Nucleation solubility effect

The development of the freeze concentration process for fruit juices has been hampered by the fact that solute concentrate is entrained by the ice crystals. This incomplete separation of the entrained concentrate from the ice results in a considerable increase of the cost of the process. In this investigation sucrose solutions were concentrated by the formation of an ice layer on the externally cooled walls of the crystallizer. The formation of the layer was initiated by secondary nuclei induced by rotating ice seeds, at subcoolings smaller than the critical subcooling needed for spontaneous nucleation. A minimum in the amount of sucrose entrapped in the ice layer was observed at a subcooling smaller than the critical subcooling for spontaneous nucleation. The effect of soluble pectins on the minimum was also studied. 

As an even more explicit example of this effect Figure 6 shows that EPM is able to reproduce fairly well the experimentally observed dependence of the particle number on surfactant concentration for a different monomer, namely methyl methacrylate (MMA). The polymerization was carried at 80°C at a fixed concentration of ammonium persulfate initiator (0.00635 mol dm 3). Because methyl methacrylate is much more water soluble than styrene, the drop off in particle number is not as steep around the critical micelle concentration (22.) In this instance the experimental data do show a leveling off of the particle number at high and low surfactant concentrations as expected from the theory of particle formation by coagulative nucleation of precursor particles formed by homogeneous nucleation, which has been incorporated into EPM. 

Water soluble impurities and their effect can be easily included in equation (1-4), through which they are going to directly affect the particle nucleation rate, f(t). If one assumes a first order reaction of an active radical with a water soluble impurity (WSI) to give a stable non-reactive intermediate, then one simply has to add another term in the denominator of equation (1-4), of the form kwsr[WSI](t)-kv, and to account for the concentration of WSI with a differential equation as follows ... 

The important consequence of eq. (6.60) is that the solubility of the solid increases with decreasing radius of crystal. Although the effect is small this illustrates the need for super-saturation on homogeneous nucleation in a liquid. Super-saturation is necessary in order to obtain nucleation since the solubility of the nuclei is higher... 

Industrial solutions invariably contain dissolved impurities that can increase or decrease the solubility of the prime solute considerably, and it is important that the solubility data used to design crystallisation processes relate to the actual system used. Impurities can also have profound effects on other characteristics, such as nucleation and growth. 

A nucleation approach was also applied to predicting the extension of solid solubility obtained in Al-alloys by rapid solidification. In this case the competition for forming either the supersaturated A1 solid solution or the equilibrium compound was taken to be nucleation controlled (Saunders and Tsakiropoulos 1988). The model was first applied to high-pressure gas-atomised (HPGA) Al-Cr and Al—Zr alloys and was innnediately able to predict a number of important general effects, all of which are consistent with experimental observation. 

On the other hand, it was found that the initial pH or excess concentration of Fe3+ ions had a strong effect on the final size of the product, as shown in Figure 1.3.8 (13). The drastic enhancement of nucleation of the hematite with increasing initial pH or decreasing excess concentration ferric ions suggests a dramatic increase of some specific ferric hydoxide complex as a precursor such as (HO)2FeOFe(OH)2 for the formation of hematite particles with increasing pH (29), since the supersaturation for the formation of hematite in terms of solubility product is identical for all pH due to the presence of Fe(OH)3 gel. 

The second study (Ref 58), published in 1976, likewise noted a 10—11% shrinkage upon solidification, and studied the solubility of various gases in the melt, as well as the rate of solidification of the supercooled liq at various rates of heat removal and at various temps. The role of nucleating agents was discussed, and HNS was shown to be effective. The established physical principles of solidification were found fully applicable t TNT... 

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