Effect of a-Factor on Growth Mechanism

In computer simulations (Gilmer and Bennema 1972) and experimental studies (Bourne and Davey 1976), the value of a-factor has been linked to the controlling growth mechanism. Depending on the value of a, three distinct growth regimes have been identified (Davey 1982a). 

It can be concluded that where mass transfer resistance is negligible, the relative growth rate and growth mechanism of a substance can be predicted with the use of the o-factor. Moreover, this approach provides a thermodynamically based method to correlate the effects of solvents that can eventually lead to methods for choosing optimal solvents. A specific example illustrating these concepts and calculations is given in the next section. 

Growth of Hexamethykne Tetramine (HMT) from Various Solvents. Detailed studies of the effect of solvents on crystal growth have been conducted using the crystalline material HMT (Bourne and Davey 1976 Bourne 1980 Davey 1986). An analysis of HMT solubility data through the use of Eq. (3.33) indicates that HMT forms an approximately ideal solution with ethanol. By contrast, large negative deviations from ideal behavior are observed in water. 

The calculation of a-factor involves two steps (1) calculation of which varies depending on crystal face and (2) calculation of thermodynamic properties [using AH and solubility with Eq. (3.32), or AH with Eq. (3.29)]. From the unit cell, the number of nearest neighbors of the HMT body centered cubic lattice is known to be 8. Since the number of nearest neighbors on the 110 face is 4, the value of for this face is = /tj/n = 0.5 (Davey 1986). 

Although no studies to date have attempted to relate impurity retention specifically to the a-factor of the solvent, it is interesting to note that HMT grows inclusion-free from ethanol (high a) at nearly all conditions, but grows with inclusions from water (low a) (Denbigh and White 1966). This is consistent with the previously 

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