Ostwalds step rule

When a compound that can form several modifications crystallizes, first a modification may form that is thermodynamically unstable under the given conditions afterwards it converts to the more stable form (Ostwald step rule). Selenium is an example when elemental selenium forms by a chemical reaction in solution, it precipitates in a red modification that consists of Se8 molecules this then converts slowly into the stable, gray form that consists of polymeric chain molecules. Potassium nitrate is another example at room temperature J3-KN03 is stable, but above 128 °C a-KNOs is stable. From an aqueous solution at room temperature a-KN03 crystallizes first, then, after a short while or when triggered by the slightest mechanical stress, it transforms to )3-KN03. 

The Ostwald Step Rule, or the rule of stages postulates that the precipitate with the highest solubility, i.e., the least stable solid phase will form first in a consecutive precipitation reaction. This rule is very well documented mineral formation via precursors and intermediates can be explained by the kinetics of the nucleation process. The precipitation sequence results because the nucleation of a more soluble... 

The "classical" theory of nucleation concentrates primarily on calculating the nucleation free energy barrier, AG. Chemical interactions are included under the form of thermodynamic quantities, such as the surface tension. A link with chemistry is made by relating the surface tension to the solubility which provides a kinetic explanation of the Ostwald Step Rule and the often observed disequilibrium conditions in natural systems. Can the chemical model be complemented and expanded by considering specific chemical interactions (surface complex formation) of the components of the cluster with the surface ... 

Ostwald proposed that when two or more new phases may form from existing phase or phases, that is, when new phases are more stable than the existing phase(s), the least stable new phase would form first and then transform into more stable phases. This is called the Ostwald rule, the Ostwald step rule, or the law of successive reactions. An alternative statement of the Ostwald rule is as follows ... 

In 1897, Friedrich Wilhelm Ostwald (1853-1932) published his now famous study of crystallization processes, which led to the Ostwald rule of stages or Ostwald step rule (Ostwald, 1897). Ostwald noticed that the course of transformation of unstable (or metastable) states into stable states normally occurs in stages,... 

In 1897, W. Ostwald formulated a general rule that remains important to this day for synthesizing materials If a chemical reaction produces several modifi cations of a compound, it is not the most stable but the most labile modifica tion that forms first, while more stable modifications are produced via further transformations of the labile one [1,2,3]. Hence, the Ostwald step rule indi cates the possibility of the formation of metastable (labile) intermediate phases on phase transitions due to the kineticaUy preferential formation of metastable phases rather than a thermodynamically stable phase immediately. 

The Ostwald step rule is, evidently, a particular case of the general requirement (see Section 1.3.3) for a sequential decrease in chemical potentials of the transformation intermediates in the course of a stepwise transformation. In the transformation of the constant composition soHd phases, the said requirement refers to chemical potentials of the soHd phases. If the state diagram of a particular matter comprises several allowed phases (the ones differing, for example, by their crystal structures, etc.), the initial phase transformation into the thermodynamically stable state at a constant temperature will be successively mediated by aU of the phases along the reaction pathway from the initial point to the stable phase. 

Transformations between phases follow the Ostwald step rule, which states When a number of phase transformations, from a less stable state to more and more stable states are possible, usually the closest more stable modification is formed and not the most stable one corresponding to the least free energy (Ostwald 1897). The phase transition from the metastable to the lowest free-energy polymorph is unavoidable due to the thermodynamic driving force to minimize free energy. 

A primary concern is polymorphic crystallization in which the Ostwald step rule is very useful (9). This rule predicts that phase changes occur step by step by way of successively more stable phases. For the relative rate of nucleation of polymorphic crystals shown in Figure 2, it follows that nucleation of the metastable forms such as a and p occurs first before the most stable p form, when nucleation occurs under a large supercooling or high supersaturation. When the amount of supercooling or supersaturation is decreased, the law is broken and the most stable form tends to nucleate at a relatively slow rate. 

The Ostwald step rule, or the mle of stages, postulates that the precipitate with the highest solubility (i.e., the least stable solid phase) will form first in a consecutive precipitation reaction. This mle is very well documented mineral formation via precursors and intermediates can be explained by the kinetics of the nucleation process. The precipitation sequence results because the nucleation of a more soluble phase is kinetically favored over that of a less soluble phase because the more soluble phase has the lower solid-solution interfacial tension (7cw) than the less soluble phase (equation 50). In other words, a supersaturated solution will nucleate first the least stable phase (often an amorphous solid phase) because its nucleation rate is larger than that of the more stable phase (Figure 13.26). While the Ostwald step mle can be explained on the basis of nucleation kinetics, there is no thermodynamic contradiction in the initial formation of a finely divided precursor. 

The empirical observation that unstable or metastable minerals form first in the weathering process, followed by progressively more stable minerals, is explained by the Gay-Lussac-Ostwald (GLO) or Ostwald step rule (cf. Sposito 1989, 1994 Steefel and Van Cappellen 1990). Steefel and Van Cappellen define the GLO step rule as follows ... 

Energy Landscape View of Ostwald Step Rule.10... 

ENERGY LANDSCAPE VIEW OF OSTWALD STEP RULE... 

---