Crystallization advantage

When a nonequilibrium crystal is mixed with water, the solution concentration attained may momentarily exceed the equilibrium solubility. Also, the rate of dissolution of nonequilibrium crystals is typically faster than that of the equilibrium crystal. Advantage is often taken of these properties of nonequilibrium crystals in drug formulations to provide a form of the drug compound that dissolves more rapidly, and even to a greater extent, than the equilibrium crystal [50]. 

We noted in Section VII-2B that, given the set of surface tension values for various crystal planes, the Wulff theorem allowed the construction of fhe equilibrium or minimum firee energy shape. This concept may be applied in reverse small crystals will gradually take on their equilibrium shape upon annealing near their melting point and likewise, small air pockets in a crystal will form equilibrium-shaped voids. The latter phenomenon offers the possible advantage that adventitious contamination of the solid-air interface is less likely. 

A related advantage of studying crystalline matter is that one can have synnnetry-related operations that greatly expedite the discussion of a chemical bond. For example, in an elemental crystal of diamond, all the chemical bonds are equivalent. There are no tenninating bonds and the characterization of one bond is sufficient to understand die entire system. If one were to know the binding energy or polarizability associated with one bond, then properties of the diamond crystal associated with all the bonds could be extracted. In contrast, molecular systems often contain different bonds and always have atoms at the boundary between the molecule and the vacuum. 

Taking advantage of the synnnetry of the crystal structure, one can list the positions of surface atoms within a certain distance from the projectile. The atoms are sorted in ascending order of the scalar product of the interatomic vector from the atom to the projectile with the unit velocity vector of the projectile. If the collision partner has larger impact parameter than a predefined maximum impact parameter discarded. If a... 

The advantages of this method are twofold (i) It is possible to observe minute changes in colour and structure before and during the process of melting. (2) It is possible to use a single crystal which, e.., is often obtained from a semi-micro sublimation. 

A small quantity of the base may be recrystallised from hot ether, and magnificent deep green crystals obtained the base is not sufficiently soluble in cold ether, however, to enable this solvent to be used advantageously in the above extractions. 

NHjCONHNHj.HCl) and 1-5 g. of crystallised sodium acetate in 10 ml. of water in a test-tube. Add 1 ml. of acetone, close the tube with a cork and shake vigorously. Allow the mixtme to stand, with occasional vigorous shaking, for 10 minutes it is advantageous to cool in ice. FUter the crystals, wash with a httle cold water, and recrystaUise from water or dilute alcohol. The m.p. of acetone semicarbazone is 187°. 

The crystals are assumed to be circular disks. This geometry is consistent with previous thermodynamic derivations. It has the advantage of easy mathematical description. 

There is extensive Hterature on nitrophosphates (65,66). A description of the Norsk Hydo nitrophosphate process, ie, using calcium adjustment by crystallization of calcium nitrate, emphasizing the environmental advantages is also available (64). 

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