Crystalline solid phase

In some cases the constants hold good up to a certain point only, when the curve suddenly changes its direction. This is attributed to the appearance of a new crystalline solid phase, and three varieties of ice have been so discovered. 

Six crystalline solid phases of fluprednisolone and an amorphous phase were characterized using XPD, infrared (IR) spectroscopy, and differential scanning calorimetry [9]. Three of these six crystalline phases were anhydrous (forms I,... 

The presence of a crystalline solid phase in a supersaturated solution often causes new nuclei to form at appreciably lower levels of supersaturation than is required for primary nucleation events. Several mechanisms are responsible for this [6] ... 

The packing arrangement of atoms or molecules in a crystalline solid phase is generally not unique, and for organic molecules in particular, it is common for two or more crystalline forms of the same substance to exist. The most familiar example in elemental terms is Graphite and Diamond. Both are composed entirely of the element Carbon, however their ciystal structures are very different, and so too are their physical properties. Calcium Carbonate is another common example with three polymorphic forms Calcite, Aragonite and Vaterite. 

In this chapter, general aspects and structural properties of crystalline solid phases are described, and a short introduction is given to modulated and quasicrystal structures (quasi-periodic crystals). Elements of structure systematics with the description of a number of structure types are presented in the subsequent Chapter 7. Finally, both in this chapter and in Chapter 6, dedicated to preparation techniques, characteristic features of typical metastable phases are considered with attention to amorphous and glassy alloys. 

Materials mined from the Earth s crust are usually highly heterogeneous mixtures of amorphous and crystalline solid phases. Crushing and grinding operations are employed to liberate individual pure grains in the 10-50 pm size range. One of the most widely used (10 tons per... 

Ultraphosphates are defined as phosphates in which the ratio Me1 is less than unity. The existence of such compounds, e.g. Ag2P60,o, PbP6Oio and Ca2PeOi7, was detected in the corresponding melts some time ago (165) and the compounds CaP4Ou and CasPeOn were described as crystalline solid phases (136). In such compounds P04 tetrahedra must be present which are linked by three of their O atoms with neighboring P atoms (349). 

As indicated in my report, we now know the rates of lateral diffusion of phospholipids in lipid bilayers in the fluid state, and in a few cases the rates of lateral diffusion of proteins in fluid lipids are also known. At the present time nothing is known about the rates of lateral diffusion of phospholipids in the crystalline, solid phases of the substances. As mentioned in my report, there are reasons to suspect that the rates of lateral diffusion of phospholipids in the solid solution crystalline phases of binary mixtures of phospholipids may be appreciable on the experimental time scale. Professor Ubbelohde may well be correct in pointing out the possibility of diffusion caused by defects. However, such defects, if present, apparently do not lead to significant loss of the membrane permeability barrier, except at domain boundaries. 

Similar effects arise from varying particle size. The transformation of a macroscopic crystalline solid phase with negligible specific surface area to a finely divided powder whose particles have a significant specific surface area produces an additional term, proportional to the specific surface area, in the expression for ArG° describing the reaction in Eq. 3.1.15 This term increases the value of the Standard-State chemical potential of the solid phase but does not alter Kdis, such that the equilibrium IAP increases also. The effect on Eq. 3.3 is to increase Kso according to the following equation 15... 

The inspection of the X-ray diffraction pattern of the crystalline solid phase allowed the conclusion that the 21.8 A spacing (the presence of which can be taken as proof of the smectic phase) was not due to residual solid material because of the absence of stronger reflections. 

The use of a cooling accessory permits XRD patterns to be obtained under subambient conditions. In pharmaceutical systems, the greatest utility of the technique is to monitor the crystallization of solutes in frozen solutions. Conventionally, differential scanning calorimetry has been the most popular technique for the characterization of frozen systems. However, as mentioned earlier, this technique has some drawbacks (i) It does not enable direct identification of crystalline solid phase(s). Moreover, it is difficult to draw any definitive conclusions about the degree of crystallinity, (ii) The interpretation of DSC curves is very difficult if there are overlapping thermal events. Low temperature XRD was found to be an excellent complement to differential thermal analysis in the characterization of water-glycine-sucrose ternary systems. " ... 

Figure 7.2. Solubility and saturation. A schematic solubility diagram showing concentration ranges versus pH for supersaturated, metastable, saturated, and undersaturated solutions. A supersaturated solution in the labile concentration range forms a precipitate spontaneously a metastable solution may form no precipitate over a relatively long period. Often an active form of the precipitate, usually a very fine crystalline solid phase with a disordered lattice, is formed from oversaturated solutions. Such an active precipitate may persist in metastable equilibrium with the solution it is more soluble than the stable solid phase and may slowly convert into the stable phase.

Since solubility is the concentration of protein in solution at equilibrium with the solid phase, the state of protein in the solid phase affects the solubility in the solution phase. Theoretical treatment of the protein solubility problem has often ignored solid phase interactions of the protein due to its complexity. A crystalline solid phase is expected to render a lower solubility than the amorphous solid phase. However the complexity and the heterogeneity of the protein in the solid state (e.g. amorphous, gel, or crystalline, or precipitates of native or denatured forms) makes it difficult to directly assess solid state effects. 

Surfactant molecules by definition have polar groups such as ions or ethylene oxide chains and nonpolar groups such as hydrocarbon or fluorocarbon chains. When they are added to water, aggregation normally occurs at fairly low concentrations to minimize the area of contact between the nonpolar groups and water. For low temperatures and molecules with long, straight hydrocarbon chains, separation into a crystalline solid phase and a dilute aqueous solution of molecularly... 

The synthesis of crystalline microporous solids is a complex reaction-crystallisation process, usually involving a liquid phase and both amorphous and crystalline solid phases. Described below (section 6) are the main elements of this transformation the induction period, nucleation processes and crystal growth. Having established this background, the mechanism underlying these changes is then examined in more detail (section 7). 

X-ray analysis of the palladium— hydrogen systems gives results which are in harmony with the view that hydrogen and palladium form two crystalline solid phases, one of which is practically pure palladium and the other a saturated solution of hydrogen in palladium. ... 

Diethoxy-2,5-dihydropyrimidine (27) has a flat ring in the crystalline solid phase, and the molecule is not involved in enamine-imine tautomerism in solution <86JOC4623>. The 4,6-diphenyl analogue (28), however, is in equilibrium with its 1,2-dihydro tautomer (29) in solution (Equation... 

When an X-ray beam falls on alums iwo processes may occur. The beam may be scaltcrcd or the beam may be absorbed with an ejection of electrons from an atom. In the case of a crystalline material the scattering of X-rays is used to determine the structure of the solid phase and the chemist applies this method to the proof of the structure of new compounds very often. But even when a regular crystalline arrangement does not exist, as in liquids or amorphous solids, scattering patterns are produced. I.ike in the crystalline solid phase the scattering of X-rays on disordered systems can be used to determine the probability of distribution of atoms in the environment of any reference atom, or in other words the frequency with which interatomic distances occur. 

There are also solids that consist neither of small, well-defined molecules nor of well-ordered infinite arrays of atoms examples are the glasses and polymers, which, for reasons of space, will not be explicitly discussed here. It is, of course, true that most molecular substances form a crystalline solid phase but, because of the relatively weak intermolecular interactions crystallinity is usually of little chemical importance, though, of course, of enormous practical significance in that it facilitates the investigation of molecular structures, namely, by X-ray crystallography. 

Figure 3.4 (Left) Phase diagram for purely hard sphere interaction potentiaf which shows only fluid (F) and crystalhne (C) phases. (Middle) A short-range (relative to particle size) interaction is added to the hard sphere potential. Then a dilute gas (G) phase can appear, as well as a metastahle hquid-hquid coexistence (L + L). (Right) The attractive interaction is long range and a complete phase diagram occurs, with gas, liquid, and crystalline solid phases. The triple hne (TL) temperature increases with increasing attractive interaction strength.

Under the influence of such a potential, the spheres easily settle into a crystalline form when the temperature is lowered or the pressure is raised. The solid phase can be either a face-centered cubic (fee) lattice or a body-centered cubic (bee) lattice, depending on the detailed nature of the potential. In the case of argon, the crystal freezes into an fee lattice, while liquid sodium freezes into a bee lattice. The transformation of simple spherical molecules (such as argon and sodium) into the crystalline solid phase is now rather well understood, largely because of the extensive use of computer simulation studies, accompanied by theoretical analysis using methods of statistical mechanics. 

At thermodynamic equilibrium the chemical potential of each component i in both liquid and solid phases has to be equal. For simple systems and certain simplifications, like pure crystalline solid phase of component b (see Walas 1985), thermodynamic considerations lead to the well-known Clausius-Clapeyron equation... 

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