Equilibrium state, amorphous solids, glass

In 1931 Simon reported that small molecules in their amorphous solid state are not in thermodynamic equilibrium at temperatures below their glass transition u. Such materials are in fact supercooled liquids whose volume, enthalpy, and entropy are greater than they would be in the equilibrium glass. (See Fig. 1). 

Solubility and speciation. Minimum requirements for reliable thermodynamic solubility studies include (i) solution equilibrium conditions (ii) effective and complete phase separation (iii) well-defined solid phases and (iv) knowledge of the speciation/oxidation state of the soluble species at equilibrium. Ideally, radionuclide solubilities should be measured in both oversaturation experiments, in which radionuclides are added to a solution untU a solid precipitates, and undersaturation experiments, in which a radionuchde solid is dissolved in aqueous media. Due to the difference in solubilities of crystalline versus amorphous solids and different kinetics of dissolution, precipitation, and recrystalhzation, the results of these two types of experiments rarely agree. In some experiments, the maximum concentrahon of the radionuchde source term in specific water is of interest, so the sohd that is used may be SF or nuclear waste glass rather than a pure radionuclide solid phase. 

Compared to crystalline materials, the production and handling of amorphous substances are subject to serious complexities. Whereas the formation of crystalline materials can be described in terms of the phase rule, and solid-solid transformations (polymorphism) are well characterised in terms of pressure and temperature, this is not the case for glassy preparations that, in terms of phase behaviour, are classified as unstable . Their apparent stability derives from their very slow relaxations towards equilibrium states. Furthermore, where crystal structures are described by atomic or ionic coordinates in space, that which is not possible for amorphous materials, by definition, lack long-range order. Structurally, therefore, positions and orientations of molecules in a glass can only be described in terms of atomic or molecular distribution functions, which change over time the rates of such changes are defined by time correlation functions (relaxation times). 

Amorphous solid substances have no fixed arrangement of their molecular bulks and are not in a definable state, either in energy content or structurally. One view is to consider an amorphous substance as an entanglement of molecules of many shapes and sizes. This is particularly true of phosphate and silicate glasses when considered as supercooled liquids. Their properties depend upon much history. They are not in a thermodynamically defined state. This means that they are not in equilibrium with their environment under existing conditions. Then there must be some other state of matter made of these same molecules (or ions) that has less energy than the amorphous system. These lower-energy systems are usually crystalline. 

---