Physical Properties of Crystalline and Amorphous Solids

Crystallinity, like most things, can vary in degree. Even single crystals typically have intrinsic point defects (e.g. lattice site vacancies) and extrinsic point defects (e.g. impurities), as well as extended defects such as dislocations. Defects are critical to the physical properties of crystals and will be extensively covered in later chapters. What we are referring to here with the degree of crystallinity is not the simple presence of defects, but rather the spectmm of crystallinity that encompasses the entire range from crystalline to fully disordered amorphous solids. Table 1.1 lists the various classes. Let s take each of them in the order shown. 

The physical properties of the solid state seen in crystals and powders of both drugs and pharmaceutical excipients are of interest because they can affect both the production of dosage forms and the performance of the finished product. Powders, as Pilpel reminded us, can float like a gas or flow like a liquid but when compressed can support a weight. Fine powders dispersed as suspensions in liquids are used in injections and aerosol formulations. Both liquid and dry powder aerosols are available and are discussed in Chapter 9 some properties of compacted solids are dealt with in Chapter 6. In this chapter we deal with the form and particle size of crystalline and amorphous drugs and the effect these characteristics have on drug behaviour, especially on drug dissolution and bioavailability. 

Measurement of the elemental composition of materials is a relatively mature art. In the natural world there are 92 elements with methods for their quantitative determination generally well established and, in many cases, the subject of internationally accepted standards. However, the physical properties of minerals and materials formed by these elements, and the manner in which they react, is not solely dependant on their chemical composition but on how the constituent elements are arranged that is, their structural form. This finite number of known elements combine into some 230 crystallographic forms with almost infinite variability induced by solid solution, degree of crystallinity, morphology and so on. Therefore, the measurement of the form and amount of the various crystalline and amorphous components is considerably more complex than the measurement of the constituent chemistry. 

The average length of the polymer chains and the breadth of distribution of the polymer chain lengths determine the main properties of PP. In the solid state, the main properties of the PP reflect the type and amoimt of crystalline and amorphous regions formed from the polymer chains. Polypropylene has excellent and desirable physical, mechanical and thermal properties when used in room temperature applications. It is relatively stiff and has a high melting point, low density and relatively good resistance to impact. 

Compare amorphous, crystalline, and polycrystaUine sohds in terms of structure. How do crystalline and amorphous solids differ in physical properties Explain the difference. 

The presence of a solvent, especially water, and/or other additives or impurities, often in nonstoichiometric proportions, may modify the physical properties of a solid, often through impurity defects, through changes in crystal habit (shape) or by lowering the glass transition temperature of an amorphous solid. The effects of water on the solid-state stability of proteins and peptides and the removal of water by lyophilization to produce materials of certain crystallinity are of great practical importance although still imperfectly understood. 

The forward search starts from the name of a chemical compound, proceeds to finding its molecular structure, and then its physical and chemical properties, such as the boiling point, melting point, density, etcetera, in a handbook. Many databases for single compounds are also organized by classes and families of similar structures. Fluid solutions represent the next level of complexity. For the most important fluids, such as water, air, and some refrigerants, we can find extensive tables for the thermal properties of mixtures. For complex fluids, such as paint and emulsion, which are difficult to characterize and to reproduce, specialized books and journals should be consulted. The properties of some crystalline solids can be found, but usually not for multicrystal composite and amorphous solids. 

Both the dimer and the photohydrate from photolysis of uracil have been isolated not only as spots on a chromatogram7 but also as crystalline or amorphous solids—the dimer by Smietanowska and Shugar45 and Swenson and Setlow48 and the hydrate by Gattner and Fahr.45 Many physical properties have not been recorded for these materials. The melting points of the principal dimer is 380°.34 The infrared spectrum is not reported. The elementary composition of the hydrate has been reported.45 ... 

Occurrence, Extraction, Refining, Applications—The Allotropy of Sulphur Changes in tho Vaporous State, Allotropy in tho Liquid. State, Allotropy in the Solid Stato, Crystalline Forms of Sulphur, Amorphous Sulphur— Colloidal Sulphur—General Physical Properties of Sulphur—Chemical Propertius—Valency—Atomic Weight—Detection and Estimation. 

The temperature at which the liquid phase and the solid phase of a given substance can coexist is a characteristic physical property of many solids. The melting point of a crystalline solid is the temperature at which the forces holding its crystal lattice together are broken and it becomes a liquid. It is difficult to specify an exact melting point for an amorphous solid because these solids tend to act like liquids when they are still in the solid state. 

The physical properties of polymers can be affected by structural irregularities present in the polymers. These structural irregularities include end groups and side branches which affect melting points, thermal stability and so on. The existence of such irregularities can be observed efficiently by solution-state NMR, because the line widths of solution peaks are typically 10-100 times narrower than those of solid-state NMR. However, by solid-state NMR, we can seek how the side groups and end groups are partitioned between the crystalline and amorphous phases, which directly relate to the physical properties of the polymers. 

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