Equilibrium condition NaCl crystals

The surface pressures for small crystals are in general much larger than the equivalent pressures for liquid droplets because the surface tensions of crystalline solids are usually much greater than those of liquids. As an example, consider an NaCl nanocrystal with r, = 5 nm for the (100) face for which tr,- = 413 dyne cm (Tasker, 1979), The surface pressure A/ = 2(415)/(5)(10) dynes/cm or 1660 bars. Thus the internal pressures of nanometer-size solid particles in equilibrium with atmospheric pressure vapors may reach thousands of bars. However, small crystals formed under dynamic gas-phase conditions in which their shape Is controlled by heat and mass transfer may not have time to attain the equilibrium shape, so the use of (9.31) and (9.32) may not be quantitatively correct in practical applications. 

Figure 7.86 shows equilibrium data for the system NaCl/Na2S04/H20 in connection with a description of a process for sequential crystallization of salt and sulfate from brine. The reader will recognize that the intermediate liquor in that process is similar to the liquor produced in triple salt decomposers. The same technique and operating conditions therefore apply to recovery of Glauber s salt in the present case. 

As is known in the case of binary systems, when their composition is close to equiatomic, d-ina-via metals form, as a rule, stable phases with Bl-type structures. But, as was observed by Vereshchagin and Kabalkina (1979) using a high-pressure treatment, it is possible to get B1 B2 (NaCl-CsCl)-type structural transitions with some oxides. The question of whether this could happen with d-met carbides was discussed by Ivanovsky et al (1988). These authors carried out LMTO band structure calculations for hypothetical TiC, VC and CrC compounds with a B2 structure. The lattice parameters were determined from the condition that the unit cell volumes of the CsCl- and NaCl-type phases were equal. In order to consider the influence of uniform isotropic compression the B2 VC calculations were carried out for crystal lattice volumes of 5 and 10% less than the equilibrium one. 

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