Crystallization of solutes

The ionic atmosphere moves continually, so we consider its composition statistically. Crystallization of solutions would occur if the ionic charges were static, but association and subsequent dissociation occur all the time in a dynamic process, so even the ions in a dilute solution form a three-dimensional structure similar to that in a solid s repeat lattice. Thermal vibrations free the ions by shaking apart the momentary interactions. 

A poly(phenylquinoxaline) was prepared for electroluminescence applications <1996SM(76)105>. Crystallization of solution donor-acceptor complexes of 2,3-dimethylquinoxaline 1,4-dioxide or phenazine 5,10-dioxide with TCNE afforded two-component solids containing weakly bound 1-D donor-acceptor arrays <1997TL7665>. A pyrazine ladder polymer was constructed from two different pyrazine units, as an optical device <1999JA8783>. The new electron-deficient macrocycle tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine was prepared from l,2-di(2-pyridyl)ethanedione and 2,3-diaminomaleonitrile for a study of its electrochemical properties <2004IC8626>. 

Add a crystal of solute to each solution to see if the solute dissolves (unsaturated) or remains (saturated), or more solute precipitates out of the solution (supersaturated). 

A saturated solution contains more solute in a given volume of solvent than an unsaturated solution. A supersaturated solution contains more solute in a given volume than would normally be present at a particular temperature. A supersaturated solution is unstable. When a crystal of solute is added to a supersaturated solution, excess solute crystallizes out of solution the remaining solution is saturated as it normally would be at that particular temperature. When a crystal of solute is added to an unsaturated solution, the crystal will dissolve. When a crystal of solute is added to a saturated solution, the added crystal will not dissolve. Most of the time, when the temperature of a solvent increases, the solubility of the solute increases in a given amount of solvent. The more colored solute is dissolved in a given amount of solvent, the more intense the color of the solution will be. Most paints are solutions of pigments (the solute) and binders (the solvent). 

In a saturated solution, an equilibrium exists between the rate of precipitation of solute particles and the rate of dissolution of solute particles. The rate of precipitation equals the rate of dissolution. The shape of a crystal of solute added to a saturated solution will change after a period of time at a constant temperature and pressure, but its mass will remain the same. The equilibrium between dissolving and precipitation is dynamic, a continuous process. 

Pure grades of a particular ammonium phosphate can be produced by crystallization of solutions obtained from furnace-grade phosphoric acid. 

There is increasing interest in the crystallization of solutes from supercritical-fluid solvents. In such instances, solubilities often are correlated by an equation of state. Such concepts are beyond the scope of the current discussion but are presented elsewhere in the encyclopedia. 

By fusing borax with boron trioxide, two definite acid borates are produced. One of them has the formula Na20,3B203, and melts at 694° C. the other melts at 783° C., and has the formula Na20,4B203.2 The crystallization of solutions with the composition ratio... 

Crystallization A process in which a liquid solution is cooled, or solvent is evaporated, to an extent that solid crystals of solute form. The crystals in the slurry (suspension of solids in a liquid) leaving the crystallizer may subsequently be separated from the liquid in a filler or centrifuge. 

The use of a cooling accessory permits XRD patterns to be obtained under subambient conditions. In pharmaceutical systems, the greatest utility of the technique is to monitor the crystallization of solutes in frozen solutions. Conventionally, differential scanning calorimetry has been the most popular technique for the characterization of frozen systems. However, as mentioned earlier, this technique has some drawbacks (i) It does not enable direct identification of crystalline solid phase(s). Moreover, it is difficult to draw any definitive conclusions about the degree of crystallinity, (ii) The interpretation of DSC curves is very difficult if there are overlapping thermal events. Low temperature XRD was found to be an excellent complement to differential thermal analysis in the characterization of water-glycine-sucrose ternary systems. " ... 

Manufacture by adding water to metasilicate or spray cooling crystallization of solutions... 

Trona may be recovered from the deposit by conventional dry, or solution mining depending on the nature and accessibility of the source [19]. Fractional crystallization of solutions of trona [20] or the sodium carbonate-rich brines separates the sodium carbonate for recovery. Sodium carbonate has also been produced from trona in a spray drier [21]. 

In the metastable region between the binodal and spinodal curves, phase separation has to occur by the mechanism of nucleation and growth. In this region, the one-phase-state is Indeed stable against small concentration fluctuations but unstable against separation into two phases of more different concentrations. Phase transformations in one-component systems like condensation, evaporation or solidification as well as the crystallization of solutes from solvents occur by the nucleation and growth mechanism. The well known phenomena of oversaturation and hindered-phase transformation can be explained by discussing the nucleation as an equilibrium reaction with the creation of the "critical nucleus" (6, 7). 

The situation becomes more complicated when more solutes and especially when particulate material is present. It is not uncommon that crystallization of solutes does not occur within a reasonable time. We will come back to this in Chapter 16. 

Processes. To obtain a food in a glassy state, water generally has to be removed without crystallization of solutes occurring. In some products this is easy, in others difficult. The following methods can be distinguished. 

Conclusion. Most foods that are aqueous solutions and/or dispersions can be freeze-concentrated without crystallization of solutes and so become mixtures of ice crystals and a glassy material. The glass is of mixed composition, and roughly 20% of its mass is unfrozen water. The frozen food is stable to all physical changes and to many chemical reactions, provided that the temperature is kept below Tg. 

In some cases, we can prepare a solution that contains more than the equilibrium concentration of dissolved solute. Such a solution is called supersaturated. It is unstable relative to the saturated solution if you add a seed crystal of solute, or just tap the container, the excess solute crystallizes immediately, leaving a saturated solution (Figure 13.8). 

In some instances it is possible to prepare a true solution that contains an excess of the equilibrium amount of solute this condition is called supersaturation. Supersaturated solutions are unstable. Iflefttmdisturbed, they may remain in this state for an indefinite period of time. However, the excess solute can be brought out of solution by a slight agitation or by the addition of any solid particle (dust, a small crystal of solute, etc.) that can act as a center for crystal growth, returning the solution to its normal satnrated state. 

Crystallization of solutions of quinol [p-dihydroxybenzene, p-C6H4(OH)2] in water or alcohol under pressure of 10-40 atm of, say, krypton, produces crystals, often up to 1 cm in length, which are readily distinguishable from the crystals of ordinary quinol (a-quinol), even visually. These crystals contain the noble gas trapped in the lattice of -quinol. When the crystals are dissolved in water, or heated, the gas is released. The crystals are stable at room temperature and can be kept for years. 

Crystallization of solutes —> enrichment in the zone 3. Evaporation evaporation of water... 

Rotating cylinders, similar in some respects to those used as rotary driers or kilns, have been used for the crystallization of solutions. The cylinder slopes slightly from the feed liquor inlet down to the crystal magma outlet. Cooling may be provided either by cold air blown through the cylinder or by water sprayed over the outside. In the former case internal baffles disturb the liquor on the inside wall and cause it to rain through the air stream. In the latter case internal scraper devices prevent excessive build-up of crystal on the walls. 

---