Solid-state properties amorphous forms

As shown in Fig. and also in the work of Barraclough and Hall, moisture uptake onto sodium chloride as a function of relative humidity is reversible as long as RHq is not attained. This is evidence that actual dissolution of water-soluble crystalline substances does not occur below RHq. This is consistent with the thermodynamic rationale that dissolution below RHq would require a supersaturated solution (i.e., an increased number of species in solution would be necessary to induce dissolution at a relative humidity below that of the saturated solution, RHq). In this regard, one should only need to consider the solid state properties of a purely crystalline material below RHo. As will be described, other considerations may be warranted for a substance that exists in multiple polymorphic forms or contains amorphous material. 

Saleki-Gerhardt A. Role of water in the solid state properties of crystalline and amorphous form of sugars. Doctor of Philosophy Thesis, University of Wisconsin-Madison 1993 104—108. 

The solubility is most often experimentally determined from the dmg concentration in the liquid phase after adding excessive amounts of a solid dmg substance to the test medium. This apparent solubility is affected by the solid-state properties of the dmg, for example, polymorphs, solvates, impurities, and amorphous content. An equilibrium with the thermodynamically most stable solid-state form, being the least... 

Solid state property differences derived from the existence of alternate crystal forms can lead to extensive differences of pharmaceutical importance, e.g., solubility, dissolution rate, and stability. It is claimed that most drug substances show polymorphism (Borka, 1991). As discussed previously, it is essential to determine which of the various forms should be used in a drug product to assure stable and reproducible formulation. A marked difference between the photostability of various crystal modifications of drug substances has been reported. This can be ascribed to differences in inter- and intramolecular binding, differences in diffusability (crystalline vs. amorphous structure), and differences in water content (crystal water, adsorbed water) (Hiittenrauch et al., 1986). 

Polymers, unlike metals, find wide application as completely amorphous solids. The two amorphous forms of greatest interest are the glasses and the elastomers or rubbers. Polymers found their earliest technical application as elastomers over 100 years ago, and it is in this form that they make a unique contribution to society. In examining the structure of the amorphous state we shall confine attention to the really significant point, which is the conformation of the molecule. It will be shown that the conformation resembles that of the molecule in dilute solution sketched at the end of the previous chapter. By means of a simple model a mathematical description of the conformation will be obtained which has fundamental and practical consequences for the mechanical properties of elastomers, the stress-strain curve in particular. 

Spray-drying processes offer several advantages when solid-state properties of drug substances need to be modified. Using this process, solubility and dissolution rates of properties of poorly soluble materials can be increased several fold and the stability of the amorphous form of the materials can be improved significantly. 

Studies of the desolvation of solvatomorphs can be conducted using VT-XRPD. For instance, after the dehydration of a hydrate phase, one may obtain either a crystalline anhydrate phase or an amorphous phase and the XRPD pattern will clearly indicate the difference. In addition, should one encounter an equivalence in powder patterns between the hydrate phase and its dehydrated form, this would indicate the existence of channel-type water (as opposed to lattice boimd water). In one study, the solid-state properties of the isomorphic desolvates of cephalexin, cefaclor, erythromycin A, and spirapril hydrochloride were investigated, with the hygroscopicity of the compounds being evaluated using a vacuum moisture balance and the structural relaxation measured using a combination of VT-XRPD and isothermal microcalorimetry. " ... 

The solubility of the drug is affected by several physiological and physicochemical factors [26], Drug properties are defined not only by the chemical structure but also by the solid material, and a drug can potentially exist in many different solid state forms which may differ in solubility. Amorphous materials tend to show much higher aqueous solubility than crystalline forms of the same compound and different crystal modifications of the same compound may also have different solubility (e.g., [25]). 

Sulfur nitride polymers [-(-S = N-)-], which have optical and electrical properties similar to those of metals, were first synthesized in 1910. These crystalline polymers, which are super-conducive at 0.25 K, may be produced at room temperature using the solid state polymerization of the dimer (S2N2). A dark blue-black amorphous paramagnetic form of poly(sulfur nitride) (structure 11.30) is produced by quenching the gaseous tetramer in liquid nitrogen. The polymer is produced on heating the tetramer to about 300°C. 

Allotropic forms of carbon. In the solid state, the element carbon exists in three different allotropic modifications—amorphous carbon and the two crystalline forms known as diamond and graphite. Amorphous carbon includes numerous common products such as wood charcoal, bone black, coke, lamp black, and carbon black. Each of these varieties of crystalline and amorphous carbon possesses properties that render it useful for a variety of purposes. 

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