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Polymorphism is the phenomenon where a compound can precipitate to form numerous crystal structures. Due to space and time limitations, we have decided to confine the scope of this paper to only include polymorphisms of crystalline solids and not include polymorphisms associated with DNA and genetic related topics. It is important in many areas of technology, that people are aware of the presence of polymorphisms and the properties of the different polymorphisms. The different crystalline structures each have different physical properties, which can change the use of the chemical. The physical properties that may differ from one polymorphism to another include: solubility, density, melting point and even color. One of the variables that affect the crystallization process is the solvent that is used in the precipitation. The solvent may cause less stable polymorphisms to form instead of those that are more stable. Predictions can be made as to which polymorphism will be formed based on the solvent and its properties. The mixing conditions also have an affect on the formation of various polymorphisms. (Meyerson) Different polymorphisms can also be formed by manipulating the solute concentrations, flows rates, and equipment configurations. Research involving polymorphisms is becoming increasingly important to the pharmaceutical industry due to the number of pharmaceuticals that are prone to polymorphisms, patents on certain polymorphisms, the differing bioavailability of the polymorphisms and the differing effects of the polymorphisms on the body. (Meyerson)
During the 1970’s, G.M.J. Schmidt coined the term crystal engineering and defined it as predicting the crystal structure of solid-state organic molecules (Sharma). In 1989, G.R. Desiraju revised the meaning as the “ ‘the understanding of intermolecular interactions in the context of crystal packing and in the legalization of such understanding and the design of new solids with desired physical and chemical properties’ ” (Sharma). Crystal engineering has grown to encompass the study of the hardness and color of solids, nanotechnology, protein receptor binding, pharmaceutical development, and polymorphisms (Sharma).
Crystals are used in many areas of science, pharmaceuticals, and materials engineering. Crystals differ from many other organic and inorganic materials because of their ability to form polymorphisms.
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The different lattice structures as well as the Bravais lattices are used to determine the internal structure of a crystal. A polymorphism is simply a crystal of the same chemical composition, but with a different structure. An x-ray method called powder diffraction commonly is used to determine the structure (Meyrson).
When a crystal transforms from one of its polymorphs to another, both the chemical and physical properties of the substance are altered. For example, solubility, hardness, shape, melting point, dissolution rate, density, optical and electrical properties, and electromagnetic spectra are some properties that are affected by a change in the crystal structure (The Internet Journal). One example of a compound that forms polymorphisms is dihydroxybenzenoic acid (DHB) which is shown in Figure 2 (“Crystal...”).
Monotropism is one of the two types of polymorphisms. When the solubility of one of the polymorphisms is always higher than the other polymorphism at temperatures below the melting point, it is considered monotropism and the polymorphism with the lower solubility is the more stable of the two (Solubility Studies). If the solubility curves intersect at a temperature below the melting point, the polymorphisms are said to display enantiotropism. The identification of the more stable of the two polymorphs depends on the relation between the transition temperature and room temperature (Solubility Studies).
An allotrope is the purest form of a polymorphism because they only contain one element (University of Virginia). Various elements such as carbon, sulfur, tin, and phosphorus exhibit allotropism (Infoplease). Carbon forms many allotropes, including diamond, graphite, buckminster fullerene, and carbon nanotube, which are pictured in Figure 3. (University of Virginia)
Carbon can form many different hollow fullerenes, two of which include the buckminster fullerene (buckyball) and carbon nanotube in Figure 3. (University of Virginia) Elemental sulfur can form numerous unit cell structures including orthorhombic, monoclinic and triclinic. (Logan)
The different structures of carbon are unique in appearance, properties and uses. Graphite forms hexagonal platelet structure containing Pi electrons within the rings. These Pi electrons give graphite good conductive properties because the electrons are free to move around the layers easily when a current is applied. The air between the layers in graphite give it soft, smooth, lubricating properties. The layers also explain the dark color of graphite because light reflects off one layer and then gets trapped within the other layers, which prohibits it from leaving the crystal structure, giving graphite a dark color. Graphite also has both a low melting and low boiling point. (Logan) Another allotrope of carbon is diamond. In a diamond the carbon atoms are in a tetrahedral unit cell structure as pictured in Figure 1. Diamond is one of the hardest materials known to man and possesses a high level of durability. It also has a high index of refraction, accounting for its valued luster. It has a high boiling and high melting point, providing a complete contrast to graphite. The buckminster fullerene is the newest discovered allotrope forming a dodecahedron unit cell structure as pictured in Figure 3. The buckminster fullerene has potential to be used as a superconductor. (Logan)
Polymorphisms can be classified in two categories: reversible (monotropism) and irreversible (enantiotropism). (Polymorphism2) Transformation occurs due to a change in conditions, most commonly a change in temperature. For example, when diamond is heated, it transforms irreversibly to graphite and at temperatures below 18oC, tin transforms reversibly into a nonmetallic state. (Polymorphism (crystallography)) The material transforms because it reaches a temperature where the current crystalline form is no longer the more stable polymorph and it transforms into the more stable of the forms at the current temperature. (Meyrson)
Transformations can result from bending of the lattice network or rotation of part of the molecule around itself. Displacive transformation of secondary coordination is a transformation in which the change is caused by a bending of the crystal lattice. A rotational disorder transformation is a transformation the results from the rotation of part of the molecule around the structure of the molecule. Transformations can also be classified as first and second order. In first order polymorphisms, there are high levels of changes between the two states; in second order polymorphs, the difference between the two states is relatively small. (Meyrson, 75-77) Another type of transformation is a martensitic transformation occurring in ceramics and metals and involving only a few atoms shift positions and no major changes occurring in the structure. Martensitic transformations are used to control the structure and properties of steel to make it more useful for its current application.
One particular field of study that deals heavily with manipulating crystals to obtain the most highly desired polymorph is crystal engineering. There are many advantages in industry to predicting and forming crystals with desired properties such as solubility, melting point, hardness, luster, and reactive potential. (Sharma) The less stable polymorph is commonly produced through supersaturation and rapid cooling where a solution is heated and then rapidly cooled and the crystals are immediately removed. The crystals that are removed are the less stable polymorph and have a minimal chance that they will transform into the more stable polymorph unless they are heated, but if they are left in the solution for an extended period of time they will likely transform back into the more stable polymorph.
The most common technique that is used to control the formation of polymorphisms is regulating the temperature because the stability of the polymorph changes with the temperature. Scientists can set the temperature to a level where the desired polymorph is the most stable of the polymorphs and the production of that polymorph will be greatly increased. (Meyrson) One example of regulation of temperature is the use zirconium dioxide. Zirconium dioxide contracts when heated and expands when cooled. Zirconium dioxide is commonly used in transformation toughening where it is used to fill cracks when it is hot and then as it cools it expands and fills the cracks to prevent propagation which can lead to serious cracking. (Meyrson)
One technique that is used to isolate a particular polymorphism is ledge-directed epitaxy (LDE). LDE uses substrates with ledge dihedral angles that are the same as those of the desired polymorph to obtain a crystal of the desired structure. (McCormick) Powder processing is also used to change the polymorphic state using grinding or milling. (The Internet Journal)
Polymorphisms play an integral role in many areas of industry and science. They are particularly important in ceramic and metal engineering, food technology and pharmaceuticals. (Polymorphism Project)
Manipulations of the polymorphisms in ceramics and metals, allows for a company to control the strength and hardness of the materials that they use for their applications. Steel can be transformed reversibly to provide for the many different types of steel that are used in industry and in households. Each form of steel has unique properties needed for different applications. (Polymorphism (crystallography)) Zirconium dioxide is used to toughen materials to prevent fracturing due to temperature change. Tin also transforms into a brittle, non-metallic solid at temperatures below 18oC and expands by 21% of its original volume. This transformation is reversible and widely used in industry. (Polymorphism (crystallography))
Polymorphisms also impact food technology, particularly fats and oils. The melting point and texture of fats is greatly affected by the crystalline structure and determines what they are best used for. (Polymorphism of Fats)
Fats typically crystallize in one of three forms, a, b and b’. The most stable of the three structures and the one that is most easily formed is the b-form. The b-form is undesirable for use in cooking because it forms fairly large crystals (5 - 25mm) resulting in graininess. The b’-form is the best for use in cooking and eating because it has the lowest melting point and has the smallest crystals (least than 1 mm). It has been determined that adding 10-15% palm oil or 20-25% cottonseed oil before hydration promotes the formation of the b’ crystals and minimizes b crystal formation in the production of canola oil. The palmitic acid in the cottonseed and palm oil is the reason for the increased stability of the b’ crystal because palmitic acid naturally crystallizes in the b’-form. Many other factors are manipulated to affect the formation of the b’-form verses the b-form. (Przybylski, Logan) Another oil that displays polymorphisms is Theobroma oil (cocoa butter). Because the stable polymorphism melts at 35oC and does not crystallize first in the cooling process, Theobroma oil must be prepared below 35oC. (University of Virginia)
Polymorphisms are very important to the pharmaceutical industry because they change the effectiveness of the drug and are often the subject of lawsuits. Ranitidine-HCl is a used for treatment of peptic ulcers and has two known polymorphic forms. Ritonavir (Novir) is a HIV protease inhibitor that has polymorphisms that have very different solubilities. In 1998, Abbott had to discontinue production of the drug until they corrected the polymorphism that they were using because it dissolved much slower than the polymorph they has used in earlier testing of the drug (Knapman).
Recently, polymorphisms have been a very important aspect in the lawsuit by Glaxo Wellcome against Novopharm about alleged patient infringement. Glaxo produces Zantac, an anti-ulcer drug, and holds the patient for one polymorphism of the drug and held the patient on the other polymorphism until it expired in 1997. (Knapman) After the patient expired, Novopharm wanted to begin producing generic Zantac using the first polymorphic form. Glaxo sued claiming that it infringed on their patient on the second crystalline form. Novopharm won the lawsuit and now produces the generic form of Zantac. (Knapman)
Chloramphenicol-3-palmitate (CAPP) is an antibiotic with three known polymorphisms. Form A is marketable while form B is stronger and provides a risk of fatal dosages and can appear due to improper storage conditions and process management. The FDA protects against this by not allowing companies to market polymorphisms without proof that it will do no harm to people.
Cortisone acetate has four polymorphic forms and only one of which is stable in water, but the other forms transform into the stable form after a short period of time. (University of Virginia) Tamoxifen citrate is used in the treatment of breast cancer and has two know polymorphisms. (Polymorphism2) Chloramphenicol has one polymorphism that is more soluble than the other and it affects the bioavailibility of the compound. (University of Virginia)
There are many ways to determine the polymorphism of a crystal structure.
X-Ray powder diffraction is the most common technique used. X-Ray powder diffraction (XRD) utilizes the way the different planes of a crystalline structure filter the beam and how the diffraction angle depends solely on the crystalline spacings. (The Internet Journal)
Scanning electron microscopy is used to compare the external crystalline structure of the polymorphisms. Other procedures that are used include Fourier transform-infrared spectroscopy (FTIR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and Raman spectroscopy. (The Internet Journal)
Polymorphisms are an integral part of technology from superconductors to steel to treatments for breast cancer. They have a great affect on the ways chemicals operate and what they can be used for. Knowledge concerning the characteristics and manipulation of these polymorphisms is of the utmost importance in industry because mistakes can be fatal or not produce a desired affect. Unfortunately, the field of polymorphisms is still developing and there are still many things that are not known. Scientists are still not able to consistently predict the presence and structure of possible polymorphisms. Scientists are also unable to determine the number of polymorphs of a particular substance. Since the study of polymorphisms affects so many aspects of industry, it is a rapidly growing field due to the lack of knowledge and the need for the knowledge from companies in all kinds of fields.
In addition to researching polymorphisms, we grew our own crystal and a polymorphism of that crystal. First, two beakers, each with 50.0 ml of water and 17.4 g glycine were prepared. Both beakers were heated to 60°C in order to dissolve all of the solid. Once the solutions were supersaturated (they dissolved more solute then they normally could at room temperature), 3.05 ml of acetic acid was added to one of the two beakers. The beaker with the acetic acid will crystallize into gamma-glycine while the other solution will form alpha-glycine. The solutions were allowed to cool slowly to room temperature as the crystals formed. The crystals were then ground into a fine powder to prepare for x-ray powder diffraction. A small amount of Vaseline was put onto a microscope slide and the powder was put onto the Vaseline so it would stick. The slide was then placed into the powder diffraction machine and x-rays were directed at the slide. When testing was over, a graph of the results was printed out. Brag’s Law is used to determine the crystal structure using the equation , where d is the interplanar distance, is the angle of the x-ray, and is the wavelength of the x-ray and is the variable being solved for. The plane in which the crystal structure exists appeared as the peak of the graph. For alpha-glycine, the peak occurred at the 110 plane where as the gamma-glycine peaked at the 130 plane. The number of the plane is a code for the structure of the crystal, for example an octahedral, square planar, or bipyramidal geometry. The alpha-glycine had a bipyramidal structure and the gamma-glycine had an octahedral structure. The structures were found using Molecule Simulation Inc. Cerius 2, a computer simulation program.
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