Amorphous solids metastable

Palladium acetate, [PdO —02CCH3)2l3, possesses a unique quality that makes it attractive for solid state decomposition studies as well as technological applications. It can be spin-coated from solution to form a homogeneous, apparently amorphous solid film. This provides large uniform areas over which we can study the effects of various irradiation sources on the chemical nature of the film. The bulky structure of palladium acetate, shown in Figure 1 (8), may offer a partial explanation of the molecule s ability to achieve an amorphous metastable phase upon rapid evaporation of solvent. 

At a given (low) temperature and pressure a crystalline phase of some substance is thermodynamically stable vis a vis the corresponding amorphous solid. Furthermore, because of its inherent metastability, the properties of the amorphous solid depend, to some extent, on the method by which it is prepared. Just as in the cases of other substances, H20(as) is prepared by deposition of vapor on a cold substrate. In general, the temperature of the substrate must be far below the ordinary freezing point and below any possible amorphous crystal transition point. In addition, conditions for deposition must be such that the heat of condensation is removed rapidly enough that local crystallization of the deposited material is prevented. Under practical conditions this means that, since the thermal conductivity of an amorphous solid is small at low temperature, the rate of deposition must be small. 

Metastable amorphous solids can in general be prepared from stable phases by bringing in excess free energy [5]. In the case of water, amorphous solids have been prepared from stable phases in all three aggregate states from the gas, the liquid, and the crystalline solid [131]. 

The noncrystalline sohds described here are amorphous and metastable. This specific thermodynamic property is because they aU originate from the liquid state. The corresponding glassy or vitreous materials are not very common in solid-state chemistry, and only a limited number of molten salts or molten alloys have the characteristics necessary to produce glasses when cooling. 

Maciejewski and Reller confirm [6] that (as yet) there is no experimental evidence for the existence of an amorphous or metastable intermediate in the dissociation of calcite. They show that the CaO formed by CaCOj dissociation under high vacuum reacts relatively rapidly at low temperatures (below 320 K) in the reverse process on CO2 readmission. Carefully controlled conditions are required to isolate and to study any highly active phases that may be formed during solid state decompositions. 

Basic crystal properties include solubility, supersaturation, metastable zone width, oil, amorphous solid, polymorphism, occlusion, morphology, and particle size distribution. Clearly. 

The strong tendency of this compound to form oils or amorphous solids at increased supersaturation indicates a relatively narrow metastable zone width. Slow simultaneous addition with a large seed area effectively maintains the supersaturation sufficiently low to prevent nucleation of fines and allows some growth. 

Microencapsulation of flavors is a technology of enclosing flavor compounds (core materials) in a carrier matrix. An amorphous or metastable solid is normally used as a carrier matrix. Microencapsulation is useful for improving the chemical stability of flavor compounds, providing controlled release of flavor compounds from microencapsulated flavor products, providing a free-flowing powder with improved handling properties and physical protection of volatile properties of flavor. 

---