst. Growth Des. 2003, 3, 125–150. (6) Oxtoby, D. W. Crystal Nucleation in Fluids. Phil. Trans. R. Soc. Lond. A. 2003, 361, 419–428. (7) Kashiev, D. Nucleation; Butterworth Heineman: Oxford, 2000. (8) Oxtoby, D. W. Nucleation of first order phase transitions. Acc. Chem. Res. 1998, 31, 91–97. (9) Anwar, J.; Boateng, P. K. Computer simulation of crystallisation from solution. J. Am. Chem. Soc. 1998, 120, 9600–9604. (10) Vekilov, P. G. Dense liquid precursors for the nucleation of ordered solid phases from solution. Cryst. Growth Des. 2004, 4, 671–685. (11) Alison, H.; G.; Davey, R. J.; Garside, J.; Quayle, M. J.; Tiddy, G. J. T.; Clarke, D. T.; Jones, G. R. Using a novel plug flow reactor for the in situ simultaneous monitoring of SAXS and WAXD during crystallisation from solution. PCCP 2003, 5, 4998–5000. (12) Bonnett, P.; Carpenter, K. J.; Dawson, S.; Davey, R. J. Solution crystallisation via a submerged liquid-liquid phase boundary: oiling out. Chem. Commun. 2003, 698–699. (13) Bernal, J. D.; Crowfoot, D. Crystalline phases of some substances studied as liquid crystals. Trans. Faraday Soc. 1933, 1032–1049. (14) Krigbaum, W. R.; Chatani, Y.; Barber, P. G. The crystal structure of p-azoxyanisole. Acta Crystallogr. 1970, B26, 97–102. (15) Carlisle, C. H.; Smith, C. H. The structure of p-azoxyanisole. Acta Crystallogr. 1971, B27, 1068–1069. (16) Chebli, C.; Brisse, F. 4,4′-azoxyanisole at 203 K. Acta Crystallogr. 1995, C51, 1164–1167. (17) Janbon, S.; Davey, R. J.; Shankland, K. The crystal structure of a metastable polymroph of para-azoxyanisole. CrystEngComm. 2008, 10, 279– 282. (18) Robinder, R. C.; Poirier, J. C. Monotropic crytalline phases of p-azoxyanisole from the nematic melt. J. Am. Chem. Soc. 1968, 90, 4760– 4763. (19) Ogorodnik, K. Z. Four Solid-Crystal Forms of Paraazoxyanisole and the Thermodynamic Relationships between Them. Crystallogr. Rep. 2002, 47, 966–969. (20) Delord, P.; Malet, G. Diffusion des Rayons X par une Phase Nematique Orientee. L’ordre a Courte Distance dans le Paraazoxyanisole. Mol. Cryst. Liq. Cryst. 1974, 28, 223–235. (21) Jaeger, F. M. Temperature dependence of the free surface energy of liquids in temperature range from-80 to 1650° centigrade. Z. Anorg. Allgem. Chem. 1917, 101, 1–214. (22) Hanson, E. G.; Shen, Y. R. Mol. Cryst. Liq. Cryst. 1976, 36, 193207. (23) Bata, L.; Broude, V. L.; Fedotov, V. G.; Kroo, N.; Rosta, L.; Szabon, J.; Umaerov, M.; Vizi, I. Mol. Cryst. Liq. Cryst. 1978, 44, 71-88. (24) Cardew, P. T.; Davey, R. J.; Garside, J. Evaluation of supersaturation in crystal growth from solution. J. Cryst. Growth 1979, 46, 534–538.
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