Pseudopolymorphs melting point

Olesen and Szabo obtained crystals from ethanol and acetone53. They found the crystals to have different solubility, melting point and x-ray diffraction patterns. Since acetone is retained in the crystalline lattice, it was indicated that the forms are pseudopolymorphs. 

If the chemical contents of a polymorph are different than other forms, it is designated as a pseudopolymorph.Most often this occurs due to the presence of differing amounts of solvent, and may alter physical properties of the crystals such as melting points and solubilities. Polymorphism and pseudopolymorphism may be observed when different experimental conditions are used for synthesis. For example, if crystals are grown by sublimation, changing the temperature will often yield different crystal structures, possibly even metastable phases that are kinetically favored. 

Each polymorph contains the same chemical contents of the respective unit cells. If the chemical contents differ, for example by the presence of different amounts of solvent, they are called pseudopolymorphs. Polymorphs may differ with respect to physical properties such as melting points, or solubilities, as also may pseudopolymorphs. Their existence often presents a serious problem in the pharmaceutical industries since physical properties of crystals are often used as criteria for quality control and thereby the effectivity of a given preparation. Polymorphs and pseudopolymorphs are usually obtained when crystals are grown under different conditions. For example, metastable crystals of the 7T-donor acceptor complex between biphenylene and pyromellitic dianhydride were obtained when crystals were grown by sublimation at high temperatures, whereas a different polymorph, stable at room temperature, was grown by the same method at a lower temperature. ... 

Solid Form Selection A drug can exist in multiple forms in the solid state. If the two forms have the same molecular structure but different crystal packing, then they are polymorphs. Pseudopolymorphs (or solvatomorphs) differ in the level of hydration/solvation between forms. Polymorphs and pseudopolymorphs in principle will have a different solubility, melting point, dissolution rate, etc. While less thermodynamically stable, polymorphs have higher solubilities they also have the potential to convert to the more thermodynamically stable form. This form conversion can lead to reduced solubility for the formulated product. One example is ritonavir, a protease inhibitor compound used to treat acquired immune deficiency syndrome (AIDS). Marketed by Abbott Labs as Norvir, this compound began production in a semisolid form and an oral liquid form. In July 1998, dissolution tests of several new batches of the product failed. The problem was traced to the appearance of a previously unknown polymorph (Form II) of the compound. This form is thermodynamically more stable than Form I and therefore is less soluble. In this case, the solubility is at least a factor of 2 below that of Form I.12 The discovery of this new polymorph ultimately led to a temporary withdrawal of the solid form of Norvir from the market and a search for a new formulation. 

When a material can crystallize into a different polymorph, the chemical nature of the species remains identical, however, the physical properties of the material can be different. For example, properties such as density, heat capacity, melting point, thermal conductivity, and optical activity can vary from one polymorph to another. Table 2.3 lists common materials that exhibit polymorphism. Looking at Table 2.3 we can see that density varies significantly for the same materials when the crystal structure has changed. In addition, the change in the crystal structure often means a change in the external shape of the crystal, which is often an important parameter in industrial crystallization that has to be controlled. Many substances crystallize into structure in which the solvent is present as part of the crystal lattice. These crystals are known as solvates (or hydrates when the solvent is water). A substance can have multiple solvates with different crystal structures as well as a solvent free crystal form with a unique crystal structure. The solvates are often referred to as pseudopolymorphs. They are not true polymorphs because of the addition of the solvent molecule(s) to the crystal lattice. Conformational polymorphism refers to the situation where the molecular conformation of the molecules of a given substance are different in each polymorph. 

The most common crystalline forms are polymorphs, hydrates, and solvates (pseudopolymorphs). Polymorphs are formed when a substance crystallizes in two or more crystal structures. Polymorphism significantly impacts on physicochemical properties of materials, such as stability, density, melting point, solubility, bioavailability, and so on. Hence the characterization of all possible polymorphs, identifying the stable (thermodynamic) polymorph, and design of reliable processes for consistent production are critical in modem day drug development. 

---