Nucleation of Ammonium Iodide Crystals from Aqueous Solutions

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Abstract
Ammonium iodide crystals nucleate at a sensible rate in small drops (10–50 micron diameter) of solution only if the supersaturation ratio S equals or exceeds a value of 1.21. This critical supersaturation ratio corresponds to an interfacial energy σ between ammonium iodide crystals and solution equal to or greater than 15.4 ergs/cm2. However, for sensible nucleation of NH4I on a freshly cleaved surface of one of the mica minerals, S is related to the disregistry δ between atom sites on the mica surface and the atom sites on a {111} plane of NH4I. This dependence can be described by the equation lnS=3.8δ2+0.0059. This equation is in agreement with the theory of Turnbull and Vonnegut relating disregistry to the potency of nucleation catalysts. However, the coefficient of δ2 is about 1 to 2 orders of magnitude smaller than the theoretical expectation. Our results are consistent with Bradley's hypothesis that S, for sensible 2‐dimensional nucleation of NH4I on its own perfect {111} planes, is remarkably small, viz. S≃1.01.