Plastic crystal fusion

In some cases there is evidence of multiple solid-solid transitions, either crystal-crystal polymorphism (seen for Cl salts [20]) or, more often, formation of plastic crystal phases - indicated by solid-solid transitions that consume a large fraction of the enthalpy of melting [21], which also results in low-energy melting transitions. The overall enthalpy of the salt can be dispersed into a large number of fluxional modes (vibration and rotation) of the organic cation, rather than into enthalpy of fusion. Thus, energetically, crystallization is often not overly favored. 

Figure 8. The temperature dependence of the heat capacity in the condensed state for adamantane [5] as measured by a scanning calorimeter. Tu, stands for temperature of transition from rigid crystal (fee) to plastic crystal (cubic) state of adamantane and Tfas stands for fusion temperature.

Plastic crystal phases in organic materials have been known since the time of Timmermans [1], and these phases are often reached via a solid-solid transition below the final melting point of the crystal. These transitions often represent the onset of rotational motions of the molecules within the crystalline lattice and the resultant phases are sometimes referred to as rotator phases [18-23]. Timmermans proposed a general rule that plastic phases have a low final entropy of fusion (A5f < 20 J K mol because the rotational component of the entropy of fusion of the fully ordered phase is already present in the plastic phase. The bulk of the... 

In the cubic phase there is plastic crystal disorder and as IVeinstock has pointed outl t the entropy of fusion of this phase is not very different from the value of 3.3 e.u. found for the noble gases. 

Naphthalene, in contrast to benzene, did not show any NMR-spectra line-width narrowing up to its melting temperature of 353 K. The mean experimental second moment was 9.1 compared to 10.1 G, estimated for the rigid crystal. Measurement of spin-lattice relaxation times indicated, however, also a slow reorientational jump motion about an axis normal to the molecular axis An activation energy of 105 kJ/mol was derived. Molecular dynamics simulations suggest that this reorientation about the axis of greatest inertia occurs with a frequency of 100 MHz within 20 K of fusion (353.6 K) Still, no plastic crystal behavior as found in cyclohexane and related compounds (see Sect. 3.1.1) is indicated for benzene or naphthalane, even close to the melting temperature. 

The ready availability of differential scanning calorimetry (DSC) and in recent years moduluated DSC have significantly increased the use of the enthalpy of fusion of crystals as a method for crystallinity determination. An illustration of this method was shown for plastic crystals in Chapter 4. The enthalpy of the solid is a combination of components from the amorphous and crystalline regions. The enthalpy at any given temperature His therefore... 

The entropy of fusion of a normal crystal can be considered as the sum of the increase in disorder due to the breakup of the crystal lattice, the configurational contribution, and that due to greater fieedom for internal motions. The configurational contributions predominate. They can be considered as the sum of the translational disorder and the effect of onset of molecular rotation. In this light a transition from a normal crystal to a plastic crystal introduces some of the disorder normally associated with melting. The sum of the entropy of transition to a plastic crystal and the subsequent entropy of fusion is within the range of the entropy of fusion of normal crystals. Molecular rotation in plastic crystals are not usually free. Thus the entropy of transition is somewhat less than what would be observed for a transition to free rotation. 

We have been particularly interested in the study of the plastic states of organic compounds,41 which are characterized by high values of AS of formation from the crystalline state, the AS of fusion (plastic-liquid transition) being much smaller. We find that the AH as well as the AS of the crystal-plastic transition generally decrease as the temperature range of stability of the plastic phase increases the AH and the AS of the plastic-liquid transition, on the other hand, increase as the temperature range of stability of the plastic phase increases.42 Neutron scattering, NMR spectroscopy, and several other techniques have been employed to study molecular reorientation in the plastic state.41 We... 

Pastry, Cake, and Biscuit Products. In general, fats play several essential nutritional, technological, functional, and organoleptic roles in most all-bakery applications. As a result of its physical properties, fat plays a major part in the production of the majority of items in the pastry, cake, biscuit, and chocolate confectionery sector for example, in the preparation of pastry cream and in the desired appearance and texture of the end product. These physical properties include, above all, the rheological properties (consistency, plasticity, texture, etc.), and the properties of fusion and crystallization depend on the type of fat, the temperature, and the working conditions of the product. 

Because of the importance of controlling the solid-state properties a great deal of interest has been generated around understanding the role of water in the formulation since water can act as a plasticizer that will deteriorate both chemical (i.e.. protein degradation) and physical (i.e., particle fusion, cake collapse, crystallization) stability. How dry this formulation needs to be depends on the glass transition of the formulation relative to the storage conditions and the so called zero temperature (55). 

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