Supercooled melts

During the production of the chapter, a current review of the RFOT theory has appeared in print [V. Lubchenko and P. G. Wolynes, Annu. Rev. Phys. Chem. 58, 235 (2007)]. In addition, microscopic descriptions of the onset of activationless reconfigurations [J. D. Stevenson, J. Schmalian, and P. G Wolynes, Nat. Phys. 2, 268 (2006)] and prefactors for viscosity and ionic conductivity of deeply supercooled melts [V. Lubchenko, J. Chem. Phys. 126, 174503 (2007)] are now available. 

This is the process of the crystallization of one or more of the constituents of glass. Generally a glass is thermodynamically unstable with respect to these crystals, but at ordinary temperatures the crystallization rate is quite negligible. Crystallization may occur when the glass is worked at high temperature. The crystals which appear in a supercooled melt are not necessarily those of the stable solid phase at the temperatiure concerned for example, cristobalite can appear at temperatures for which tridymite is the stable crystalline... 

The primary nucleation process is divided into two periods in CNT one is the so called induction period and the other is the steady (or stationary) nucleation period (Fig. 2) [16,17]. It has been proposed by CNT that small (nanometer scale) nuclei will be formed spontaneously by thermal fluctuation after quenching into the supercooled melt, some of the nuclei could grow into a critical nucleus , and some of the critical nuclei will finally survive into macroscopic crystals. The induction period is defined as the period where the nucleation rate (I) increases with time f, whereas the steady period is that where I nearly saturates to a constant rate (fst). It should be noted that I is a function of N and t,I = I(N, t). In Fig. 2, N and N mean the size of a nucleus and that of the critical nucleus, respectively. The size N is defined... 

Fig. 2 Illustration of the induction and the steady (stationary) periods during the nucleation process. Small clusters exist in the supercooled melt at t = 0. During the induction period (t < r,), isolated nuclei of size N, smaller than the critical nuclei (named nanonuclei or embryo), are formed. The nuclei grow larger and larger with increase of time and some of them attain a much larger size than the critical size, N ...

Lipid nanodispersions (SLN and NLC) are complex, thermodynamically unstable systems. The colloidal size of the particles alters physical features (e.g., increasing solubihty and the tendency to form supercooled melts). The complex structured lipid matrix may include hquid phases and various lipid modifications that differ in the capacity to incorporate drugs. Lipid molecules of variant modifications may differ in their mobility. Moreover, the high amount of emulsifier used may result in liposome or micelle formation in addition to the nanoparticles. 

To detect dynamic featnres of colloidal preparations, additional methods are required. Nuclear magnetic resonance spectroscopy allows a rapid, repeatable, and noninvasive measurement of the physical parameters of lipid matrices withont sample preparation (e.g., dilution of the probe) [26,27]. Decreased lipid mobility resnlts in a remarkable broadening of the signals of lipid protons, which allows the differentiation of SLN and supercooled melts. Because of the different chemical shifts, it is possible to attribute the nuclear magnetic resonance signal to particnlar molecnles or their segments. 

Bunjes H., Siekmann B., and Westesen K., Emulsions of supercooled melts a novel drug delivery system, in Submicron Emulsions in Drug Targeting and Delivery, Benita S., ed., Harwood Academic Publishers, Amsterdam, 1998, 175. 

Typically, resolution by entrainment involves a sequence of alternate crystallizations of both enantiomers interrupted by loading the mixture with racemic material. A continuous resolution may be performed with supercooled melts in the apparatus shown in Figure 3. 

The problem of morphological instability was solved theoretically by Mullins and Sekerka [20], who proposed a linear theory demonstrating that the morphology of a spherical crystal growing in supercooled melt is destabilized due to thermal diffusion the theory dealt quantitatively with and gave linear analysis of the interface instability in one-directional solidification. 

Physical properties of solid polyphosphate glasses and their melts are also in accord with the conclusions drawn from chemical studies. The X-ray diffraction pattern shows the polyphosphate anions to consist of long chains of P04 tetrahedra (32) and the same conclusion is reached by studying the double refraction of fibers formed by rapidly drawing supercooled melts of Graham s salt (101). 

The Kelvin equation helps explain an assortment of supersaturation phenomena. All of these —supercooled vapors, supersaturated solutions, supercooled melts —involve the onset of phase separation. In each case the difficulty is the nucleation of the new phase Chemists are familiar with the use of seed crystals and the effectiveness of foreign nuclei to initiate the formation of the second phase. 

Knowledge of the mode of crystn of TNT is essential because it underlies the widespread practice of melt-pouring employed in the preparation of the commonly-used composite expls, such as Composition B. Samples of TNT obtained by sublimation onto a condensing surface held at a temp (78°) close to the mp, or by freezing melts at temps close to the mp, consist solely of the simple monoclinic form (Ref 26). Crystn from solvents at room temp, or from strongly supercooled melts, yields primarily monoclinic variant forms. Orthorhombic TNT is formed by crystn from solvents at low temps. At least seven morphological types of TNT have been identified (Ref 48). Two types have been identified by nuclear quadripole resonance (NQR) (Ref 66) a phase transition was noted at... 

Fig. 6.2. The kinetic curves of isothermal saturation of PrOH with various samples of AKOPr j at 20°C m.p. 140°C (I), 118°C (11, HI), supercooled melt (IV) [1642],...

Fig. 5.10 DSC measurements on individual samples of polymorphs R, Y, OP and ON of 5-XII (see caption for Fig. 5.9 for definition of terms), recorded at 10 °C min , each exhibiting homogeneous melting. Inset DSC trace of a mixture of the four modifications, recorded at 0.5 °C min, showing better separated melting endotherms and exotherms resulting from crystallization from supercooled melts, (from Yu et al. 2000, with permission.)...

Fig. 5.14 Infrared absorption spectra in the region of the CN stretch for 5-XII. Bottom, solid state spectra of forms (left to right) R, ORP, YN, ON, OP and Y. Middle, solution spectra in CCI4 (0.90 mM). Top, supercooled melt at 22 °C. (From Yu et al. 2000, with permission.)...

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