Metastable crystalline form

Numerous disperse dyes are marketed in a metastable crystalline form that gives significantly higher uptake than the corresponding more stable modification. The molar free enthalpy difference can be used as a criterion of the relative thermodynamic stabilities of two different modifications [53]. Certain dyes can be isolated in several different morphological forms. For example, an azopyrazole yellow disperse dye (3.52) was prepared in five different crystal forms and applied to cellulose acetate fibres. Each form exhibited a different saturation limit, the less stable modifications giving the higher values [54]. 

This law is particularly useful as it allows the total entropy of a substance to be obtained if sufficiently low-temperature enthalpy or heat-capacity measurements are available. However, questions remain as to its validity when considering metastable crystalline forms and the law, as stated, would not apply to defect-stabilised structures and amorphous phases. 

Cocoa butter NF is defined as the fat obtained from the seed of Theobroma Cacao Linne (Family Sterculiaceae) (44). Cocoa butter softens at 30°C and melts at 34°C. It contains four different forms alpha, beta, beta prime, and gamma with melting points of 22°C, 34°C to 35°C, 28°C and 18°C, respectively. The beta form is the most stable and is desired for suppositories. The biggest challenge with the polymorphism of cocoa butter is the impact of the manufacturing process on the characteristics of the suppository itself. When cocoa butter is hastily melted at a temperature greatly exceeding the minimum required temperature and then quickly chilled, the result is metastable crystalline form (a crystals), which may not even... 

At atmospheric pressure water normally crystallizes as ice-i, which has a hexagonal structure like that of tridymite. It may also be crystallized directly from the vapour, preferably in vacuo, as the cubic form ice-i with a cristobalite-like structure provided the temperature is carefully controlled (—120° to —140°C). (This cubic form is more conveniently prepared in quantity by warming the high-pressure forms from liquid nitrogen temperature.) Ice-i,. is metastable relative to ordinary ice-ij, at temperatures above 153°K. The existence of a second metastable crystalline form, ice-iv, has been firmly established for DjO but less certainly for HjO. A vitreous form of ice is formed by condensing the vapour at temperatures of -160°C or below. 

Akao et al. investigated the dehydration of trehalose dihydrate to yield form II under supercritical fluid conditions (21). Trehalose form II is a metastable crystalline form of trehalose anhydrate and can be readily converted into the dihydrate by exposure to a moist environment at room temperature (22). Trehalose form III is another anhydrous polymorph. The phase transition behavior, detected by Fourier transform infrared (FTIR) spectroscopy and confirmed by first-derivative euclidean distance analysis (FDE), was found to be dependent on the extraction time, temperature, and pressure of SCCO2. At 20 MPa, an increase in temperature from 70°C to 90°C augmented the dehydration rate of trehalose dihydrate. Thus it appears that a temperature higher than 70 °C at 20 MPa is required for dehydration of trehalose dihydrate. The polymorphic forms obtained at different temperatures and pressures are summarized in Table 1. 

A clathrate is thus an inclusion compound which is a solid solution of the guest atoms or molecules in a metastable crystalline form characterized by isolated large spherical voids of the host species. These structural criteria differentiate clathrates from the other intercalation compounds subsequently described (see 16.3 and 16.4), in which the available voids exhibit a uni- or two-dimensional arrangement (tunnel and sheet structures). 

Elemental sulfur occurs in several allo-tropic forms. All allotropic forms of liquid and solid sulfur consist of polyatomic molecules, simple rings or unbranched helical chains. Below 160 °C, depending on pressure, the stable molecule is the eight-mem-bered ring that constitutes the lambda component of the liquid, the alpha and beta are crystalline forms stable below 1.5 kilobars, five crystalline forms are possibly stable at higher pressures, and there also exist many metastable crystalline forms. Sulfur vapor dissociates at higher temperatures from Sg and S4 to S2. Above about 2000 °C, the gas consists only of sulfur atoms (Hampel and Hawley 1973, Nehb and Vydra 2002)... 

In the pressure range 0-22 kbar there occur, as we have seen, five stable and three metastable crystalline forms of ice, together with a vitreous modification. By contrast, from this pressure up to 200 kbar, the highest yet investigated, only a single form. Ice VII, freezes from the liquid (Pistorius et al. 1963) and only one additional allotrope. Ice VIII, has been identified (Whalley et al. 

Figure 6.3 The Gibbs function diagram for a monotropic liquid crystal. represents the equilibrium crystal, and Gy a possible metastable crystalline form. The equilibrium pathway on heating (marked by arrows) leads to direct isotropization at Ty. Heating along pathway Gy gives a metastable enantiotropic behavior, with a mesophase at Ty and isotropization at T,ci. (Redrawn from data in [4])...

Gilbert, R.A. and Garrett, A.B. (1956) The equilibria of the metastable crystalline form of beryllium hydroxide. Be(OH)2 in hydrochloric acid, perchloric acid and sodium hydroxide solutions at 25 °C. J. 

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