Polymorphism (materials science)

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In materials science, polymorphism is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements. The complete morphology of a material is described by polymorphism and other variables such as crystal habit, amorphous fraction or crystallographic defects. Polymorphism is relevant to the fields of pharmaceuticals, agrochemicals, pigments, dyestuffs, foods, and explosives.

When polymorphism exists as a result of difference in crystal packing, it is called packing polymorphism. Polymorphism can also result from the existence of different conformers of the same molecule in conformational polymorphism. In pseudopolymorphism the different crystal types are the result of hydration or solvation. This is more correctly referred to as solvomorphism as different solvates have different chemical formulae. An example of an organic polymorph is glycine, which is able to form monoclinic and hexagonal crystals. Silica is known to form many polymorphs, the most important of which are; α-quartz, β-quartz, tridymite, cristobalite, coesite, and stishovite. A classical example is the pair of minerals, calcite and aragonite, both forms of calcium carbonate.

An analogous phenomenon for amorphous materials is polyamorphism, when a substance can take on several different amorphous modifications.

Background[edit]

In terms of thermodynamics, there are two types of polymorphism. For a monotropic system, a plot of the free energy of the various polymorphs against temperature do not cross before all polymorphs melt—in other words, any transition from one polymorph to another will be irreversible. For an enantiotropic system, a plot of the free energy against temperature shows a crossing point before the various melting points, and it may be possible to convert reversibly between the two polymorphs on heating and cooling or through physical contact with a lower energy polymorph.

The first observation of polymorphism in organic materials is attributed to Friedrich Wöhler and Justus von Liebig when in 1832 they examined[1] a boiling solution of benzamide: on cooling the benzamide initially crystallised as silky needles but on standing these were slowly replaced by rhombic crystals. Present-day analysis[2] identifies three polymorphs for benzamide: the least stable one, formed by flash cooling is the orthorhombic form II. This type is followed by the monoclinic form III (observed by Wöhler/Liebig). The most stable form is monoclinic form I. The hydrogen bonding motif is the same for all three phases, however they differ strongly in their pi-pi interactions.

Reasons[edit]

Polymorphs have different stabilities and may spontaneously convert from a metastable form (unstable form) to the stable form at a particular temperature. They also exhibit different melting points, solubilities (which affect the dissolution rate of drug and consequently its bioavailability in the body), X-ray crystal and diffraction patterns.

Various conditions in the crystallisation process is the main reason responsible for the development of different polymorphic forms. These conditions include:

  • Solvent effects (the packing of crystal may be different in polar and nonpolar solvents)
  • Certain impurities inhibiting growth pattern and favour the growth of a metastable polymorphs
  • The level of supersaturation from which material is crystallised (in which generally the higher the concentration above the solubility, the more likelihood of metastable formation)
  • Temperature at which crystallisation is carried out
  • Geometry of covalent bonds (differences leading to conformational polymorphism)
  • Change in stirring conditions

Despite the potential implications, polymorphism is not always well understood. In 2006 a new crystal form was discovered of maleic acid 124 years after the first crystal form was studied.[3] Maleic acid is a chemical manufactured on a very large scale in the chemical industry and is a salt forming component in medicine. The new crystal type is produced when a co-crystal of caffeine and maleic acid (2:1) is dissolved in chloroform and when the solvent is allowed to evaporate slowly. Whereas form I has monoclinic space group P21/c, the new form has space group Pc. Both polymorphs consist of sheets of molecules connected through hydrogen bonding of the carboxylic acid groups; but, in form I, the sheets alternate with respect of the net dipole moment, whereas, in form II, the sheets are oriented in the same direction.

1,3,5-Trinitrobenzene is more than 125 years old and was used as an explosive before the arrival of the safer 2,4,6-trinitrotoluene. Only one crystal form of 1,3,5-trinitrobenzene was known in the space group Pbca. In 2004, a second polymorph was obtained in the space group Pca21 when the compound was crystallised in the presence of an additive, trisindane. This experiment shows that additives can induce the appearance of polymorphic forms.[4]

Ostwald's rule[edit]

Ostwald's rule or Ostwald's step rule,[5][6] conceived by Wilhelm Ostwald, states that in general it is not the most stable but the least stable polymorph that crystallises first. See for examples the aforementioned benzamide, dolomite or phosphorus, which on sublimation first forms the less stable white and then the more stable red allotrope.

Ostwald suggested that the solid first formed on crystallisation of a solution or a melt would be the least stable polymorph. This can be explained on the basis of irreversible thermodynamics, structural relationships, or a combined consideration of statistical thermodynamics and structural variation with temperature. Ostwald's rule is not a universal law but is only a possible tendency in nature.

Polymorphism in binary metal oxides[edit]

Structural changes occur due to polymorphic transitions in binary metal oxides and these lead to different polymorphs in binary metal oxides. Table below gives the polymorphic forms of key functional binary metal oxides, such as: CrO2, Cr2O3, Fe2O3, Al2O3, Bi2O3, TiO2, SnO2, ZrO2, MoO3, WO3, In2O3.[7]

Metal oxides Phase Conditions of P and T Structure/Space Group
CrO2 α-phase Ambient conditions Rutile-type Tetragonal (P42/mnm)
β-phase RT and 14 GPa CaCl2-type Orthorhombic
RT and 12±3 GPa
Cr2O3 Corundum phase Ambient conditions Corundum-type Rhombohedral (R3c)
High pressure phase RT and 35 GPa Rh2O3-II type,
Fe2O3 α-phase Ambient conditions Corundum-type Rhombohedral (R3c)
β-phase Below 773 K Body centered cubic (Ia3)
γ-phase Up to 933 K Cubic, spinel structure (Fd3m)
ε-phase -- Rhombic (Pna21)
Bi2O3 α-phase Ambient conditions Monocllinic (P21/c)
β-phase 603-923 K and 1 atm Tetragonal
γ-phase 773-912 K or RT and 1 atm Body centered cubic
δ-phase 912-1097 K and 1 atm FCC (Fm-3m)
In2O3 Bixbyite-type phase Ambient conditions Cubic (Ia3)
Corundum-type 15-25 GPa at 1273 K Corundum-type Hexagonal (R3c)
Rh2O3(II)-type 100 GPa and 1000 K Orthorhombic
Al2O3 α-phase Ambient conditions Corundum-type Trigonal, (R3c)
γ-phase 773 K and 1 atm Cubic (Fd-3m)
SnO2 α-phase Ambient conditions Rutile-type Tetragonal (P42/mnm)
CaCl2-type phase 15 KBar at 1073 K Orthorhombic, CaCl2-type (Pnnm)
α-PbO2-type Above 18 KBar α-PbO2-type (Pbcn)
TiO2 Rutile Ambient conditions Rutile-type Tetragonal
Anatase Above 1073 K Tetragonal (I41/amd)
Brookite High pressure phase Orthorhombic (Pcab)
ZrO2 Monoclinic phase Ambient conditions Monoclinic (P21/c)
Tetragonal phase Above 1443 K Tetragonal (P42/nmc)
Fluorite-type phase Above 2643 K Fluorite-type (Fm3m) cubic structure
MoO3 α-phase 553-673 K & 1 atm Orthorhombic (Pbnm)
β-phase 553-673 K & 1 atm Monoclinic
h-phase High pressure and high temperature phase Hexagonal (P6a/m or P6a)
MoO3-II 60 kbar and 973 K Monoclinic
WO3 ε-phase Up to 220 K Monoclinic (Pc)
δ-phase 220-300 K Triclinic (P1)
γ-phase 300-623 K Monoclinic (P21/n)
β-phase 623-900 K Orthorhombic (Pnma)
α-phase Above 900 K Tetragonal (P4/ncc)

Polymorphism in pharmaceuticals[edit]

Polymorphism is important in the development of pharmaceutical ingredients. Many drugs receive regulatory approval for only a single crystal form or polymorph. In a classic patent case the pharmaceutical company GlaxoSmithKline defended its patent for the polymorph type II of the active ingredient in Zantac against competitors while that of the polymorph type I had already expired[citation needed]. Polymorphism in drugs can also have direct medical implications. Medicine is often administered orally as a crystalline solid and dissolution rates depend on the exact crystal form of a polymorph. Polymorphic purity of drug samples can be checked using techniques such as powder X-ray diffraction, IR/Raman spectroscopy, and utilizing the differences in their optical properties in some cases.[8]

In the case of the antiviral drug ritonavir, not only was one polymorph virtually inactive compared to the alternative crystal form, but the inactive polymorph was subsequently found to convert the active polymorph into the inactive form on contact, due to its lower energy and greater stability making spontaneous interconversion energetically favourable. Even a speck of the lower energy polymorph could convert large stockpiles of ritonivir into the medically useless inactive polymorph, and this caused major issues with production which ultimately were only solved by reformulating the medicine into gelcaps and tablets, rather than the original capsules.[9]

Cefdinir is a drug appearing in 11 patents from 5 pharmaceutical companies in which a total of 5 different polymorphs are described. The original inventor Fujisawa now Astellas (with US partner Abbott) extended the original patent covering a suspension with a new anhydrous formulation. Competitors in turn patented hydrates of the drug with varying water content, which were described with only basic techniques such as infrared spectroscopy and XRPD, a practice criticised in one review[10] because these techniques at the most suggest a different crystal structure but are unable to specify one; however, given the recent advances in XRPD, it is perfectly feasible to obtain the structure of a polymorph of a drug, even if there is no single crystal available for that polymorphic form. These techniques also tend to overlook chemical impurities or even co-components. Abbott researchers realised this the hard way when, in one patent application, it was ignored that their new cefdinir crystal form was, in fact, that of a pyridinium salt. The review also questioned whether the polymorphs offered any advantages to the existing drug: something clearly demanded in a new patent.

Acetylsalicylic acid has an elusive second polymorph that was first discovered by Vishweshwar et al.;[11] fine structural details were given by Bond et al.[12] A new crystal type was found after attempted co-crystallization of aspirin and levetiracetam from hot acetonitrile. The form II is stable only at 100 K and reverts to form I at ambient temperature. In the (unambiguous) form I, two salicylic molecules form centrosymmetric dimers through the acetyl groups with the (acidic) methyl proton to carbonyl hydrogen bonds, and, in the newly claimed form II, each salicylic molecule forms the same hydrogen bonds, but then with two neighbouring molecules instead of one. With respect to the hydrogen bonds formed by the carboxylic acid groups, both polymorphs form identical dimer structures.

  • Paracetamol powder has poor compression properties; this poses difficulty in making tablets, so a new polymorph of paracetamol was found which is more compressible.
  • Due to differences in solubility of polymorphs, one polymorph may be more active therapeutically than another polymorph of same drug.
  • Cortisone acetate exists in at least five different polymorphs, four of which are unstable in water and change to a stable form.
  • Carbamazepine (used in epilepsy and trigeminal neuralgia) beta-polymorph developed from solvent of high dielectric constant ex aliphatic alcohol, whereas alpha polymorph crystallized from solvents of low dielectric constant such as carbon tetrachloride.

Interesting occurrences of polymorphs[edit]

Walter McCrone stated that "every compound has different polymorphic forms, and that, in general, the number of forms known for a given compound is proportional to the time and money spent in research on that compound."[13]

Crystal polymorphs can disappear. There have been cases of laboratories growing crystals of a particular structure and when they try to recreate this, the original crystal structure isn't created but a new crystal structure is. Also, findings of one crystal structure intermittently polymorphing over time into another have been recorded. The drug paroxetine was subject to a lawsuit that hinged on such a pair of polymorphs.[14] An example is known when a so-called "disappeared" polymorph re-appeared after 40 years. These so-called "disappearing" polymorphs are most likely metastable kinetic forms.

Polytypism[edit]

Polytypes are a special case of polymorphs, where multiple close-packed crystal structures differ in one dimension only. Polytypes have identical close-packed planes, but differ in the stacking sequence in the third dimension perpendicular to the these planes. Silicon carbide (SiC) has more than 170 known polytypes, although most are rare. All the polytypes of SiC have virtually the same density and Gibbs free energy. The most common SiC polytypes are shown in Table 1. ZnS and CdI2 are also polytypical.[15]

Table 1: Some polytypes of SiC.[16]

Phase Structure Ramsdell Notation Stacking Sequence Comment
α-SiC hexagonal 2H AB Wurtzite form
α-SiC hexagonal 4H ABCB
α-SiC hexagonal 6H ABCACB The most stable and common form
α-SiC rhombohedral 15R ABCACBCABACABCB
β-SiC face-centered cubic 3C ABC Sphalerite or zinc blende form

See also[edit]

References[edit]

  1. ^ F. Wöhler, J. Liebig, Ann. Pharm. 1832, 3, 249 – 282. doi:10.1002/jlac.18320030302
  2. ^ Polymorphism in Benzamide: Solving a 175-Year-Old Riddle Juergen Thun, Lena Seyfarth, Juergen Senker, Robert E. Dinnebier, and Josef Breu Angew. Chem. Int. Ed. 2007, 46, 6729 –6731doi:10.1002/anie.200701383
  3. ^ Graeme M. Day, Andrew V. Trask, W. D. Samuel Motherwell and William Jones (2006). "Investigating the latent polymorphism of maleic acid". Chemical Communications 1 (1): 54–56. doi:10.1039/b513442k. PMID 16353090. 
  4. ^ Thallapally PK, Jetti RKR, Katz AK (2004). "Polymorphism of 1,3,5-trinitrobenzene induced by a trisindane additive". Angewandte Chemie International Edition 43 (9): 1149–1155. doi:10.1002/anie.200352253. PMID 14983460. 
  5. ^ Ostwald, W. (1897). "Studien über die Bildung und Umwandlung fester Körper. 1. Abhandlung: Übersättigung und Überkaltung". Zeitschrift für Physikalische Chemie 22: 289–330. 
  6. ^ Threlfall, T. (2003). "Structural and thermodynamic explanations of Ostwald's Rule". Organic Process Research and Development 7 (6): 1017–1027. doi:10.1021/op030026l. ISSN 1083-6160. 
  7. ^ "Polymorphism in nanocrystalline binary metal oxides", S. Sood, P.Gouma, Nanomaterials and Energy, 2(NME2), 1-15(2013).
  8. ^ Thomas, Sajesh P.; Nagarajan, K.; Row, T. N. Guru (2012). "Polymorphism and tautomeric preference in fenobam and the utility of NLO response to detect polymorphic impurities". Chemical Communications 48: 10559-10561. doi:10.1039/C2CC34912D. 
  9. ^ Bauer J, et al. (2004). "Ritonavir: An Extraordinary Example of Conformational Polymorphism". Pharmaceutical Research 18 (6): 859–866. doi:10.1023/A:1011052932607. PMID 11474792. 
  10. ^ Polymorphisms and Patent, Market, and Legal Battles: Cefdinir Case Study Walter Cabri, Paolo Ghetti, Giovanni Pozzi, and Marco Alpegiani Org. Process Res. Dev.; 2007; 11(1) pp 64 - 72; (Review) doi:10.1021/op0601060
  11. ^ Peddy Vishweshwar, Jennifer A. McMahon, Mark Oliveira, Matthew L. Peterson, and Michael J. Zaworotko (2005). "The Predictably Elusive Form II of Aspirin". J. Am. Chem. Soc. 127 (48): 16802–16803. doi:10.1021/ja056455b. PMID 16316223. 
  12. ^ Andrew D. Bond, Roland Boese, Gautam R. Desiraju (2007). "On the Polymorphism of Aspirin: Crystalline Aspirin as Intergrowths of Two "Polymorphic" Domains". Angewandte Chemie International Edition 46 (4): 618–622. doi:10.1002/anie.200603373. PMID 17139692. 
  13. ^ W. C. McCrone, in Physics and Chemistry of the Organic Solid State, eds. D. Fox, M. M. Labes and A. Weissberger, Interscience Publishers, London, 1965, vol. 2, pp. 725-767.
  14. ^ "Disappearing Polymorphs and Gastrointestinal Infringement"
  15. ^ C.E. Ryan, R.C. Marshall, J.J. Hawley, I. Berman & D.P. Considine, “The Conversion of Cubic to Hexagonal Silicon Carbide as a Function of Temperature and Pressure,” U.S. Air Force, Physical Sciences Research Papers, #336, Aug 1967, p 1-26.
  16. ^ "The basics of crystallography and diffraction", Christopher Hammond, Second edition, Oxford science publishers, IUCr, page 28 ISBN 0 19 8505531.

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