Polymorphic transitions

Parrinello M and Rahman A 1981 Polymorphic transitions In single crystals a new molecular dynamics method J. Appl. Phys. 52 7182-90... 

Fig. 3. The main crystallographic forms of siUca stable at atmospheric pressure. The vertical directions represent the facile, displacive polymorphic transitions, whereas the horizontal directions represent the sluggish reconstmctive transitions (44).

M. Parrinello and A. Rahman, Polymorphic transitions in single crystals A new molecular... 

Polymorphic transition may occur very rapidly (e.contact with the second form, accelerate the process. 

Table 1 Some Drugs That Undergo Polymorphic Transition When Triturated...

A rate enhancement effect due to secondary nucleation has been identified in the solution-mediated transformation of the 7-phase of (i)-glutamic acid to its / -phase [82]. In this study, the kinetics of the polymorphic transition were studied using optical microscopy combined with Fourier transform infrared, Raman, and ultraviolet absorption spectroscopies. The crystallization process of n-hexatriacontane was investigated using micro-IR methodology, where it was confirmed that single... 

Figure 4 Schematic representation of polymorphic transitions in trihalide salts of BEDT-...

Freitas, C. and Muller, R. H., Stability determination of solid lipid nanoparticles (SEN) in aqueous dispersion after addition of electrolyte. J. MicroencapsuL, 16, 59-71, 1999. Bunjes, H., Westesen, K. and Koch, M. H. J., Crystallization tendency and polymorphic transitions in triglyceride nanoparticles. Int. J. Pharm., 129, 159-73, 1996. Freitas, C. and Muller, R. H., Correlation between long-term stability of solid lipid nanoparticles (SLN) and crystallinity of the lipid phase. Eur. J. Pharm. Biopharm., 47, 125-32, 1999. 

As we have already stated, the Gibbs free energy modification connected with a polymorphic transition is zero. The univariant equilibrium along the transition curve is described by the Clapeyron equation ... 

Incidentally, it must be noted that, although most olivine compounds are stable (at P = 1 bar) up to the melting temperature, in some compounds polymorphic transitions are observed at temperatures below the melting point (for instance, Ca2Si04 has a transition to a monoclinic polymorph at 1120 K). Forsterite at 1 bar melts at 7) = 2163 K, whereas fayalite melts at a much lower temperature Tf = 1490 K). The significance of this difference, as shown by Hazen (1977), must be ascribed to different polyhedral thermal expansions for polyhedra containing Mg and Fe cations. Hazen (1977) pointed out that, if the curve of volumes of the members of the (Mg,Fe)2Si04 mixture is extrapolated at various... 

Polymorphic transitions render the thermodynamic description of feldspar end-members rather complex. According to Helgeson et al. (1978), the transition between the monoclinic and triclinic forms of NaAlSi308 may be regarded as the overlap of two high-order transitions (cf section 2.8), one associated with displacive structural modifications and the other with ordering on tetrahedral sites. The analysis of calorimetric data by Holm and Kleppa (1968) concerning the enthalpy... 

Figure 5.64 Effect of cooling rate on Si02 polymorphic transitions. From Putnis and McConnell (1980). Reproduced with modifications by permission of Blackwell Scientific Publications, Oxford, Great Britain.

Figure 9. Model of the polymorphic transition from the B-type to the A-type starch, in the solid state. The parallel double-helices which form the duplex are labelled 0 and 1/2 (this indicates their relative translation along the c axis). The water molecules are shown as dots.

Solidification of the particles may not be the final step in the formation process of solid lipid particles. Lipidic materials exhibit rich polymorphism [69,70], which may also occur in the dispersed state. In nanoparticles, the polymorphic behavior of the matrix lipids may, however, differ distinctly from that in the bulk material. Polymorphic transitions are usually accelerated in the nanoparticles compared with the bulk lipids [2,62]. In some cases, polymorphic forms not observable in the corresponding bulk materials were detected in lipid nanoparticles [1,65]. Because polymorphism can affect pharmaceutically relevant properties of the particles, such as the drug incorporation capacity [65], corresponding investigations should also be included in the characterization process. As long as polymorphic or other crystalaging phenomena have not terminated, the particle matrix cannot be regarded as static, and alterations of the particle properties may still occur. 

In DSC the sample is subjected to a controlled temperature program, usually a temperature scan, and the heat flow to or from the sample is monitored in comparison to an inert reference [75,76], The resulting curves — which show the phase transitions in the monitored temperature range, such as crystallization, melting, or polymorphic transitions — can be evaluated with regard to phase transition temperatures and transition enthalpy. DSC is thus a convenient method to confirm the presence of solid lipid particles via the detection of a melting transition. DSC recrystaUization studies give indications of whether the dispersed material of interest is likely to pose recrystallization problems and what kind of thermal procedure may be used to ensure solidification [62-65,68,77]. 

Since a polymorphic transition (a/p) of the mixed iron and cobalt molybdate occurs in the temperature range of the catalytic reaction (10,13,14), and since the high temperature form (p) can metastably be maintained at low temperature,... 

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