The Structure of Supercooled Water and the Mechanism of Homogeneous Nucleation of Ice Ih

Abstract

Abstract The mechanism of the phase transition of supercooled water to ice Ih at −36 ºC is discussed on a molecular level. The structure of liquid water is described by the interstitial model first proposed by Samoilov. It is consistent with the results of X-Ray and neutron scattering and the experimental results on the diverse properties of liquid water. In order to illustrate our reflections several numerical calculations have been performed using exclusively experimental data. From the temperatur dependence of the nucleation rate the minimum number of water molecules, which constitute the critical germ, and the volume of the critical germ are determined. The occupation numbers of the regular tridymite lattice and the vacancies of the interstitial lattice are evaluated. The negative entropy of nucleation is discussed. It can be split in two contributions. They refer to the change of the statistics by the the transition from the protonucleus to the nucleus and the changes of the degrees of freedom of motion. Considering the simple model, the calculated value fits well to the experimental value of the nucleation entropy. The transition state, which is a key point in the classical nucleation theory, is discussed on a molecular level. It has a lifetime of 0.57 × 10−12 s. The nucleation volume is of the order nm3. One can expect that in this small volume density fluctuations exist in which especially the molecules in interstitial positions are involved. They are rich in energy and easily moveable. Statistical calculations deliver the deviation ±5 from the average value 23 of the number of molecules at interstitial positions in the nucleation volume. Corresponding density fluctuations have been observed by Huang et al. by very fast measurements of small angle X-ray scattering.