Dependence of ice nucleating ability on misfit

Abstract

RAMASAMY*Volmer developed the theory of nucleation of condensed phase on a plane foreign substrate and called it heterogeneous nucleation [1]. Laboratory and field experiments have established that AgI particles serve as effective substrates for ice nucleation. The similarity between the lattice structure of AgI and that of ice is probably a prominent factor which makes it a good catalyst [2][3][4]. The ice-nucleating ability of AgI has been found to improve when bromine atoms replace some of the iodine atoms in the lattice, thereby decreasing the misfit [5]. AgI and CuI solid solutions have been tested for decreasing the mismatch between their lattice parameters and that of ice, thereby increasing the nucleation catalysts [6]. In a recent work, we have successfully used solid solutions of AgI, AgBr and CuI as catalysts to improve further the nucleation temperature of ice [7]. Volmer's theory of crystal growth has been adapted to obtain the critical supersaturation ratio by Turnbull and Vonnegut who demonstrated that the structure of crystals and the substrates that catalyse their formation resemble each other closely in the case of coherent nucleation catalysis and predicted a supercooling of 2.5 ° C for ice and AgI system [8]. They assumed that the formation of ice was perfectly coherent on AgI so that the actual disregistry is almost zero and the ideal disregistry is almost equal to the strain.In the present work, the misfit strain is calculated using the actual disregistry between the substrate lattice and ice lattice rather than the ideal disregistry as has been done previously [8]. The observed improvement in the ice-nucleation catalysis when the disregistry decreases [7] is explained by taking into account the actual misfit strain and its effect on the volume and surface free energies in the formation of the critical nucleus. The present study, therefore, is about a semi-coherent interface where the mismatch is partly taken up by strain within the ice phase and partly by dislocations at the interface. Also, the values of supercooling AT have been computed for various values of disregistry and they show remarkable agreement with the experimental values reported by different workers for solid solutions [5][6][7].Lattice parameter calculations and estimation of freezing temperatures for ice and the AgI-AgBr-CuI system have been reported by Palanisamy et al. [7]. Samples (13) of the system having different molar compositions were prepared by vacuum fusion. X-ray studies of these samples revealed that solid solutions of AgI-AgBr-CuI were formed. The highest nucleation temperature of -0.48 ° C has been reported for the 50-25-25 mol % composition of the above solid solution. A summary of the results is given in Table I. The experimental data of Vonnegut and Chessin for AgI-AgBr combinations [5] and Chessin et al. [6] for AgI-CuI solid solutions have also been considered to test the validity of the present study.Turnbull and Vonnegut have suggested that the interfacial strain between the low-index planes of ice an...

A Molecular Mechanism of Ice Nucleation on Model AgI Surfaces

“…where δ is the lattice mismatch, and a ice and a AgI are the lattice constants of ice and the silver iodide surfaces, respectively. Although previous researchers often only considered the lattice mismatch between hexagonal ice and AgI surfaces, 1,9,31,32 here we consider the lattice mismatch with both hexagonal ice and cubic ice, for reasons that will become clear below. Hexagonal ice, I h , is the stable form of ice under atmospheric conditions and cubic ice, I c , is a metastable form of ice.…”

“…The lattice mismatch can be quantified with the equation where δ is the lattice mismatch, and a ice and a AgI are the lattice constants of ice and the silver iodide surfaces, respectively. Although previous researchers often only considered the lattice mismatch between hexagonal ice and AgI surfaces, ,,, here we consider the lattice mismatch with both hexagonal ice and cubic ice, for reasons that will become clear below.…”

“…Important conclusions can be reached from Table , Figure , and Supporting Information, Figure S2. First, although hexagonal ice is often the only polymorph of ice considered when discussing heterogeneous ice nucleation on silver iodide surfaces, ,, cubic ice frequently was observed in the simulations, and cubic ice nucleated exclusively on one of the surfaces of silver iodide. Second, contrary to the earlier suggestion, the lattice mismatch between ice and a surface of silver iodide is insufficient to predict the ice nucleation ability of the surface.…”