Kinetics of polymorphic transformation

All SFC processes operate at above the critical temperature (Tc) of supercritical fluids. Temperature is a critical controlling variable of the SFC process based on both thermodynamic and kinetic considerations. First, solubility is a function of temperature, and this will determine the supersaturation ratio or the driving force for the crystallization of individual polymorphs. Second, the kinetics of polymorphic transformation is governed by the Arrhenius law and is also temperature dependent. The rate constant of the conversion is related to the activation energy and the mass transfer process involved (i.e., diffusion, evaporation, or mixing in supercritical fluids). 

The kinetics of polymorphic transformations in melts and solutions are discussed in section 6.5. 

L.E. O Brien, P Timmins, A.C. Williams and P. York, Use of in situ FT-Raman spectroscopy to study the kinetics of the transformation of carbamazepine polymorphs, J. Pharm. Biomed. Anal., 36, 335-340 (2004). 

McHale JM, Auroux A, Perrotta AJ, Navrotsky A (1997) Surface energies and thermodynamic phase stability in nanocrystalline aluminas. Science 277 788-791 Molteni C, Martonak R, Parrinello M (2001) First principles molecular dynamics simulations of pressure-induced stiuctural transformations in silicon clusters. J Chem Phys 114 5358-5365 Murray CB, Norris DJ, Bawendi MG (1993) Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites. J Am Chem Soc 115 8706-8715 Onodera A (1972) Kinetics of polymorphic transitions of cadmium chalcogenides under high pressure. Rev Phys Chem Japan 41 1... 

The additives or impurities have a great influence on the kinetics of the transformation. Additives can be subdivided into mineralizers and stabilizers according to their effects on materials. Mineralizers are substances that accelerate the changes of the basic material. For y- these substances accelerate sintering and polymorphous transformations during the calcination and decrease... 

Crystallization of Blends The first polymer blend was made from two polymeric mbbers in 1846, but polymer blend technology and a scientific understanding of the underlying principles controlling the compatibility (or lack of) in polymer mixtures (alloys as they have been named recently) has taken place only in the latter part of the current century. Many blends are non-crystalline but our interest in this document is focused on the kinetics of phase transformations of binary and ternary systems that receives more attention annually. Some of these systems can be very complicated, often comprised of multiple phases that m involve homopolymers, copolymers, mesophases and the like. Polymorphism and even isomorphism may occur... 

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... 

Diffusive transformations (including polymorphic transitions) abound in solid state chemistry. Component diffusion is always involved. However, the kinetics of a... 

Kaneniwa et al. (1988) studied the transformation of phenylbuta eftram to thea-form in ethanol at 4C by DSC and XRD. The reaction was essentially complete in 4 days. Kaneniwa et al. (1985) also studied the transformation kinetics of indomethacin polymorphs in ethanol and Lt the data to nine different kinetic models. The data Ltthe Avrami equation best, which assumes two-dimensional nuclear growth. The transformation ofdfferm to they-form was a function of temperature with an activation energy of 14.2 kcal/mol. 

Richardson, M. R., Yang, Q., Novotny-Bregger, E. and Dunitz, D. J. (1990). Conformational polymorphism of dimethyl 3,6-dichloro-2,5-dihydroxyterphthalate. II. Structural, thermodynamic, kinetic and mechanistic aspects of phase transformations among the three crystal forms. Acta Crystallogr. B, 46, 653-60. [178, 215] Rieper, W. and Baier, E. (1992). Neue Kristallmodifikationen von C.I. Pigment Yellow 16. European Patent EP 054072. [261t]... 

Due to the difference of the polymer adsorption onto different faces of different crystal forms, polymer adsorption onto the crystal face has played an important role in crystal polymorphic transformationJ Garti and Zour have studied the effects of surfactants on the polymorphic transformation of glutamic acid. Glutamic acid has two crystal forms, a and p, with the p-form being more stable than the ot-form. Those surfactants that preferentially adsorb onto the surface of the ot-growing crystals retard the transformation of the ot-form to the p-form. A Langmuir analysis indicates that the kinetic coefficient of crystal polymorphic transformation is related to the volume of the surfactant adsorbed at the crystal surface. 

Otsuka, M. Ohfusa, T. Matsuda, Y. Effects of binders on polymorphic transformation kinetics of carbamazepine in aqueous solution. Colloids Surf, B 2000, 145-152. 

Much of the work to date on particle size effects on phase transformation kinetics has involved materials of technological interest (e.g., CdS and related materials, see Jacobs and Alivisatos, this volume) or other model compounds with characteristics that make them amenable to experimental studies. Jacobs and Alivisatos (this volume) tackle the question of pressure driven phase transformations where crystal size is largely invariant. In some ways, analysis of the kinetics of temperature-motivated phase transformations in nanoscale materials is more complex because crystal growth occurs simultaneously with polymorphic reactions. However, temperature is an important geological reality and is also a relevant parameter in design of materials for higher temperature applications. Thus, we consider the complicated problem of temperature-driven reaction kinetics in nanomaterials. 

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