Metastable immiscibility

Charles, R. J. Wagstaff, F. E. 1968. Metastable immiscibility in the B203-Si02 system. Journal of the American Ceramics Society, 51, 16-20. 

We note that for aqueous solutions of some salts with ions of higher valence such as BaCl2 [73] or U02S04 [74] such a liquid-liquid immiscibility is indeed observed. Moreover, for some other salts such as MgS04 a metastable liquid-liquid immiscibility at elevated temperatures [76] seems to be suppressed by a retrograde salt solubility that rapidly decreases as the temperature is increased. 

Consequently, it can be concluded for the mixtures of LLL-MMM, LLL-PPP, LLL-SSS, MMM-PPP, and PPP-SSS that the TAG binary mixtures are miscible in metastable polymorphs of a and p forms when the difference in the number of carbon atoms of the fatty acid moieties. An, equals 2, whereas immiscible mixtures are found in all polymorphic forms when An is larger than 2. Results obtained for these mixture systems may indicate a relationship between polymorphism and phase behavior of the binary mixtures of the saturated-acid TAGs in such a way that rotational freedom of hydrocarbon chains and entropy of methyl-end stacking are crucial factors determining the polymorph-dependent phase behavior. 

The phenomenon of metastable immiscibility, i.e. separation into two liquid phases below the Uquidus temperature, was first established in the system Na O— —B2O3 —S1O2, and also practically exploited in the manufacture of Vycor glass (see later). Turner and Winks had already found in 1926, that B2O3 and Na20 can be... 

The demixing of the immiscible components in an initially one-phase melt proceeds rapidly due to a thermodynamic instability, this is spinodal decomposition, or a thermodynamic metastability (i.e., nucleation and growth of a new phase). However, this distinction of the character of the liquid-liquid phase transition is largely academic for the purposes of this work, because in either case the system rapidly produces droplets of a new phase (i.e., for off-critical mixtures). The demixing process itself was not studied in detail in this work. However, further details of this process may be found elsewhere (21). The production of droplets of a new phase in these immiscible systems is extremely rapid compared with the subsequent coarsening of the system to the final morphology. 

Figrue 4.6 Idealizedphase diagram for a binary system exhibiting metastable immiscibility... 

Critical temperatures often follow a simple trend for systems of related compositions. The critical temperature of alkali silicate melts, for example, decreases in the order of increasing radius of the alkali ion present, such that lithium and sodium silicate melts clearly exhibit metastable immiscibility, while the existence of immiscibility in potassium silicate glasses has not been conclusively established. There is no evidence for the existence of phase separation in the rubidium or cesium silicate systems. 

Similar trends exist for many other systems. In general, the critical temperature within series such as the alkaline earth silicates or borates will decrease with increasing radius of the alkaline earth cation. In the former case, the suppression of the critical temperature with increasing radius of the alkaline earth cation is great enough that barium silicate melts exhibit metastable immiscibility, while the other alkaline earth silicates exhibit stable immiscibility. 

Figrue 4.8 Metastable immiscibility diagram for the sodium silicate system... 

The effects of phase separation on viscosity are more subtle for compositions exhibiting metastable immiscibility. In many cases, the fine scale of the morphology is such that no indication of phase separation is observed by the naked eye. The measured viseosity will be a funetion of the compositions of the equilibrium liquids, the connectivity of each phase, and the scale of the morphology. If the phase with the higher viscosity has a connected structure, the viscosity of the lower viscosity phase will be of little importance since the measured viscosity will be determined by the less mobile phase. On the other hand, if the phase with the higher viscosity exists only as isolated spheres within a matrix of the less viscous phase, the measured viscosity will be near that of the more fluid phase. Any thermal treatment which alters the connectivity... 

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